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Musings of an Energy Nerd

Radiant Barriers: A Solution in Search of a Problem

A well-insulated home doesn’t need a radiant barrier

Where do radiant barriers make sense?

A radiant barrier is a shiny panel or flexible membrane used in construction. Although radiant barriers have no R-value, they can be used as part of a building assembly — for example, an assembly made up of a radiant barrier and an air space — to slow heat transfer.

The sale and distribution of radiant barriers has always attracted a disproportionate share of scam artists, many of whom promise impossible energy savings. The explanations made by these hucksters usually include multiple references to space vehicles and NASA. Having been swayed by this type of misinformation, a few builders have adopted an almost religious belief in the magical powers of radiant barriers.

So, what’s the real scoop on these products?

Low-e surfaces don’t emit much radiant heat

A radiant barrier is a thin sheet of reflective material, often aluminum, applied to a substrate such as kraft paper, plastic film, cardboard, or plywood. By definition, a radiant barrier has a low emissivity (0.1 or less). Radiant barriers reduce radiant heat transfer across the space which they face. The lower a material’s emissivity, the more effective it is at reducing radiant heat transfer.

Although radiant barriers can be made from a variety of materials, there is no such thing as radiant barrier paint. No one has yet invented a paint that achieves an emissivity of 0.1 or below. (For more information on low-e paints, see ‘Insulating’ Paint Merchants Dupe Gullible Homeowners.)

Radiant barriers that aren’t facing an air space don’t work. If it’s sandwiched between a layer of sand and a concrete slab, it’s a conductor, not an insulator.

Although a radiant barrier has no R-value, it can help boost the R-value of an adjacent air space. According to ASHRAE Fundamentals, a vertical 3/4-inch air space has an R-value of…

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  1. Jamie Kaye | | #1

    Great article Martin.

    Great article Martin.

    Around my area, Hilton Head Island, SC, there are radiant barriers being sold for $1.50 sq. ft. installed. This makes no sense when you can have open cell foam installed for about the same price and get the ducts in the envelope and provide a thermal and air barrier at the same time. It seems like a no brainer, but people are still making the choice to install these barriers. Their money could be spent so much more wisely.

    Opportunity brings out the people selling these products and most of them don't understand building science or even know what it means. It is groups like yours that help get the real word out about what steps people should be taking if they really want to improve their homes performance and achieve energy efficiency gains.

    I am a loyal reader of yours, and of GBA in general, and can not say enough about the quality of content you guys provide.


  2. Curtis Dean | | #2

    Radiant Barriers article
    Thank you, thank you, thank you! This is a topic that has been haunting me for several weeks. We have a company that is inviting senior citizens to lunch and trying to sell them a radiant barrier that they say will cut their utility bills (not just heating and cooling, the entire bill) by 40%. Your article is the best I've seen about the value (or lack thereof) of radiant barriers. Here in northern Iowa, it makes no sense.

  3. Kris Knutson | | #3

    Another great report!
    Dear Martin,

    Your reporting on radiant barriers is among the best online; backed with science, reports, written in straightforward language and addressing many practical applications. Thank you!

    I am glad you mentioned radiant barrier paint. I researched this several months ago after seeing some outrageous claims on a manufacturer's website. They quoted reports from national labs claiming substantial savings, etc, and had a copy of the report on their website. I looked at the report and something was fishy, so I went looking for the article hosted on the author's website. Perhaps not surprisingly, the author's version of the report did not match with the manufacturer's and the projected savings calculations were quite different! I read the real report and found that in many climates, the net annual effect of radiant barrier paint is an energy and money loser. Minimal summer cooling savings are overshadowed by winter solar gain, which is of course, beneficial everywhere.

  4. Robert Hronek | | #4

    A couple of years ago I was
    A couple of years ago I was researching energy efficiency. On a hot sunny day the roof gives off its heat via radiation to the attic floor vs heating the air. Radiant barriers seemed like a good way to block the heat transfer. I was trying to find a way to value the benefit of the adding a radiant barrier.

    In my research what I discovered is what you have said. The better the insualtion the less benefit derived from the radiant barrier. I never did the math as it seemed unecessary.

    What readers should understand is that if they feel they would benefit from a radiant barrier what they realy need is to air seal and to add more insualtion. They will gain much more doing the insualtion then they would from adding a barrier.

    I have seen installers around here getting a $1.50/SF for installing foil, It is very lucrative for them. In bulk foil can be bought for $0.15-0.20/SF. Around here the perferred method of instalation is to lay it on top of the insualtion. A very easy and simple instaltion.

  5. Pam | | #5

    Hemp Houses
    Hey, Martin. Off topic, but did you see this article about the hemp houses in NC:

    What do you think? The silver bullet perhaps? They are saying that 1 tonne of hemp sequesters 2 tonnes of carbon.... and that there are a bundle of other benefits PassivHaus quality....

  6. Kevin Dickson, MSME | | #6

    Back to an unshiny radiant barrier idea
    Of course, most of these heat transfer type problems can be analyzed with a computer simulation, usually that work has been done.

    Here's an idea that can save homeowners the replacement cost of roof shingles, and prevent the sun's radiant energy from overheating the roof and the attic.

    Henry's makes a white elastomeric roof coating that goes on for about $0.15 per sq. ft. At least one of them is recommended for asphalt shingles. It should improve the reflectivity of the roof from 12-35% to 75-85%. The other benefit is that you would never need to replace your shingles. Recoating would be required every 10-20yrs.

    On many houses it might not be aesthetically acceptable, but on many two story homes without a steep roof slope, you just plain can't see the roof.

    Voila, a money and energy saving radiant barrier idea that also combats global warming, since the heat is primarily reflected back to deep space.

    There is a government tax credit for using this product in this application, so Martin, would you put it on your recommendation list, or is there a downside I'm not seeing?

  7. GBA Editor
    Martin Holladay | | #7

    Response to Kevin Dickson
    Kevin Dickson,
    You asked, "is there a downside?" Well, there are several possible downsides.

    1. Just because a product is sold as a "white elastomeric roof coating," doesn't mean it is a radiant barrier. By definition, a radiant barrier must have an emissivity (emittance) of 0.1 or less. I looked at the emissivity of the Henry Company product, and its emissivity is 9 times higher than that -- its emissivity is 0.9. So it's not a radiant barrier.

    2. In a heating climate, you don't want to cool off your attic. Any roof coating that lowers attic temperatures will raise energy costs in a home in a heating climate.

    3. What problem are you trying to solve? If you want to prevent heat from being transmitted from your attic to your conditioned space, the money would be better spent on attic insulation than on a roof coating.

  8. Brad Buser | | #8

    Foil on Polyiso Insulation
    Where does foil faced polyiso insulation fall in light of this article. I've designed wall sections where I feel the foil made significant contributions. For example, consider a single story commercial building with metal studs, foil faced polyiso sheathing, 2" inch airspace (ventilated), and dark brick.

    In this design the brick would absorb and emit significant amounts of radiant energy and by my analysis, prevents significant amounts of heat from entering the building since it has the obligatory air space for brick weeping that is also required for the foil to work correctly. I might be not shiny behind the air space after mortar dropping, but seems better than nothing in this case and fairly effective. Thoughts anyone?

  9. GBA Editor
    Martin Holladay | | #9

    Response to Brad
    As I hope my article makes clear, radiant barriers work. The question is, are they the most cost-effective way to improve a home's thermal performance?

    If you are buying 2-in. polyiso, the radiant barrier comes free. If the foil faces an air space, you can pick up an additional R-3 on your wall assembly. That's good.

    As I wrote in the article, the effect of a radiant barrier on a poorly insulated wall is more significant than on a well insulated wall. You describe a wall with only R-13 insulation -- so the additional R-3 is a fairly significant improvement.

    If you were designing an R-40 wall, the extra R-3 wouldn't be as significant.

  10. wz6DMEmbAP | | #10

    Nonshiny radiant barriers

    We may just be having a semantic disagreement. In a cooling climate, preventing solar energy from heating the roof and the attic is desirable. The reflectivity of a white roof is 0.9. So I'm saying that a white roof is a radiant barrier because it reflects 90% of the sun's radiant heat. The reflectivity of "Shasta White" shingles is closer to 0.30, and "Onyx Black" is 0.03.

    Is it more effective per dollar spent than attic insulation at keeping the house cool? Probably, but an energy simulation could tell us for sure. Since it's cheaper than shingle replacement, this is one radiant barrier that is cost-effective.

  11. GBA Editor
    Martin Holladay | | #11

    Response to Kevin
    You are using the phrase "radiant barrier" loosely.

    You're right: a roof coating can help lower attic temperatures. However, I doubt that the investment in a roof coating makes economic sense compared to investing the same dollars in thicker attic insulation. And in a heating climate, the roof coating will increase your energy bills.

  12. Greg Follet | | #12

    Great article - have been arguing this point with my insulation guy for years. On the lighter side, anyone else catch the radiant foil/barrier ads all over the place? Funny is good.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Greg Follet
    The Google advertising program has some kind of automatic algorithm that pairs advertisers with Web pages associated with the products being sold.

    Hey, we'll take the income.

  14. Rod Brewer | | #14

    The Performance Value is in the Application of a Radiant Barrier
    Over the years I have listened to and read opinions from building professionals, consultants and advisors. Although well meaning in their desire to design and build energy efficient homes, they totally misunderstand the nature of heat gain or loss in a building. R-values and mass insulation has become their Holy Grail on how to fix the problem of heat gain or loss. In a relatively well-sealed interior, heat gain or loss takes place between the difference in the interior air temperature and the surface temperature of the interior envelope of the building to include ceilings, walls, windows and doors. (Warm surface temperatures radiate heat into the interior air space, Cold surface temperatures conduct heat from the the interior air space) The interior envelope (drywall, wood, metal, glass, etc) temperature is primarily impacted by the temperature of the building materials in contact with the exterior envelope and the temperature of air space between the exterior and interior envelope. All building materials conduct or radiate heat, including mass insulation, depending on sources of radiant heat and air temperatures.

    It has been my experience that most buildings are designed with inadequate mass insulation. Once the interior envelope temperature is breached by an exterior temperature differential greater of less than the interior envelope surface temperature, heat gain or loss begins to take place. The rate at which this transfer takes place is accelerated by the degree of differential (i.e. exterior temperatures of 120 degrees vs 32 degrees for heat gain or loss respectively). Or in other words the r-values never seem to be high enough, thus the need for air-conditioning or heating.

    Think about it for just a moment. If mass insulation is so great by itself, then why are our energy bills so high. There are claims by many in the building industry especially regarding the "green products" that are exaggerated, but one wrong does not deserve another. Radiant barrier if applied properly will reduce the amount of radiant heat that is conducted by the building envelope. For example, if a radiant barrier is applied above the roofing felt before a tile roof is installed, radiant heat from the bottom surface of the tile is reflected into the air space between the tile and the roof deck. (Whereas without the radiant barrier, the heat is conducted by the asphalt felt and wood roof deck and radiated into the attic air space raising attic temperatures to as high as 150 degrees.

    There are many other successful radiant barrier applications.

    Radiant barriers installed under the roof deck, supplemented by a solar fan to increase the amount of active attic ventilation, have proven to reduce significantly the amount a radiant heat reaching the attic floor. Remember the more radiant heat that is conducted by the mass insulation the greater the interior heat gain.

    Cold climate installations required a completely different mind-set. To reduce heat loss, a perforated radiant barrier should be located between the ceiling and the mass insulation with an air space between the ceiling and the barrier. This type of insulation is not an easy retrofit and is more apropos to new construction. Install the radiant barrier on the underside of the ceiling joists. Add 3/4" battens to the the bottom of each joists. Attach ceiling material to batten leaving an air space between the radiant barrier and the ceiling material. I actually prefer an air space above the radiant barrier as well, thus creating an actual barrier to reduce radiant heat loss from the ceiling. The same construction method can be used to reduce heat loss through exterior walls.

    There have been several studies that substantiate the use of radiant barriers in the building envelope. More research should be done by independent groups. The influence of the mass-insulation industry and the utility companies unfortunately has proven to be the real barrier between true energy efficient buildings and the continued design and building of energy consuming buildings.

  15. Sally Leong | | #15

    window applications

    I just wanted to point to the potential for using radiant barriers for insulation of windows. There are many DIY claims of energy savings in homes and RVs and campers. Covers for windshields have been around for decades. I have done some research with thermal imaging on single pane windows and have found differential temperatures of ~18 degrees F up or down from window temperature.

    We have designed in air gaps in our products to maintain radiant function of the foil per recommendations of DOE.

    We hope to conduct third party testing in the coming year.

    I have met many people including myself who have newer windows with seals that have failed. This is a serious problem for retrofit with the cost of new windows that fail in 10 years. I am forced to replace them because I cannot see through them due to grey film that has formed between the panes.

  16. GBA Editor
    Martin Holladay | | #16

    Response to Rod Brewer
    Rod Brewer,
    Your arguments in favor of radiant barriers are consistent with those of what I call "religious believers." I have no doubt that nothing I write, and no research data, will undermine your faith.

    As I have already written, radiant barriers work. The only problem is that their effect is feeble in a well insulated building. They also cost much more to install than insulation.

    You're right about one thing, though: most U.S. building are poorly insulated. That's why GBA has consistently championed the need for more insulation.

  17. GBA Editor
    Martin Holladay | | #17

    Response to Sally Leong
    You wrote, "I have met many people including myself who have newer windows with seals that have failed. This is a serious problem for retrofit with the cost of new windows that fail in 10 years. I am forced to replace them because I cannot see through them due to grey film that has formed between the panes."

    The problem you describe is a real one, but it has nothing to do with radiant barriers. When the airtight seals separating two panes of glass in a double-glazed window fail, moist air can enter the gap between the panes, leading to condensation and clouding. The solution, as you correctly point out, is to replace the glazing.

    You are also correct that window glazing can benefit from a lowering of emissivity -- precisely because windows are the least-well-insulated component in most building envelopes. The best way to lower the emissivity of your windows is to order low-e glazing. Covering windows with aluminum foil is not an option for most homeowners -- although your suggested trick has limited value for vehicle owners and owners of RVs.

  18. ED GILSON | | #18

    Both the Florida Solar Energy Center and Texas A&M University tests shows for homeowners in sunbelt states are better off to install radiant barrier over adding more mineral insulation. Both reports state that R-19 and radiant barrier is the equivelent of R-30 or better. The US Dept of Energy efficiency and Renewable Energy states the following." Radiant barrier greatly improves the performance of mass insulation - Why? because regular insulation contains air or gfas filter air spaces that retard primarily conductive heat flow. Radiant barrier reflects a large portions (95%), of incoming radiation, and transfers very little thermal energy to the house's interior. Without radiant barrier, your roof radiates solar generated heat to the insulation below it. The insulation absorbs the hear, and gradually transfers it to the material it touches, principally the ceiling. This heat transfer make your air condition run longer and consumes more energy. RIGHT FROM THE US DEPT OF ENERGY. If radiant barrier saves so little energy saving, why does the state of California in it's Title 24 program list radiant barrier as an excepted insulating product? Why does the Hawaiin insulation code state that if you have a light colored roof and radiant barrier, no other insulation is required. It also states R-19 is suffient insulation. Hawaii has no winters but all their heat gain is via the suns radiation. Why would the state of Florida give building credits to contractors who install radiant barrier. The city of Austin Texas has always given cash rebates for installing radiant barriers. Lasting...who says radiant barrier works
    (1) Phillip Fairey, Florida Solar Energy Center, 19th Internaional Thermal Conductivity Conference, 1985
    (2) Deigning and installing radiant barrier Systems, Florida Solar Energy Center 1984
    (3) Performance testing of radiant barriers by JAMES HALL TVA, Chattanooga, TN 1986
    (4) Use of passive radiation barrier in ventilated attics, Samior Moujaes Ph.D University of Nevada
    (5)An ecvaluation of the placement of radiant barriers on thier effectiveness of reducing heat transfer in attics, S.D Katipamula dnd D. O"Neal , at Texas A&M university, 1986
    (6)Cooling energy measurements of unoccupied sigle family houses with attics containing radiant barriers by W.P. Levins and M.A. Karnitz , Oakridge National Laboratories 1986
    (6) Influence of Infrared radiant barrier on attic hat transfer by S/D Katiparnula and D. ONeal, W.D. Turner asnd W.e. Murph at annual symposium ofn improving buildong Energy Efficiency in hot humid climates 1985
    (7) In-site measurement of attic radiant barrier performance in hot arid climates H. Wu Arizona Statre University.
    (8) data presented by RIMA.
    Do we need more?? Again, don't paint the radiant barrier industry as a whole because of a very few retailers of radiant barrier who make outrageous claims or over zealous pricing. THEY ARE IN THE MINORITY. And you will find these same types in all fields,,Claims by some retailers of mineral insulation fit this same catagory...

  19. Lee | | #19

    Worked for me
    My house is steel truss, 4-12 roof pitch, purlins on 24"centers, foil/bubble/foil laid over the purlins and metal roofing directly over that. I am in southern Arizona. The roof is red. The top surface is blistering hot when the sun's out yet inside the garage you can feel no heat radiating down from the roof as you would if the foil/bubble/foil was not there. Over the living area I have added 12" fiberglass bats so there is no heat coming through from the roof, period. Foil/bubble/foil is simply aluminized layers on both sides of what appears to be bubble wrap.

  20. gred | | #20

    radiant barriers
    Good discussion of this. I myself went to a free dinner to hear the pitch this summer, and was able to steer a couple friends away from spending $1.50/sf. One person bought their own, added more insulation and foil.
    I have a question though. Foil-faced roof sheathing would only be effective at reflecting heat coming up from the house, not at keeping heat from the roof covering from entering the attic because there is no air space between the shingles/plywood and the foil. There is is, of course, an air space in the attic. Therefore it seems that this is a better strategy for heating climates, as it would prevent some heat loss from the house. In an A/C situation there should be an air space above the foil to reflect the roof heat away from the living space.
    I told my DIY friends to install it on the bottom of the roof rafters - creating an air space on both sides.

  21. GBA Editor
    Martin Holladay | | #21

    Response to Ed Gilson
    Ed Gilson,
    Your post is a mix of truths, half-truths, exaggerations, and outright falsehoods.

    1. You're right that anyone living in a hot-climate house with thin (R-19) attic insulation will benefit from a radiant barrier. However, installing that radiant barrier will cost more than upgrading the attic insulation to a reasonable level.

    2. You wrote, "Radiant barrier reflects a large portions (95%), of incoming radiation, and transfers very little thermal energy to the house's interior." Well, this is kind of true. Reflecting radiant energy helps, but it's no miracle cure. An assembly consisting of a radiant barrier and its adjacent air space can have an R-value of about R-3. That's something -- but it certainly doesn't stop all the heat flow through the assembly.

    3. You wrote, "Without radiant barrier, your roof radiates solar generated heat to the insulation below it. The insulation absorbs the heat, and gradually transfers it to the material it touches, principally the ceiling. This heat transfer make your air condition run longer and consumes more energy." You're right that plywood or OSB roof sheathing can radiate heat to the top of the insulation on your attic floor. However, your implication that the attic insulation doesn't slow heat transfer is mistaken. The whole reason that a hot-climate house needs R-30 insulation on the attic floor is to slow down heat flow. If you have a good R-30 layer of insulation, properly installed, it does an excellent job of slowing heat flow through the insulation. Although it's always possible to measure some heat flow -- even through R-30 insulation -- the rate of heat flow is very, very, small. By the way, it's also possible to measure heat flow through an assembly with a radiant barrier.

    Heat flow can be slowed but not stopped. The way we measure heat flow through a building assembly is by lab tests. These lab tests determine the building assembly's U-factor or R-value. The lower the U-factor and the higher the R-value, the better the building assembly is at slowing down heat flow.

    4. Concerning your questions about whether California's Title 24 defines radiant barriers in a certain way: Title 24 determinations are a political question. Such determinations have little to do with physics.

    5. There's an easy answer to your point about the use of radiant barriers in Hawaii: Hawaiian buildings are barely insulated! Have you been to Hawaii? The reason people refer to Hawaii as "paradise" is that outdoor temperatures are comfortable year 'round. Many homes are uninsulated. If you have an uninsulated building assembly, a radiant barrier is effective at slowing heat flow.

    6. Concerning your list of references and technical papers: I never said that radiant barriers don't work. I said that their effect is feeble and they are expensive to install. It's better to spend your money on air sealing and insulation than radiant barriers.

  22. GBA Editor
    Martin Holladay | | #22

    Response to Gred
    You wrote, "Foil-faced roof sheathing would only be effective at reflecting heat coming up from the house, not at keeping heat from the roof covering from entering the attic because there is no air space between the shingles/plywood and the foil."

    You're wrong. When sun shines on the roof sheathing, the roof sheathing gets hot. If the roof sheathing includes an integral radiant barrier, the shiny side points down, toward the air space. The amount of radiant heat that is emitted by the underside of the roof sheathing is determined by the emissivity of the surface facing the attic. A shiny surface has a low emissivity, so the radiant barrier slows radiant heat transfer from the roof sheathing to the objects that the sheathing faces -- for example, the top of the attic insulation on the floor of the attic.

  23. Glen | | #23

    Radiant Barrier in Houston

    I agree with your general comments that radiant barriers are well oversold to their real effect on your energy bills, particularly on the retrofit applications.

    My house was built in 2003 and lets call it an "upgraded" builder home, but far from anything in Fine Homebuilding. I have R30 blown-in attic insulation fairly well done, passively ventilated (ridge/soffit vents) roof with HVAC system in attic.

    Radiant barrier decking (LP's product) was used on the house portion and regular sheathing used on my detached garage. The house and garage are oriented the same, therefore essentially get the same sun exposure. As an experiment (one of many on-going), I tested the air temperature at the bottom of the roof rafters (2x6) in mid-July, full sun, about 3:00 in the afternoon for both attics. The radiant barrier reduce the attic temperature about 10 degrees (115 vs 125) as compared to my garage.

    So the question is, how much does the lowering of 10 degrees have on my HVAC system? Also, there is some leakage in my house to the attic, so 115 temperature is probably being tempered a few degrees as compared to an unconditioned garage below. This is beyond my knowledge, but would be worth knowing.

    If I had it to do over again and had a ventilated attic I would still use the radiant barrier decking (in hot climates) since the cost is minimal and there is some benefit. As you noted, the benefit is small. Is a ventilated attic in a hot-humid climate the best solution obviously not, but it is better that not having it.

    Lastly, my water heater is also in the attic, so one side effect is almost no standby heat loss from the water heater to the attic at 115 degrees:)

  24. GBA Editor
    Martin Holladay | | #24

    Response to Glen
    I agree that for your house -- or any hot-climate house with HVAC equipment and ductwork in an unconditioned attic -- it makes sense to use radiant barrier roof sheathing. That's what my article states.

    Once a builder has made the mistake of putting HVAC equipment in the attic, all possible remedies are expensive. If you can afford it (some day?), it would make sense to use closed-cell spray foam to create an unventilated conditioned attic, in order to bring your HVAC equipment into your home's conditioned space. But the high cost of this work probably can't be justified until energy prices rise a bit.

  25. Don | | #25

    Radiant barriers
    In 1956 I designed my first house in rural Connecticut putting into effect what I had learned in architecture school (indeed a long time ago)...that radiant heat losses account for approximately 2/3 the total heat loss.. The walls of the house were insulated with a product produced by Reynolds Aluminum... Arriving in long flattened rolls of aluminum, they were cut to length and then popped open by pulling along the flanges at the edges. This produced three layers of aluminum with two 1" air spaces between and closed sides. Flanges were fastened to studs... I visited the house about 5 or 6 years ago, a half century after it was built... no one complained about heat losses and the texture 111 sheathing/finish siding was in fine condition.
    For my own house built in Maine I used a radiant barrier on the warm side of R-19 walls and R-30 second floor ceilings below a ventilated attic. So far, so good.

  26. GBA Editor
    Martin Holladay | | #26

    Response to Don
    As I hope you know by now, the statement that "radiant heat losses account for approximately 2/3 the total heat loss" has no basis in physics. Let's hope that by now, this falsehood is no longer part of the standard architectural curriculum.

    For different components of a house, radiant heat loss plays a larger or smaller role. For the average house, radiant heat loss is significant only for windows, with single-glazed windows obviously being the worst offenders. Low-e double glazing (or better yet, triple glazing with two low-e coatings) has gone a long way toward addressing radiant losses through windows.

    In a well insulated wall or roof assembly, as my article points out, radiant effects are almost non-existent.

  27. Wayne Jones | | #27

    OK, I get it
    I have been designing radiant barrier sheathing on multifamily apartments in NC for the last 3 or 4 years, and have always gotten a lot of anecdotal evidence that they work well at reducing cooling load, (or at least all the contractors like to work underneath them in the summer). This has been in combination with R-38 attic insulation, ducts in partly unconditioned space (they get pretty well covered up by the blown in fiberglass). I do see with your explanation that their positive (or negative in winter) benefit becomes more marginal when coupled with the high insulation level. Thank you.

  28. GBA Editor
    Martin Holladay | | #28

    Burying ducts in blown-in fiberglass
    If you are burying ducts in blown-in fiberglass, I hope they are first insulated with duct insulation equipped with a vapor barrier. Otherwise, the ducts will be dripping wet during the summer months.

  29. Ivan Druker | | #29

    "Up North a sun warmed attic helps lower heating bills"
    I take issue with this line in your article. Assuming a properly ventilated and insulated attic space, the space above the attic insulation is "outside" space, protected from the elements by the roof but still "outside". Therefore the temperature of the attic space (above the insulation) should have absolutely no bearing on the temperature inside the building and your heating bills.

  30. GBA Editor
    Martin Holladay | | #30

    Response to Ivan Druker
    The better your ceiling air barrier, and the thicker the insulation layer on your attic floor, the closer your house will come to the theoretical situation you posit -- namely, that the temperature of the attic is irrelevant.

    That's theory. If we can build a house to Passivhaus standards of airtightness, and if we install R-60 ceiling insulation, the performance of our house will be close to that theoretical situation.

    Nevertheless, insulation slows heat transfer; it doesn't stop it. Heat flow can be measured through R-60 insulation.

    Moreover, the vast majority of U.S. homes don't come close to this type of Passivhaus performance. Even up north, where we have seriously cold winters, there are lots of homes with ceilings insulated with sloppily installed R-19 batts. For these homes, that attic temperature definitely matters.

    Finally, it's important to remember that just because an attic has soffit vents and ridge vents, does NOT mean that the attic temperature is the same as the outdoor temperature. On a sunny afternoon in March, an attic will be warmer than the outdoor temperature. That can help lower your heating bill.

  31. Chris | | #31

    The biggest problem with
    The biggest problem with exaggerated claims of savings from radiant barriers is that once people feel they've been hoodwinked, they're less likely to believe other more effective energy efficiency practices.

    Air sealing and properly-installed insulation should be the "low hanging fruit" that can make radiant heat transfer a minor issue. I guess they just don't look as shiny and exciting to people who haven't researched best practices, though.

  32. Gavin Farrell | | #32

    Minor nit-pick
    Martin, in the article you state:

    "Since the drywall is at room temperature, it isn’t emitting any radiant heat. (Radiant heat transfer only becomes significant when a radiating surface is at a significantly higher temperature than surfaces in the room or air space which it faces.)"

    The second part is correct for interior comfort, but the first part is I believe technically incorrect. All materials above absolute zero emit radiant heat to some degree, room temperature drywall is definitely emitting radiant heat at all times.

  33. GBA Editor
    Martin Holladay | | #33

    Response to Gavin Farrell
    Gavin Farrell,
    In winter, let's say the drywall is at 70 degrees F. My skin is at 90 degrees F. So my skin is radiating heat to the drywall, and the drywall is gaining heat that has radiated from my skin to the drywall.

    The sofa, the table, and the books in the room are at 70 degrees F. Everything except me, the dog and the cat are at room temperature. The sofa is not gaining heat from the drywall; nor are the books. However, my skin, my cat, and my dog are all radiating heat to the drywall. The drywall is being heated by our radiant bodies.

    Now, if I go the refrigerator and take out a cold 6-pack of beer, and put the 6-pack on the table -- NOW the drywall is radiating heat to the 6-pack. The beer is warming up, through several mechanisms: conduction from the table, convection from the 70 degree air, and through radiation from the warm drywall.

  34. Gavin Farrell | | #34

    aarg, sorry to nit-pick.
    That fact that solid objects in the same room are different temperatures doesn't mean that one or the other isn't still radiating heat. They're ALL still radiating some amount of heat all the time, though the net exchanges might favor one or the other as things find temporary equilibrium. Even the cold beer is radiating heat, although, as you say, it is accepting more heat through various mechanisms than it is giving off. Put on the infra-red goggles. Just because the beer is colder than its surroundings doesn't mean it still isn't radiating heat.

    So, basically my 'splitting hairs' criticism was that to say: "Since the drywall is at room temperature, it isn’t emitting any radiant heat" is factually incorrect. It definitely is. If it wasn't, how could it suddenly contribute to warming up the cold beer? It only emits heat when there is something colder that it nearby? No, the drywall, so long as it has temperature above absolute zero is always radiating heat. It would be better to say something like: "Since the drywall is at room temperature, a human being standing next to it will not feel any discomfort from any significant difference in radiant heat"

  35. Riversong | | #35

    Sorry Gavin
    But you're wrong.

    An object above absolute zero can radiate heat only if there is another cooler object to receive it. The amount of radiant transfer is proportional to the fourth power of the temperature difference and inversely proportional to the square of the distance.

    To suggest otherwise is similar to saying that an object conducts heat all the time but other objects can "feel" it only if they are cooler. Heat is a measure of the transfer of non-kinetic energy from one object to another.

    The zeroth law of Thermodynamics states that two objects are in thermal equilibrium when they transfer no energy to or from one another.

  36. Riversong | | #36

    Now for Martin...
    Martin uses the same kind of misleading and sensationalist "journalism" that he accuses radiant barrier "religious fanatics" of employing.

    He responded to one commenter: "Your post is a mix of truths, half-truths, exaggerations, and outright falsehoods." I would say the same thing of Martin's "musings".

    One simple distortion is that "radiant barriers have no R-value". One of the most prevalent radiant barrier materials being used today in construction and renovation, including HVAC work, is bubble foil. By Martin's own admission, bubble foil has a measurable R-value, albeit minimal unless part of an assembly (which is typically how the product is tested and advertised). This R-value is almost entirely due to the reduction of radiant transfer across the air space created by the bubble foil, just as the dramatic increase in window R-value with lowE coatings is due to the reduction in radiant transfer between the glass surfaces.

    But to state flatly that "there isn’t any need to install a radiant barrier in a well-designed home" or "new homes in a cold climate…shouldn't use radiant-barrier roof sheathing" is to ignore their very real and field tested value.

    There is overwhelming evidence that a radiant barrier underneath the sheathing or framing of a hot-climate house will significantly reduce attic temperatures and hence the delta-T and conductive heat transfer through the ceiling, regardless of the R-value of installed mass insulation. A Florida Solar Energy Center study, for instance, measured a 26% reduction in ceiling heat flux with the addition of a radiant barrier under the roof, and Martin himself quotes a range of values exceeding that.

    If the delta-T across the ceiling is reduced by 26%, then the heat flow will be reduced by that percentage regardless of the R-value of the ceiling insulation. So while it's true that, the greater the insulation the less the return on investment of a radiant barrier, it's simply untrue to state that it has little to no value.

    In a cold climate, radiant gain in winter is far less of an issue since roofs are often covered with snow, but radiant gain in summer is as much of a problem as in hot climates and perhaps more so since many northern homes lack air conditioning.

    And to use a simple mathematical formula for reducing the value of this radiant ceiling flux reduction by the proportion of total heat transfer attributable to the ceiling ignores the scientific data that human discomfort is far more impacted by warm ceilings than by any other overly warm surface. So a cooler ceiling means a more comfortable environment with a higher AC setpoint and consequent energy and cost savings.

    It is also disingenuous to use inflated installed prices to "prove" that it is more cost-effective to add bulk insulation than to install a radiant barrier under the rafters in an existing attic. If the attic has a floor and is used for storage (as is often the case in older homes), then there is no easy way to add bulk insulation. And foil-faced kraft paper (what used to be called builder's foil) is selling right now at my local lumber yard for less than 8 cents a square foot. Installation labor, for stapling this up is minimal and a task that almost any homeowner could manage themselves.

    Martin's repeated reference to an R-3 value of an air space ignores the rest of the ASHRAE data which shows that the most effective place to add a radiant barrier for prevention of heat loss is in a floor. A 4" air gap with one foil surface, for instance, with heat flow downwards will offer almost R-9 and with two facing foil surfaces nearly R-11.

    For this reason I routinely install foil/bubble/foil under an in-floor radiant heating system 4" from the subfloor. This is one of the two most effective and cost-effective places to use a radiant barrier. The other is under the roof in any climate, unless the roof covering is white.

    While it's true that there is much disinformation being propagated about the value of radiant barriers, challenging that with similar distortions and misinformation only undermines the credibility of the author and the credibility of the green building movement.

  37. bHqUcamKme | | #37

    Radiant Barrier-High Cost, Low Effectiveness
    I just did an energy audit this morning for a couple who succumbed to a radiant barrier sales guy at a county fair three weeks ago. They spend $2,150 to cover the floor of their 1008 square foot attic with a radiant barrier (EnergyShield AB). I went into the attic and peeled back the blanket to find a mere 5" of cellulose. For less than a third of the cost, they could have insulated to R49, received a 70% rebate from the utility I work for, and gotten the rest covered through the tax credit. SIGH.

  38. GBA Editor
    Martin Holladay | | #38

    Response to Robert
    I stand by my reporting.

    Foil-faced bubble wrap is composed of polyethylene plastic surrounding bubbles of trapped air; on one side of the polyethylene bubble wrap, a thin radiant barrier is adhered to the polyethylene. The radiant barrier, as I correctly reported, has no R-value. Foil-faced bubble wrap -- like other faced building materials with a radiant barrier adhered to other materials, like foil-faced polyisocyanurate or foil-faced EPS -- does have a measurable R-value. But a radiant barrier does not.

    However, a radiant barrier assembly does have a measurable R-value.

    The heat flux numbers you quote, as I pointed out in my blog, refer to heat flux through a poorly insulated attic floor. It's now illegal to build homes with R-19 insulation on the attic floor. That's why I pointed out that "there isn’t any need to install a radiant barrier in a well-designed home." I also pointed out that if your builder made the unfortunate decision to put HVAC equipment or ducts in your attic, then you had better install radiant-barrier roof sheathing.

    Plenty of studies, including studies conducted by Danny Parker at the Florida Solar Energy Center, as well observations by people like Curtis Dean, who just posted a comment, confirm my statement that insulation improvements are cheaper than the installation of an attic radiant barrier in an existing home.

  39. Riversong | | #39

    Reporting or Advocacy
    Martin, I would not call your blog post "reporting". Your increasingly strident "musings" are sensationalistic, deliberately deprecating towards those you disagree with, use scientific data and mathematical formulations selectively, include conclusions not supported by your own evidence, and seem to massage the facts to fit a predetermined conclusion.

    You cannot counteract propaganda and misinformation with more of the same. What is particularly disconcerting is that your readers seem to accept your distortions with the same alacrity with which too many have bought into the commercial hype.

    The green building movement should be based on sound science and conclusions that are the logical results of an objective analysis of data and field experience. That is not what I read in your latest blogs.

    I expect hype from hucksters and salespeople. I hold you and the other editors and authors here to a much higher standard.

  40. Believer in Alaska | | #40

    using the wrong standard.
    using the R-Value standard to judge efficiency is not correct. Of course a thin sheet of foil has no R-Value, but the ability to reflect the heat either back into the house or away from the house is the important issue. A simple example is the mylar tents forest fires hunker under when caught in flames. They survive a flash over not because of the R-Value but reflectivity/emissivity.

    The same as you can't compare thermal mass in log homes with R Value. There is physics at work that compensate for or overcome R-Value. Radiant Barriers work, use them and don't forget the 3/4" air gap.

  41. GBA Editor
    Martin Holladay | | #41

    Response to Believer
    I agree with you: radiant barriers work.

    The best way to assess the performance of a radiant barrier is to measure or calculate the R-value of the total assembly in which it is included. Engineers and laboratory technicians do this all the time; R-value tests are an appropriate way to test the performance of an assembly including a radiant barrier.

    But, as I've written several times before, anyone thinking of investing money in a radiant barrier should realize that the money would be better spent on insulation. That way you'll get more bang for your buck.

  42. Gavin Farrell | | #42

    Sorry Gavin - Response to Riversong
    'An object above absolute zero can radiate heat only if there is another cooler object to receive it.'

    The molecules in an object have no idea about the proximities of other matter around them and whether or not it is cooler or warmer in relation to them, all they know is that they have energy ('temperature') and one of the natural things objects with energy do is slowly release that energy via radiation. It gets fired away in all directions. The amount of radiation it emits is proportional to how hot or cold the object is and what properties it has. It doesn't matter if it is colder and 'warming up' in relation to other things near it, hotter, or at 'equilibrium' with the things immediately around it. It will be emmitting radiant energy in some form if it has temperature.

    I love your articles Martin, they are always extremely interesting and informative. Thanks for all the interesting research & compilation.

  43. chris | | #43

    ISO order
    I'm planning on using a combination of 1'' XPS and 1'' poly ISO in a rafter cavity which also serve as a ventilation cavity. I was considering the ISO board be the most interior part of the stack because of its low perm rating.

    If I were to put the XPS in the interior most part of the stack wouldn't the foil block the XPS from drying into the vent gap?

  44. GBA Editor
    Martin Holladay | | #44

    Response to Chris
    You need to provide more information.
    1. What's your climate?
    2. Why are you mixing two types of foam board?
    3. Will your rafter cavities also include other types of insulation such as fiberglass batts or cellulose?
    4. What total R-value are you aiming for?
    5. Will you be installing multiple stacks of foam board, or just one layer each?
    6. Do you have any plan to address thermal bridging through the rafters?

  45. chris | | #45

    Zone 6
    have 2x4 rafters. ISO

    Zone 6
    have 2x4 rafters. ISO only comes in 1'' around me, thus will stack 1.5'' XPS & 1'' ISO
    ISO & XPS will only be in cavity, which will also serve as ventilation cavity getting air from soffit to ridge
    This combination gets me the most R value (considering I only can get the 1'' ISO)
    1 layer of each......1'.5'' XPS and 1'' ISO
    I understand the thermal bridging concern .....project limited in scope and funds

  46. GBA Editor
    Martin Holladay | | #46

    Second response to Chris
    Q. " If I were to put the XPS in the interior most part of the stack wouldn't the foil block the XPS from drying into the vent gap?"

    A. The XPS will not be wet, so the XPS will not need to dry. If your insulation forms an air barrier, that's a good thing. The permeance of the foam is irrelevant in this application. Just be sure that everything is installed in an airtight manner.

  47. Chris Sanders | | #47

    Interesting Martin .......I'm
    Interesting Martin .......I'm not suggesting its going to be wet . However because the ISO foil is extremely vapor retardant, don't you want it to be at the warm-in-winter side of the stack, keeping any moisture away from the cooler part of the rafter. This is why it seems by putting the XPS in first with the foil iso OVER it would block the XPS from drying into the vent gap.

    With this is mind does it not make sense to have the ISO layer as the interior most layer with the XPS over it?

  48. GBA Editor
    Martin Holladay | | #48

    Response to Chris
    There won't be any conceivable circumstances, nor any conceivable way for you to stack your foam, that will allow your XPS to "dry into the vent gap." But it doesn't have to -- the XPS isn't wet.

    When you stack up your foam and seal the edges to make the foam airtight, it acts like a single, vapor-impermeable sandwich. You're not looking for vapor diffusion through the foam, and vapor diffusion isn't going to happen.

  49. Chris Sanders | | #49

    Order when layering
    Ok understood Martin. Looking a few posts back for the details... When layering 1' ISO (one side of the ISO has a foil facer) and 1.5'' of XPS bewteen a 2x4 rafter space that will also serve as ventilation cavity in a cathedral/slant wall of a one and half story house. What should the inner most board be XPS or the ISO.
    Per your comments above now I'm thinking 1)drywall 2) XPS then3) ISO with the foil facer facing outwards towards the backside of the roof decking. Both layers will be cut 14'' wide vs 14.5'' so they may be foamed in place against the sides of the rafters to create a tight air barrier. Please advise on the correct order of layering the rigid board for this application.

  50. GBA Editor
    Martin Holladay | | #50

    Fourth response to Chris
    The only possible difference between how you stack the two types of foam would be if there's only one way that allows the foil to face an air space. You'll get a slightly higher R-value if the foil faces an air space than if it doesn't.

    It doesn't matter if the air space with the foil is between the drywall and the foam, or between the foam and the roof sheathing. But if you have a choice, the foil should face an air space.

    And by the way, if at all possible, you should include continuous rigid foam under your rafters to address the thermal bridging problem.

  51. Chris Sanders | | #51

    Because I will only be installing 2.5'' total of rigid board in 3.5'' cavity indeed their will an air space/ ventilation cavity between the foam and the roof sheathing. I guess my order working from the inside to the exterior would indeed be Drywall, XPS the ISO with foil side facing the airspace/back of sheathing, would it not?

  52. M. Johnson | | #52

    You have some valid points,
    You have some valid points, but it is wrong to dismiss the phenomenon of HVAC systems with ductwork located in unconditioned attics. In Texas where I live this is a nearly ubiquitous practice. If we put in more than a certain amount of insulation, it would become impossible to use the attic for storage, or even to walk safely to the AC for service. I am not going to build my own house to meet your criteria of what is right, the houses I buy have been built already, and what is productive is to accept what is already done and discuss what will improve things. People standing under a radiant barrier immediately notice the lesser heat radiated, and I do believe the FSEC reporting which consistently says that radiant barrier reduces AC consumption 8-12% in homes normal for that state.

  53. GBA Editor
    Martin Holladay | | #53

    Response to M. Johnson
    M. Johnson,
    It's certainly interesting to see how regional differences in construction practices play out. Here in northern New England, it's not unusual to install 20 inches or more of attic insulation. If you want to use your attic for storage, you have to build up your framing and install a plywood deck above the insulation.

    But as a general rule, we don't put HVAC equipment or ducts in our attics up north. When I explain to someone in Texas that it's perfectly possible to build a house with forced-air heat and air conditioning WITHOUT any ducts in the attic, they usually say, "That's impossible," or "That's not going to happen in Texas."

  54. Chris Byers | | #54

    Gavin is 100% correct
    Sorry Robert and Martin, Gavin is 100% correct on this one.
    Any object above absolute zero ALWAYS radiates heat. This is a property of matter; it happens independently of the temperature of the object it is next to. Robert, you lack a basic understanding of the principles you state so I don’t know quite where to start.

    1. Robert you state that “heat is the measure of the transfer of non-kinetic energy from one object to another” which couldn’t be more false, in fact the exact opposite is true. Heat is defined on the molecular scale as the kinetic motion of molecules. Think of billiard balls moving very fast and imparting their kinetic energy on to the next ball, this is exactly the same as water on the molecular scale. Ice has less heat because the molecules are in a lattice and move very slowly, during waters transition to the gas phase the molecules have enough kinetic energy to escape the bulk water and leave as steam.
    2. Also your statement about thermal equilibrium is also grossly misunderstood. The definition of the word equilibrium defeats your very point (see equilibrium law). There is ALWAYS some rate of heat transfer between objects above absolute zero; only at equilibrium is the NET-RATE of heat transfer between the two objects zero.
    3. Additionally Robert, Martin is correct about R-value and radiant barriers. Not knowing the basic principles of radiant energy transfer reflect in your incorrect conclusions with regard to R-value and radiant barriers. I refer to your statement about a quote from Martin, “One simple distortion is that ‘radiant barriers have no R-value’.” To split hairs even further, there is no such thing as radiant heat, only radiant energy. The two are not synonyms. Radiant energy consists of photons, it is the interaction of these photons with matter that produces heat (and foil reflects photons). R-value is a measure of thermal resistance to heat flow; since foil is a conductor then it does not have a resistance to thermal heat flow. Thus the foil itself provides no R-value by definition (because it reflects photons not heat). However, by reflecting photons from objects radiating energy (foil is not directly in contact with) it reduces absorbtion of the photons which cause heat. This results in an observed R-value increase of the matter it is attached to. R-value determination experiments were never designed to measure differential photon absorbtion.

    Martin is correct and has been very careful with how he states his conclusion about radiant barriers. I will try to be more blunt. Since one cannot simply equate radiant barrier to insulation R-value I will use the term energy which encompasses heat and radiation and use an arbitrary value to illustrate the point. Properly installing a radiant barrier will reduce energy transfer by some arbitrary value 5, but for the same amount of money one can purchase insulation that will reduce energy transfer by 5Y (where Y is a number > 1). So the point Martin is making is if forced to chose, why not spend the same amount of money on something that will do a better job?

    To beat the dead horse…yes, good insulation (like foam or cellulose in an air sealed cavity) does a better job than a radiant barrier. This is because good insulation stops and/or slows two forms of heat transfer whereas a radiant barrier only slows the other one. Obviously this depends on the total R-value of insulation used.

  55. Ward | | #55

    "Where do radiant barriers make sense?"
    All this talk about attic space and we seem to have lost site of a very simply solution where radiant barriers do exactly what they are designed for - radiate! Placing radiant barriers behind radiators, especially for example, those exposed to uninsulated concrete exterior walls. Like most products radiant barriers have a specific application.

  56. GBA Editor
    Martin Holladay | | #56

    Ward, I disagree
    A radiant barrier is not designed to radiate. A radiator is designed to radiate -- as well as to encourage convection.

    When you place a radiant barrier on the wall behind a radiator, it lowers the emissivity of the wall, thereby reducing heat transfer from the radiator to the wall.

  57. phil | | #57

    rad. barr.
    I have read with interest the comments flowing back and forth. and good exchanges on a subject that we can learn from. I would interject that we are spending too much time and placing too much enphasis on "R value". the comfort level in a home and the energy we buy to acheive what we desire are the base to work off of. R value is just a tool to help predict that and not a true science. Insulation sales love the word and we all buy into it because it keeps bringing us back to insulation and more sales. and it is easy for advisors to say, add more insulation and all will be well. the new product is spray foam and we can argue the merits of open or closed but we still sell insulation. and to talk about what a well insulated home needs doesnt address most home in america. also when advisors talk about heat gain in positive terms they display a total lack of understanding of those in extreme southern climates and the opposite is also true. many homes we see in Texas are underinsulated, HVAC in the attic and its going to be that way the life of the home. adding insulation in the attic is a good thing but doesnt fill all the needs. in the mix of things they can do radiant barr. is a positive. maybe not the first and yes often misquoted and over priced.

  58. GBA Editor
    Martin Holladay | | #58

    Response to Phil
    I disagree with your statement, "R value is just a tool to help predict that and not a true science." The R-value of an assembly is strictly defined and can be measured; it corresponds to the requirements of the scientific process.

    If a building assembly includes a radiant barrier, then the affect of that radiant barrier will be reflected when the R-value of the building assembly is measured. Of course, other factors -- especially air leakage -- also strongly affect the thermal performance of a building assembly.

  59. Ward | | #59

    I stand corrected...
    I stand corrected, I agree that a radiant barrier is not designed to radiate, but rather reflect. Please excuse the improper terminology, but I do take issue with the idea that radiant foils do not improve energy efficiency in some applications and that they are perfect solutions for some things...
    Example: Cash strapped renter wants to improve energy efficiency of his/her apartment in an uninsulated concrete building.
    Martin, to achieve a comfortable environment for the tenant, the idea is EXACTLY to "reduce heat transfer from the radiator to the wall". The purpose of the radiant foil is exactly to lower the emissivity of the wall as you suggest because you want the heat to reflect off the wall and HEAT THE AIR, not the wall...

  60. GBA Editor
    Martin Holladay | | #60

    Response to Ward
    No one's disagreeing with you. I was just pointing out that radiant barriers don't radiate much. They reduce radiation; they don't increase it.

    Fortunately, it's illegal to build an uninsulated concrete residence. But for tenants who live in such a building, I agree -- aluminum foil might help.

  61. Sam C | | #61

    Radiant Barrier Confusion
    I am a bit mystified here. If radiant barriers reflect heat only if they are shiny and only with an air space, how can foil faced roof sheathing over an unconditioned attic in Texas reflect outdoor summer heat to make the attic cooler? If there is no air space between the roofing and the sheathing, the top side of the sheathing will not reflect outdoor heat. With an air space on the attic side, it seems that the heat in the attic will merely be reflected back into the attic. In this discussion, it sounds like it makes no difference which side the air space is located… the reflective foil knows by magic which way the homeowner wants the heat reflected. Can someone clarify this?


  62. GBA Editor
    Martin Holladay | | #62

    Response to Sam C
    Hot surfaces radiate heat; if there are cooler objects within line-of-sight range of the hot surface, the cooler object will get warmer, due to the heat radiating from the hot surface.

    Low-e surfaces (basically, shiny surfaces) radiate heat at a lower rate than high-emissivity surfaces. So if the emissivity of the surface is lowered, it won't radiate as much heat.

    Plywood and OSB have a higher emissivity than shiny aluminum. If your roof sheathing is plywood or OSB, and the roof sheathing is hot, it will radiate heat to the attic floor. If you substitute aluminum foil for the bottom surface of the plywood, you've changed the emissivity of the surface. Now it's a low-e surface, so it radiates less heat.

    As you guessed, radiant barrier roof sheathing won't work unless there is an air space on the attic side. If you don't have an air space adjacent to the aluminum foil, it won't work.

  63. Sam C | | #63

    Radiant Barrier Confusion
    Thanks for your response. Maybe what is confusing to me, at least, is the "shiny surface" which not only reflects light, but also reflects heat. The interior of my portable kerosene heater, for example, has a chrome backing which reflects the heat from the kerosene burner towards the front much like a shiny metal roof reflects heat from the sun back into the sky. So is this reflection of heat from a shiny metal surface a different property from the low emissivity that reduces radiant heat from a shiny metal surface or are these the same properties working in slightly different ways?

  64. GBA Editor
    Martin Holladay | | #64

    I'd have to look at your kerosene heater
    I'm not quite sure what your kerosene heater looks like. Depending on where any insulation is located, and the emissivity of the surfaces, the designers of the heater might have anticipated which parts of the heater should stay relatively cool, and which parts should stay hot, in order to direct the heat where it does the most good.

    It's hard to say whether your kerosene heater is intelligently designed or stupidly designed without examining it.

  65. Sam C | | #65

    Kerosene heater
    The kerosene heater (a KeroSun) is sort of like a miniature fireplace with a back wall and open front. The kerosene burner (a round perforated metal affair with cloth wick) is like the burning logs sitting in the middle of the fireplace. The back wall is polished metal which very effectively focuses heat towards the front. The top is an open grill so heat also escapes vertically from the burner, but the front facing heat is the most noticeable.

  66. Paloma | | #66

    Radiant barrier not necessarily a vapor barrier
    I'm sorry, I skipped reading all comments, but wanted to point out, that there definitely are vapor open radiant barriers in the market. Check out DuPont Tyvek Enercor, Dorken Delta Titan and others...

  67. Ted | | #67

    Radiant Barrier in Southern Climate
    Hello Martin,

    My home is located in southern North Carolina which typically has 5-6 months where the A/C is running the majority of the day/night and 4-5 months where the heat (Gas) is running. I have a two story home with a trussed roof and two furnace units with air handlers and insulated flexible ducts run within the attic. I am trying to determine if there would be a significant enough benefit for me to install radiant barrier foil to the bottom of the rafters in my attic and on the gable end walls along with an increase in loose fill cellulose insulation to about R-40+ in most areas (the platform where the furnaces are installed and a storage area will be around R-33 to R-38).

    The ducts are elevated far above the attic floor in many instances and attached to truss supports with strap material. As such, I do not have an option of burying the ducts in blown-in insulation. As previously mentioned, it does have a thin layer of insulation and an outer plastic wrap built around the ducts.

    If I install it myself, it looks like I can purchase enough material to do the job for under $300. I spent approximately $600 to purchase enough blown-in insulation to increase attic floor insulation R-values to R-20 to R-30 (depending on the area of the attic).

    If I understood your article correctly, it sounded like there were benefits to radiant barrier foil installations in attics that contained HVAC equipment and ductwork. Do you think I would achieve a reasonable payback for installing it in my instance? Your time and input are much appreciated.

    Best Regards,


  68. GBA Editor
    Martin Holladay | | #68

    Response to Ted
    It's hard to predict the payback period for your proposed $300 investment, because there are too many variables in the equation. It's safe to say that installing the radiant barrier as you propose will help lower your attic temperature (at least until the radiant barrier gets dusty).

    You're in a tough spot, because your ductwork and furnaces are located in the wrong place -- outside of your home's thermal envelope.

    You might consider replacing as much of your attic ductwork as possible with new insulated flex duct. In the past, flex duct was often insulated to just R-4; it is now possible to buy R-8 flex duct. If you do this work, be sure you know how to seal duct seams to prevent leakage.

  69. Mark I. | | #69

    I agree with M. Johnson. This website should be renamed It is pointless to debate whether running ductwork in conditioned space in Texas is feasible. Because 99% of the homes here were/are built with ductwork located in unconditioned attics. As homeowners of existing Texas properties, we can all abandon our improperly ducted homes in favor of northern-construction-standard homes (cost prohibitive and a wasteful consumption of resources) or do the best with what we've got. I measured a 20 degree drop in attic temp when I installed radiant barrier on the underside of rafters. By doing this installation along a roof pitch, though my top side may collect dust over time (reducing reflectivity), the bottom side should remain relatively dust free (maintaining a low emissivity). Of course I am also upgrading the 30 year old ductwork to reflective r-6 in manageable increments. I've added some additional blown insulation. I am incrementally sealing the envelope of the home. 15 of my 16 windows are now double paned. Check out the latest US census. The country is migrating south. So sites like this and elitist magazines like This Old House that seem to regularly ignore southern climates should begin doing the opposite. I like your articles Martin. I just don't like the 'sorry charlie' tone when it comes to how existing homeowners in Texas are dismissed. Can't we get a little love?!?

  70. GBA Editor
    Martin Holladay | | #70

    Response to Mark I.
    Mark I.,
    I commend you for all the retrofit work you are doing in your attic. It sounds like you are doing all the right things.

    I support the use of radiant barriers in unconditioned attics with ductwork. That's why I wrote in my article, "Radiant-barrier roof sheathing is effective at lowering attic temperatures. Since it doesn't cost much more than ordinary roof sheathing, it makes sense to install it on new hot-climate homes."

    Thanks for the reminder that GBA could do a better job at presenting information from a Southern perspective. We're trying. For example, we just invited Allison Bailes of Georgia to write a guest blog. See it here:
    Is There a Downside to Lumpy Attic Insulation?

  71. Mark I. | | #71

    Thanks Martin
    Please understand that I'm also not trying to turn a camaro forum into a forum about mustangs. The title of this forum is clear: building, not retrofitting. And the passive house movement may very well have been born out of the need to retain heat in colder climates. But this site (and your blogs) do such a good job of pushing homeowners to explore ways to use their home as a tool for energy consciousness and conservation. When I see a 'poorly performing' northern home's utility costs, it is often fractional compared to the 1kWh/sq ft/month that even moderately 'efficient' existing homes in Texas consume during peak summer months. I've tracked my consumption over the past two years. During mild months, we consume roughly a third of the energy as in peak summer, so I know the HVAC running is our primary enemy. I personally am even further challenged by having only one zone in a two story home. I've actually adapted a motorized duct damper that closes heat distribution to most of the upstairs when the system calls for heat, since I want to work with rather than against convection and have the heat pushed downstairs and let it filter up. I also ignored all four HVAC contractors who said it was impossible to add an upstairs air return to our steep roofline victorian. I wouldn't take no for an answer, and found a way to add a 12"x12" return by harvesting the upper shelf of a wall cabinet to get the duct from the upper attic to the side attic where the air handler is locating. This all being done after I had one of those utility-subsidized blower door and duct pressure tests, which revealed that my air return was unsealed and wide open to the joist cavities between my two floors (I had to army crawl inside my walls with precut plywood and Great Stuff to rectify this issue).

    As I said previously, these are incremental steps. My point in advocating for more southern perspective (thanks for the Allison Bailes link) is that I think the southern states are the real energy hogs. Just as Al Gore would do us all a favor by preaching climate change to China rather than California, I think forums like this should aggressively counsel the southern inhabitants, where I believe the homes are the most egregious energy offenders.

    And as for radiant barrier, it can be as big a scam as solar. A buddy of mine (before I could warn him) paid to have it put all over his attic floor. It was ridiculously costly, and once the dust really sets in, it will merely serve as a heat conductor in the peak of summer, and I believe make his A/C bills even higher. In my case, I spent $350 in materials to treat 1200 sq ft by stapling to the rafter undersides. I intentionally WON'T treat the northern/shaded attic space because the return on that investment (less of a temp differential) would be poor.

  72. GBA Editor
    Martin Holladay | | #72

    More on Southern energy bills
    Mark I.,
    It sounds like you and I are on the same page. Thanks for sticking with GBA in spite of evidence that we often display Northern bias.

    You're exactly right that Texas homeowners with attic ductwork don't need preaching -- they need solutions. And you're right that a properly installed radiant barrier can be part of the solution.

    It's probable, though, that Texas new-home builders (like new-home builders up north) still need some preaching. I'll probably continue to preach about the folly of installing ductwork in unconditioned attics, even if I sound repetitive and crazy to some ears in Texas.

    To make a dent in those huge summer electric bills you are alluding to, we'll eventually have to get everyone's ductwork inside their homes' conditioned envelopes. It may take a generation, but it's a worthy goal.

    Keep up the dialog. It's good to have readers tell us when we could be doing a better job.

  73. Mark I. | | #73

    Improving on Radiant Barrier
    Thanks Martin. You folks are already doing a terrific job evangelizing to readers of this site. And I appreciate your Herculean patience when some people come onto this forum with a bullhorn instead of a peacepipe when sharing their alternate views.

    Back on topic of your original blog, for that attic space on my north/shaded side of my home, I wanted to get your thoughts. This space contains my air handler. It is a side attic on my 2nd story accessed through a door in my daughter's room. It has about 10 ft of porch overhang. Consider it shaped like a wedge of cheese (rectangle block cut diagonally across opposite corners, where the base of the wedge is partly over conditioned space and partially not, the vertical side is a wall shared with conditioned space, and the hypotenuse is the roofline obviously). If I'm reading your blogs correctly, I could 1) do closed-cell foam and not have to create the furring strips ventilation for the decking underside, but I'd never see a roof leak until it was in an advanced state or 2) do open cell foam with the hassle of the air gap but moisture penetration would be more visible more quickly if the roof leaked. Based on the assumption that there is really no way I can get that attic space's envelope completely sealed (stick frame x 30 year old home x 40 year old homeowner = some impossible to reach nooks and crannies), would you recommend one type spray foam insulation over the other, or is it pointless if I can't get the attic envelope sealed 100%? Not only is the envelope sealing a consideration, but of course so is our hot and humid Houston climate AND the importance that you counsel us to hit the rafter boards with foam to reduce thermal bridging.

    Frankly, I'm also beginning to wonder if all the efficiency improvements I've made on my 2500 sq ft home (even to the point of retrofitting air-tight recessed can trim kits) means my next leap in reduced energy consumption might come from a lower tonnage system. It's a 10 year old 5-ton 12-SEER American Standard with no signs of kicking the bucket anytime soon. But if the unit was sized appropriate to the home's efficiency level 30 years ago, then I might have a home that now is better served by a 4 ton. I would imagine a 4-ton 16 SEER with dual stage compressor would put the most noticeable dent in utility costs at this point. Thoughts? Perhaps this latter question has been answered (when a homeowner has done enough efficiency improvements to have crossed into a new HVAC sizing zone), in which case just forward me the link.

  74. GBA Editor
    Martin Holladay | | #74

    Sealing an attic over a porch
    Mark I,
    I'm sorry that I can't make useful recommendations concerning your utility room / attic without a site visit. There are just too many variables. Here are some points to consider, however:

    1. It's possible to install open-cell spray foam directly against your roof sheathing if you want. In your climate, you don't even need vapor-retarder paint. Whether or not you'll be able to locate roof leaks with such an installation, however, is hard to say. Roof leaks are tricky, and you may have a rafter bay full of very wet spongy foam before you notice that anything is wrong.

    2. Both open-cell and closed-cell spray foam should be protected on the interior by a thermal barrier like drywall. Different jurisdictions have different requirements, so discuss this with your local building inspector.

    3. If some of the attic floor projects over an unconditioned porch, you'll have to come up with a plan to insulate the floor of your attic as well as the ceiling.

    4. You should be able to calculate the annual energy savings attributable to switching from a 12 SEER to a 16 SEER air conditioner. My guess is that the cost to install a 16 SEER unit is high -- we're talking big bucks. Sharpen your pencil and do the math; but when most homeowners hear how much a 16 SEER unit costs, they swallow hard and say they'll wait until the old unit dies.

  75. Mark - Dallas | | #75

    Radiant Barrier in the south
    Martin, thanks for your insight and information. I had a couple questions. my home is in Dallas and I recently changed out the roof which included radiant barrier on the decking. When I measure the decking temp in the summer its is surprisingly cool but the roof rafters are surprisingly hot. (That foil is a great conductor). Overall I didn't really lower my attic temps in the process. I am guessing its because the new roof system has too little exhaust ventilation. Does that sound right?

    When the attic stays hot it seems to me eventually the insulation gets thoroughly heated and transfers that heat into the living space, albeit slowly. Rather than argue the physics, is there a temp on the ceiling sheetrock in the house that says I am getting good performance out of my attic? Most areas run 82 degrees on a 100 degree day with the house at 75

  76. GBA Editor
    Martin Holladay | | #76

    Response to Mark in Dallas
    There are two possibilities:
    1. You have ductwork in your attic, or
    2. You have no ductwork in your attic.

    If you have ductwork in your attic, passive attic ventilation (soffit vents and ridge vents) might lower your attic temperature a little, but they won't do much. Don't install a powered attic ventilator (a fan), because it will use more energy than it saves.

    If you have no attic ductwork, don't even worry about your attic temperature.

    The best way to keep your ceiling cool is to increase the depth of your attic insulation. If your ceiling is now 82 degrees, then your insulation is insufficient. Your ceiling temperature should be the same as your indoor air temperature.

    In your climate zone, the 2006 IRC calls for a minimum of R-30 attic insulation. That means at least 8 inches of cellulose or 14 inches of blown fiberglass. Of course, more insulation is better than less. There's nothing wrong with R-40 or R-50 insulation -- thicker insulation will lower your cooling costs.

    No one has yet invented insulation that stops heat flow. All insulation slows heat flow, it doesn't stop it. So, on one level you are correct that heat can flow from a hot attic to the ceiling. The whole point of the insulation is to slow down that heat flow as much as possible.

    Insulation works. The way we measure the effectiveness of insulation is by comparing R-values. A layer of R-30 insulation is twice as effective as a layer of R-15 insulation; it will only allow half the heat flow of the R-15 insulation.

  77. Nelson | | #77

    1. Attic Insulation 2. Auditorium Cooling
    1. House Insulation
    I'm looking to better insulate the attic of a 1200 sq foot ranch in upstate New York. It has 8 inch fiberglass batts unfaced. I had been thinking about a radiant barrier- sounded like a good idea- but If I'm hearing you correctly the best investment would be additional cellulose - another 6 inches? over the existing insulation. I think that would do two things: increase the R value, and decrease air infiltration through the fiberglass. It sounds as though adding a radiant barrier could help, but at a disproportionate cost compared to the benefit. For example, I could put a barrier under the roof, and it would lower the attic temp but it wouldn't significantly reduce my energy use in the winter. And a barrier over the insulation wouldn't reduce the attic temp in summer and would only minimally decrease the heat loss in winter.

    2. Auditorium Cooling
    I was intrigued by the conversation about roof coating to make it more reflective because our small church auditorium has a cathedral ceiling (no hope of additional insulation). The main driver of being able to keep the auditorium comfortable is whether the sun is out. Starting in April, even at 40 degrees, if the sun is out, we can't keep the temp stable. No sun, we're comfortable. Oh, the roof faces east and west (peak runs north to south).

    I was somewhat taken aback by your comments that "Up north, a sun-warmed attic helps lower heating bills." Um, well, not in upstate NY, not unless you want ginormous icicles. A well insulated roof (usually a cold, well ventilated attic) is the way to avoid ice dams, water backing up under shingles, and icicles. So making the roof absorb less heat is always a good thing for us (and even in the attic-less situation, the roof is often covered with snow during the cold months so the roof heat absorptivity is a non-issue)

    Which leads me to question: any good resources to recommend on attic ventilation, both methods and amount needed? I continue to run into arguments over the efficacy of ridge, gable, turbine, powered, standardand venturi vents. My current conclusion for my climate is: ridge- gets covered with snow here, there are concerns about clogging with dust, and also of the actual air flow. gable- messes up air flow patterns. turbine- can be noisy, they wear out, questions about efficacy. Powered- it seems that's a waste of energy in most situations. Standard- probably good. Venturi- intriguing, wondering if worth the extra money.

    Thanks very much for the blog. It's been enormously useful

  78. GBA Editor
    Martin Holladay | | #78

    Response to Nelson
    1. Yes, you're right -- your attic needs AT LEAST 6 inches of cellulose on top of the existing fiberglass. Before you blow in the new insulation, however, it's important to peel back the fiberglass batts and seal all of the existing air leaks. You can search this site for good information on attic air sealing, or you can call an experienced contractor to help you.

    2. A contractor can easily add insulation to the roof of you church auditorium. Rigid foam insulation is easy to install on top of the existing roof sheathing. Of course, that means installing new roofing as well.

    3. You're wrong to dispute my point about radiant barrier roof sheathing in cold climates. If your roof is snow-covered and you have ice damming problems, the sun is not warming your roofing -- nor it is warming your roof sheathing, nor is it warming your attic. The sunlight is hitting the snow, not your roofing! If there's snow on your roof, a radiant barrier is irrelevant.

    Later in the season, in March and April, sun can shine on your roof shingles and warm your attic. That's useful up north. A radiant barrier will slightly increase your heating costs -- not enough to make a big deal about, especially if you have adequate insulation on your attic floor -- but it's safe to say that your added heating costs will probably exceed any savings during the air conditioning season.

    3. When it comes to attic ventilation, stick with soffit vents and a continuous ridge vent. Don't worry too much about snow on your ridge -- there won't be snow on your ridge all year long unless you're at the South Pole, and you'll still get a little bit of ventilation under the snow. Remember, other details of your roof construction and thermal envelope construction -- especially air sealing and insulation thickness -- are far more important than ventilation details.

  79. eyQ93v9DFh | | #79

    1. Attic Insulation 2. Auditorium Cooling
    Thanks for the quick response.
    1. I have talked to a contractor about sealing and and insulating my attic. And my response is the same as your's about radiant heat: it's not cost effective!My walls are insulated, my air leakage is not far over the minimum or whatever it's called (1400 cfm vs 1000, the point at which I have to start introducing fresh air). I have a fairly well insulated house- I'm currently spending <$900 per year on gas (almost all heating). Any reduction in use are the "cheap therms" above fixed costs, so if I can get a, say, 25% reduction (which the contractor will guarantee IF I put in a new furnace and seal/additionally insulate my attic), I won't save 25% on my bill because I'm whacking off cheap therms, so maybe it's 20% in dollars- perhaps $180/yr. I will save a little on my cooling costs. So for $7000 I have a payback schedule of optimistically 30 years. For the attic insulation alone, they want $3600, which is mostly labor for sealing air leaks with no guaranteed savings. This makes no economic sense. I'm guessing- and I'd appreciate feedback- that I'll get 80-90% of the value by sealing any obvious leaks and putting in the blown cellulose (which will reduce some of the existing air leakage)

    2. You say a contractor can easily add insulation to the roof of our church auditorium- "but, of course, that means installing new roofing, as well." But our roof is 10 years old, so putting a $15000 roof on to save a few hundred dollars a year makes no sense. I keep hoping for a not perfect, but economically feasible solution. If there was a coating we could put on for $1000 and cut the heat gain in the sun would really help our comfort and perhaps save upgrading our current AC, a big expense.

    3. Let me say this a different way. (Oh, and I never meant to imply a radiant barrier would help when there's snow on the roof- sorry if I did.It's covered with snow!)
    I still can't see an overall advantage to roof heat absorption, at least here. Here's my thinking: remember where I am- we have a significant snow cover mostly from December through March (yes, we get more snow that any other major city in the country). We have problems with ice dams, etc, and everything I hear says the way to take care of those is a COOL attic- keeping interior heat away from the roof. I agree with you on that. And the same construction techniques that remove heat coming up from the living area will remove any heat coming down from the roof. So how is absorbed heat from the sun in winter of any value, unless I rig some kind of system to shut down the ventilation when the roof is snow free and open up when snow covered?

    Slightly different situation in same climate- for the church roof w/ cathedral ceiling, minimal ventilation, mostly snow covered during winter (by observation- it's not meltin real fast so the insulation is doing something):
    -in cold winter months- snow covered, heat absorption is a non issue
    -summer months- any heat absorption is bad;
    -fall and spring- the problem here is that in 40+ weather with sun on the roof we can't keep the temp down in our auditorium with our current a/c. Until you pointed it out, I hadn't realized we might be getting some heat absorption benefits when we're not running a/c *because* it's not insulated and vented properly, but the a/c cooling problems in our minds outweigh any small heating benefits. When not in use, we keep the building quite cool, so in the fall/spring, the heat runs very little.

    4. Ridge vent
    Again, we may have more snow cover that when you're thinking. And when there's snow cover is precisely when we need the most ventilation, is it not? As to the statement that "other details of your roof construction and thermal envelope construction -- especially air sealing and insulation thickness -- are far more important than ventilation details" I presume there's some research on that? I understand they are important and that I don't want a lot of heat escaping into that space, but esp. in a somewhat suboptimal situation such as my roof, might not really good ventilation help. BTW, any rules of thumb on amount of ventilation? I've heard everything from 1" per square foot to a third or a quarter of that. And some say it's divided between soffit and roof, and some say the amount for each.

    Thanks again. You surely put a lot into this!!

  80. GBA Editor
    Martin Holladay | | #80

    Response to Nelson
    1. It's your choice on the attic retrofit work. I agree that many proposed energy retrofit measures don't pencil out when it comes to anticipated energy savings.

    2. You didn't tell me what type of roofing you have on your church. But if your church doesn't want to improve the insulation because of the cost of roofing replacement, then you should talk to a roofer about the possibility of installing a coating. Such reflective coatings only make sense on certain types of roofing; obviously, you can't install a coating on asphalt shingles, for instance.

    3. I've never tried to argue that solar gain through roofing is a significant factor in lowering heating bills. For anyone with an unconditioned attic, I always say: do the air sealing work, put down a thick layer of insulation, and then don't worry whether you attic is hot or cold -- because it won't matter.

    4. If you want more information on my position on solving ice damming problems, see Prevent Ice Dams With Air Sealing and Insulation.

  81. qwrkQY5TWy | | #81

    Religious Believer of Insulation
    Mark from Dallas just had a great point, when the attic stays hot insulation becomes thoroughly heated and then transfers this heat into the living envelope. Martin responded to this by saying that insulation that stops the transfer of heat has yet to be invented, which is partially true. Instead of adding another 6"-8" of insulation, as advised, which would only add some additional delay to this heat transfer process, you could spend much less and install something that will REFLECT 90% of the sun's radiation from being absorbed/passing through your current insulation and into the home.

    At this point go outside into the hot southern sun and put a sheet of aluminum foil up over your head and block the suns RADIATION. The temperature of the air around you changes very little when compared to before the shading but yet you feel much more comfortable. This must be why construction workers are using this low budget method to keep cool.

    Moving on to Nelson's issue with his church being so effected by the sun's rays, not the cool outside temperature which is being insulated from the living envelope, but the radiation from the sun. It seems as if Martin finally realizes the losing battle his insulation has in this situation so he advises Nelson to look into reflective coatings, not a radiant barrier, but reflective coatings. Can't support that "religious" word, I'm sure. The church isn't being heated by conduction or convection, that leaves the run's radiation as the culprit. The atmosphere and the building's roof isn't absorbing all the radiation!

    My final point that unfortunately leads me to question Martin's "religious beliefs" is when he refers to radiant barriers as a utility cost increase in the winter, when everyone in the North could otherwise be benefiting from the sun heating the attic and in turn heating the living space. How exactly will this warm attic heat our homes when we have your amazing R-50 insulation keeping it in the attic and away from our living envelope dying for heat?? Martin, at the very least, stick to you guns and stop trying to bad mouth radiant barriers.

    Of course I can already hear you typing "I never said radiant barriers didn't work", "they just don't compare to the savings that could be had with MORE insulation!" R value too low! We aren't worried about boasting how high our homes R value is, we want to be comfortable in our homes!

    I spent $300 on radiant barrier material and installed it over the small amount of blown in insulation on the deck of my 1200 sqft attic and last summer felt much better because of it, we didn't even have to use our window A/C units as we did for 3 months the year before, only ceiling fans. $300 wasn't too expensive for the comfort I felt and will feel this coming summer. If this means I have to check the dust accumulation I may have every few years then so be it, no big deal! The attic temp did seem hotter than before the barrier was installed, which may lead to installing a solar attic fan but again it is worth the comfort.

    I agree with Martin when he says that there are situations where increased insulation is the way to go, but I'm not very religious so I can also say that there are definite benefits to radiation barriers as well.

    From CT

  82. GBA Editor
    Martin Holladay | | #82

    Response to Griffin DeLancy
    You wrote, "I spent $300 on radiant barrier material ... and last summer felt much better because of it."

    If it makes you feel better, that's good. It's your house, so you can install whatever you want.

    You wrote, "when the attic stays hot insulation becomes thoroughly heated and then transfers this heat into the living envelope." No, you are wrong about that. Assuming that the inside of the house is at a different temperature from that attic -- after all, if they are at the same temperature, you don't need any insulation -- then it is impossible for all of the attic insulation to reach the attic temperature.

    What you get is a gradation of temperatures across the depth of the insulation. If the ceiling is 75°F and the attic is 125°F, then the bottom of the insulation will be at about 75°F, and the top of the insulation will be at about 125°F, and the center of the insulation will be at about 100°F. You'll have a gradation of temperatures through the insulation layer. The thicker the insulation, the slower the heat transfer from the attic (the hot side of the insulation) to the ceiling (the cool side of the insulation).

    Concerning the church, my first recommendation was to add insulation to the roof assembly. Nelson said that would be impossible. Installing a reflective coating would help a little, but it would be inferior to insulation.

    I certainly agree with you that if you have R-50 insulation in your attic (and no ducts), then the presence or absence of a radiant barrier is totally irrelevant. You seem to think that is an argument that proves me wrong -- but that's what I've been saying consistently all along.

  83. qwrkQY5TWy | | #83

    Response to Martin
    1. Your confused regarding my statement "I spent $300 on radiant barrier material ... and last summer felt much better because of it." I did not feel better because of the fact that I did the job myself or spent little money but because of the fact that the environment inside of my home was much more comfortable. I hope that most of the others reading understood it that way.

    2. Your confusing me now on how insulation works, and I am very confident that I grasp its purpose. you disagree with me and Mark from Dallas' statement on "when the attic stays hot insulation becomes thoroughly heated and then transfers this heat into the living envelope." But lets say you have 2" of insulation and you subject it to the conditions you have state above, 125°F attic temp, 75°F ceiling temp. The insulation will slow the heat transfer but eventually it too will become 125°F and in turn start raising the temperature of the ceiling and in turn the living envelope. Your typical solution is to keep throwing inches upon inches of insulation in your attic, raising your R value and slowing the heat transfer rate enough to make it nearly impossible to saturate the entire insulation in attic temp. Even still energy can neither be created of destroyed so that heat isn't going away due to the fact that the insulation you speak of has no properties that will deflect of REFLECT the energy.

    Radiant Barriers DO REFLECT this radiant energy back into the air and objects above, which in this case is the attic, or shingles.

    3. And back to your church. Your first recommendation of adding insulation to the roof assembly would do little to NOTHING for Nelson's high temperature's during the cool yet sunny seasons of the year because insulation is only an INSULATOR and his church's issue is not the conduction of heat but the radiation of heat that is making them feel uncomfortable. Like i said there is a use for both insulation and reflective materials but in this case insulation would be inferior to a radiant barrier or reflective roofing material.

    And to finish, an attic with R-50 insulation would still fail to stop most of the radiation that is emitted from the sun, traveling 93 million miles to earth and without such radiation there would be no energy to be conducted or insulated by your R-50 insulation.

    But by all means, keep piling it on

  84. GBA Editor
    Martin Holladay | | #84

    Response to Griffin
    I'm sorry, but you are wrong about the temperature of the insulation in your example. You can continue to re-state your false assertion here, but that won't make it true.

    You wrote, "But let's say you have 2 inches of insulation and you subject it to the conditions you have stated above, 125°F attic temp, 75°F ceiling temp. The insulation will slow the heat transfer but eventually it too will become 125°F and in turn start raising the temperature of the ceiling and in turn the living envelope."

    The entire depth of the insulation will not achieve 125°F -- assuming it is really insulation, and not a conductor (like a 2-inch thick slab of solid copper). Two inches of insulation will provide a thermal barrier between the ceiling temperature (75°F) -- which is also the temperature of the bottom of the insulation -- and the attic temperature (125°F), which is also the temperature of the top of the insulation.

  85. qwrkQY5TWy | | #85

    Response to Martin
    So with 2" of insulation I should never experience and conductive heat loss? Whats the need for more insulation?

    I guess I will try to communicate this to you more specifically so that you can not dance around the truth.

    The sun is keeping your attic at a constant 125°F and unfortunately you live in an area where this is the case year-round until the end of time.

    Your A/C unit is keeping your building at a nice 68°F, and then it malfunctions and stops working.

    If you decided to "sweat it out" your attic would CONTINUE transferring heat through you attic insulation, even though its R-50 insulation! and begin raising the temperature of the bottom side of your attic insulation since your ceiling is no long cooled by your A/C.

    So now this heat that has always been transferred through your insulation and then cooled by your ceiling exposed to conditioned air is able to warm your ceiling and then your air is warmed by the ceiling and on and on until.... your attic temperature and your living envelope have equalized at the same temperature. Since the sun in your area is relentless, your living envelope will conform to your attic temperature of 125°F.

    If this is incorrect then please explain in detail where I am wrong, don't just say I'm wrong.

    Insulation resists conducting heat, but it is only resisting, not blocking. If you put a piece in the oven at 125°F I'm pretty sure it will heat throughout until its 125°F. Please correct me if I'm wrong and try not to avoid any technical details you may not want to share.


  86. GBA Editor
    Martin Holladay | | #86

    Still another response to Griffin
    Q. "So with 2 inches of insulation I should never experience any conductive heat loss?"

    A. No, there would still be heat loss and heat gain through the insulation. The R-value of the insulation and the home's air leakage rate will tell you what you need to know to calculate the rate of heat loss and heat gain. (Of course, you also need to know the solar heat gain coefficient, orientation, and area of the windows.) Insulation with a low R-value allows heat loss and heat gain at a high rate, while insulation with a higher R-value allows heat loss and heat gain at a slower rate.

    Q. "What's the need for more insulation?"

    A. To slow down the rate of heat flow.

    Q. "Your A/C unit is keeping your building at a nice 68°F, and then it malfunctions and stops working."

    A. I agree that if your air conditioner stops running, your house will be uncomfortable in hot weather. If you want to design a house that is comfortable in such circumstances, you would want small windows (especially facing west) with low-SHGC glazing. Ideally you would want the windows to be shaded. You would want your house to have high thermal mass, thick layers of insulation with a high R-value, and a thermal envelope with a low rate of air leakage. When the weather cooled off -- for example, at night -- it would be a good idea to open windows or turn on a whole-house fan.

    Q. "Insulation resists conducting heat, but it is only resisting, not blocking."

    A. Right. The laws of physics say that heat will always flow from an area of high heat toward an area of low heat. The insulation slows down the flow. The thicker the insulation and the higher the R-value, the slower the heat flow.

    Q. "If you put a piece [of insulation] in the oven at 125°F I'm pretty sure it will heat throughout until it's 125°F."

    A. You're right, it will. But the 2 inches of insulation on the attic floor is not in an oven. The bottom of the insulation is in direct contact with the drywall ceiling at a temperature of 75°F. So the fibers on the bottom layer of the insulation are also at 75°F, because of this direct contact. As long as the air conditioner keeps the drywall ceiling at 75°F, the fibers at the bottom of the insulation layer will also be at 75°F. If you don't want your air conditioner to work as hard, you should make the insulation thicker to reduce the heat flow rate.

  87. user-940601 | | #87

    Response to Griffin
    Griffin, I see your point but it needs to be modified a bit. It seems to me too that Martin may be dancing around your question so I will try to help. I think part of the issue is a misunderstanding of the intent of the question and the answer. Warning, this is quite long.

    I disagree with Martin, your statement of "when the attic stays hot insulation becomes thoroughly heated and then transfers this heat into the living envelope" is exactly correct. However, Martin's following description of the heat gradient across the insulation is also correct. The real answer is quite complicated, so I will make a series of simplifications to help illustrate the main idea, just keep in mind that these tend to break down at some point.

    Using the 2” foam insulation example (this almost removes convection from consideration) with 125 degree on the outside and 75 degree on the inside. If these two temps are held constant then an equilibrium gradient will be established where 1” into the insulation the temp is 100 degrees. Yes, heat is flowing from the outside to the inside, it is just being removed by the A/C, this keeps the inside surface at 75. When the A/C is off will the inside surface of the insulation increase? Yes, because the same amount of heat is entering the house it is just no longer being removed, but the inside will never reach 125 degrees. Assume a house where it is 100 degrees outside, 125 in the attic with 2” attic floor foam and 75 inside. Remember there will always be at least two sides of the house that are shaded, a wall opposite the sun and the foundation. Both of these act as a heat-sink, the foundation as an infinite one which in Texas is kept around 70-75 degrees year round. This establishes another gradient across the floor and ceiling of the room (and the insulation).

    To illustrate Martin’s point that insulation slows heat transfer. When the insulation is heated it radiates and conducts heat in all directions equally (assuming a spherical piece of insulation, there will be differences when diverging from this shape), conduction occurs with surrounding insulation, wood framing and even the adjacent air. Lets say we are talking about a piece of foam like a 1” sheet of OSB where the sides are extremely small compared to the top and bottom surfaces and can thus be ignored. If 1 BTU were pumped into the foam from the attic then something slightly less than half that would be released into the conditioned space, but lets call it 50% and use it as our reference for a 1” thick piece of foam (disclaimer: These are not actual values and are only used for illustrative purposes, any resemblance to actual values is purely coincidental). So for a 2” thick piece of foam with the same 1 BTU applied will only release 25% or 0.25 BTU into the inside space (Note: this is extremely simplified because the heat emitted from the lower 1” piece back to the upper 1” piece will then again have 50% emitted to the lower piece of which 50% of that will emitted to the inside and 50% back to the upper piece…so on and so forth). So it will take about 2 BTUs from the attic to get 1 BTU to the inside through 1” foam and about 4 BTU for 2” of foam. Since heat flow is dependent on time, this process is not instantaneous and thus insulation slows and reduces the amount of heat flow across it in one direction. So as you can see, more insulation will reduce the heat flow to the inside yet there will always be some.

    Now as far as radiant barriers, I do not doubt that they can have a large impact on heat transfer into a building. But let me clarify, by large I mean, if installed properly, near 97% reduction in radiant heat absorbed by the building. Heat flows in 3 ways, conduction, convection and radiation. If for example, 30% of a buildings heat load is radiant heat (I just picked an arbitrary number close to one third, I do not know the real percent) then close to 29% of that can be removed if the house is wrapped in a radiant barrier and installed properly. But what is installed properly? I’m glad you asked because this is the important part and this is why heat reductions are no where near what is claimed. Installed properly is installed as the outermost part of the building envelope, this is the only location where close to 97% of radiation can be blocked. Even if installed as a wall and roof foil-faced OSB facing the interior, the radiant energy has already been converted to all three forms and is conducting through the rafters and wall studs and convecting through air currents in the attic. Lets say the OSB emits 30% of its energy as radiation, so 97% of (30% (total heat load that is radiant) x 24% (OSB radiation minus 6% because 20% of the OSB surface is covered by studs)) = ~7% reduction in overall heat load using these assumptions and if all walls and the ceiling are covered in foil-faced OSB. This is in agreement with the range that Martin suggested.

    The only way to see large reductions is if a radiant barrier is the outermost part of the envelope to reflect radiant energy before it is converted to other forms. But nobody likes shiny houses that reflect sunlight, that’s dangerous for driving and not a durable surface. So if “installed properly” will we see a ~30% reduction in cooling bills? Sure, but we can’t build a house that way. However, low-e walls and roofs may be another option but it is unlikely that any will be as good as foil and not degrade over time.

    I hope this helps.


  88. XhQbazw3Af | | #88

    Florida Attic Question
    Like the Texas comments I have my ducts in the attic above my blown in Cellulose of R-30. I live in a block home and have few windows not shaded. Luckily during the few winter heating days the house is sealed enough that ambient heat creates most of my required heating. FYI HIP style roof for Florida winds!

    I saw the numbers of only 2-10% savings on cooling costs with a radiant barrier. My question is that assuming that the duct work is not in the attic? I believe my ducts to be sealed well. However, I notice the far side of the house stays warmer until late into the night. My assumption is the hot attic is heating the air before it travels the 40'. Additionally my builder was kind enough to put returns in all the rooms in the attic above the insulation so my hot attic is hitting me twice.

    So this past month I got quotes on geothermal HVAC. I thought great eliminate Hot water bill and get cheaper A/C. First quote was 23K (not gonna happen). Got a second quote at 13K. However, that would use shallow well open loop design and I have talked to a Florida PE who said DO NOT DO IT! The impure ground water will ruin the equipment.

    So I looked at Closing up the attic completely with foam. Before air handler, I was told 6K to remove cellulose and seal in attic and get HVAC inside the envelope. 6K a bit hard to afford in current economy. Then Foam guy says keep vented attic and go with foam and attic will drop to no more than 10 degrees more tan ambient temp at only $2500. (I like this but sounds to good to be true) But would like your opinion.

    Lastly, I am thinking on my house I can add radiant barrier for under say $700 (includes price of gift cards for my pals that help me install). I understand that I will save money with radiant but was wondering if the 2-10% was based on having the ducts in the attic or not?

    So to recap:

    Could you provide opinion on savings with duct work (return and diffuse) in attic with radiant?

    Could you provide opinion on foam that still maintains vented style?

    Thanks in advance for your help.

  89. GBA Editor
    Martin Holladay | | #89

    Response to Rob Mills
    Unfortunately, there is no way to provide accurate energy savings estimates over the Internet, without a site visit.

    It sounds like your attic is a good candidate for retrofitting a radiant barrier under the rafters, especially if you and your friends can do the labor.

    I don't understand this sentence: "Then Foam guy says keep vented attic and go with foam and attic will drop to no more than 10 degrees more tan ambient temp at only $2500."

    What does "go with foam" mean? Where does the foam guy plan to install your foam? On the underside of your roof sheathing? On your ducts? Somewhere else?

  90. XhQbazw3Af | | #90

    Reply To Martin

    i appreciate your help. I understand trying to give an accurate figure without a site visit. I was just trying to decide if radiant barrier was a) worthwhile investment and b) how it would compare to foam that would still maintain a vented attic all things being equal.

    The Foam contractor to has proposed (told) me that if he were to spray the underside of the roof sheeting (I would use Open Cell so I could detect any leaks that may arise later) that although it remained vented the attic would be between 10-15 degrees above the outdoor ambient temperature. So if it was a 90F summer day then may attic would be between 100-105F. ($2500 Solution)

    I was also told the $5000 dollar foam solution was to total seal the attic with foam and that the attic temp would be approximately 10 degrees F above the indoor ambient temp. So if I set my air at 76F then the attic containing my ducting would be about 86F.

    Now I really have no idea what type of heat reduction that the radian Barrier would provide. My assumption (I know assumptions are dangerous) is that if for example the outside air is 90F my attic is probably currently (need to verify) probably 160F and that with a radian barrier I may be able to get that down to 115-120F.

    Now if I am wrong that the radian barrier will be that effective maybe I am better going foam while still keeping the attic vented. However, I was wondering if foam while the attic is still vented could really keep the attic that close (within 10-15F) to the outdoor ambient temperature.

    I am trying to just make a decision based on an all things equal do I foam and keep it vented or just radian barrier with the venting. I mean if neither would provide real benefit then maybe I need to look at ceiling the attic and providing a humidity (air handler) solution to the attic.

    I just am trying to get an educated guess at the best bang for the buck without breaking the bank. I got to tell you this site is a godsend. Thanks again for your advice!

    Completely unrelated, cheers to you for your help to others overseas. Time and effort is a greater donation one can make to others than any amount of money!

  91. GBA Editor
    Martin Holladay | | #91

    Another reply to Rob Mills
    I don't think it makes any sense to spend thousands of dollars to install spray foam on the underside of your roof sheathing if you are still ventilating your attic. Once you decide to insulate under your roof sheathing, it's time to seal your attic vents.

    If you are considering spending $2,500, I think the best way to spend the money would be on sealing duct leaks (if any) and replacing ductwork with new ductwork with better duct insulation -- or, if you want to keep your existing ductwork, to aim the spray-foam nozzle at the ductwork, not the roof sheathing.

    It makes sense to insulate your ductwork to a very high level. But you have to find a spray foam contractor who knows what he or she is doing -- some one who has used spray foam to insulate ductwork.

  92. cpfWzwrPKu | | #92

    No one knows it all and one size never fits all
    I am a 40 year solar professional and have worked around many different types of construction and insulation techniques.
    I would neither discourage nor encourage the use of radiant barrier use based upon my scientific knowledge of the product but I can certainly tell you what my experience shows.
    I have installed many solar energy systems around the USA and in several instances in New Orleans, I was able to see what happens when you put bubble type radiant barrier on roof rafters.
    Without the material installed, you wouldnt even be able to work in the attic in the afternoons because of the extreme heat build-up. After the installation of the material, it was actually comfortable in the attic.
    The result of this dramatic change was that the fiberglass insulation in the floor of the attic was not taking on heat. I knew a building contractor out west who had his crews go by his house when they had left over fiberglass insulation and put in in his attic.
    After putting about 28" of fiberglass in the ceiling he quickly discovered his (zoned) air conditioning unit in that part of the house didnt shut off till around three am. Meaning, the fiberglass insulation was storing heat and transferring it through his ceilings, sheet rock, into the conditioned space.
    Now, anyone who has the vaguest idea of basic thermodynamics knows that heat is attracted to cold so the stored heat in the attic insulation was being transferred into his living space.
    Point being that in his experience, with the extra fiberglass insulation, he was readily able to see what dynamics are occurring in the cooling mode/summer months.
    There are two lessons here. Radiant barrier material placed on the inside of roof rafters keeps out a huge amount of heat. Also, without it, any amount of insulation in the ceiling will store more heat from the day into the night.
    Based on this personal experience, in the cooling mode at least, there seems to be a thermodynamics pattern which would obviously result in an energy savings in the summer mode.
    Anyone who argues these facts is inexperienced, uneducated or just plain stupid.

  93. GBA Editor
    Martin Holladay | | #93

    Response to Samuel Hay
    You wrote, "Any amount of insulation in the ceiling will store more heat from the day into the night. ... Anyone who argues these facts is inexperienced, uneducated or just plain stupid."

    You are wrong. I will argue your "facts." I'll leave it to GBA readers to determine how inexperienced, uneducated, and just plain stupid I am.

    Insulation slows the transfer of heat from one side of the insualtion to the other side. The effectiveness of the insulation at slowing heat transfer can be measured; in the U.S., insulation effectiveness is reported as R-value. The higher the insulation R-value, the better it is at slowing heat transfer.

    If your attic is hot and the interior of your house is cold, then you want a layer of insulation on your attic floor to slow the transfer of heat through your ceiling. The higher the R-value of that insulation, the better it is at slowing the transfer of the heat from the attic to the house.

    If you insert a series of temperature probes in the attic insulation, you will see a gradation of temperatures in the insulation. Let's say your attic is at 140°F and your house is at 75°F. The top layer of the insulation will be at 140°F, the bottom will be at 75°F, and the middle of the insulation will be at about 107°F.

    It is simply untrue that "any amount of insulation in the ceiling will store more heat from the day into the night." A layer of insulation is not a pile of rocks; it is more like a down quilt.

    Now, if your fiberglass batts aren't working well, there are three possibilities:
    1. They are poorly installed, with many gaps or irregularities.
    2. They aren't thick enough.
    3. They are installed over a leaky ceiling.

    The solution to these problems is to learn how to install insulation correctly, install it deep enough, and learn how to do air sealing work. (You might also consider using a cellulose, a more effective product.)
    If you take my advice, your investment will yield better rewards than the purchase of a radiant barrier.

  94. cpfWzwrPKu | | #94

    Response to Martin Holladay
    I developed an electronics company that manufactured, along with proportional solar controllers, a 10 station temperature monitor. You cant even believe how many places I have stuck sensors :)
    My point was that I have PERSONALLY WITNESSED the results of the addition of radiant barrier on roof rafters in New Orleans, LA.
    I would not, will not, advise anyone anywhere what type of insulation they should use. Period. Especially if I did not inspect the structure, existing insulation, location, orientation, etc etc.
    Fiberglass insulation does build up heat within when located in an area which has sunny, hot days.
    Would you dispute that fact?
    The more heat the fiberglass insulation contains, the more heat has to be removed by the air conditioning system through the sheet rock as heat is attracted to cold.
    Would you dispute that fact?
    Im not saying not to use fiberglass although it is not my choice. Im simply saying that in my own field experience ie; get down out of your ivory tower and get your hands dirty, the application of radiant barrier on rafters cools the attic and therefore reduces the A/C load.
    Would you dispute that fact?
    MY insulation of choice is foam. R values being what they are, any one can understand that when you hold a cup of coffee at over 200f with only 1/8" of foam keeping you from getting scalded there is something very positive going on there which directly relates to the problem at hand.

  95. GBA Editor
    Martin Holladay | | #95

    Second response to Samuel Hay
    Samuel Hay,
    Q. "Fiberglass insulation does build up heat within when located in an area which has sunny, hot days. Would you dispute that fact?"

    A. Yes, I would dispute it. The top of the insulation will be close to the attic temperature, while the bottom of the insulation will be close to the temperature of the ceiling drywall. That's why they call it insulation -- because it insulates.

    Fiberglass batts aren't the best choice for insulating an attic floor -- I prefer cellulose -- but they will work if they are carefully installed.

  96. cpfWzwrPKu | | #96

    Martin Still doesnt get it.
    Ok to oversimplify. I have used temp monitors that I manufactured to test just about everything in construction techniques you could ever think of.
    NOW: A residence with an active HVAC system will show what you profess. However one which is static, without any HVAC operational, will show the heat stagnates in the insulation at an almost constant temperature. Hot on top, hot on bottom.
    So again, my point is that the reason your example indicates the bottom of the insulation is cooler than the top is NOT because it is insulating anything. It is, quite simply that as I stated, the HVAC system is pulling the heat from the batts through the sheetrock and into the living space and spending a lot of money to remove the heat from the structure.
    The only way to keep this from happening is it keep the heat from ever getting into any part of the structure in the first place. The insulation and the studs and the sheet rock will all absorb any heat that is at a greater temperature than they are.
    If you want to see some photos of my insulation system get me an eaddy and I will show you how to build a house.

  97. GBA Editor
    Martin Holladay | | #97

    Samuel, you're still wrong
    For some reason, you are convinced that insulation doesn't insulate. But it does.

    However, there is no reason to argue with someone who doesn't believe in physics. If you prefer a radiant barrier (or "insulating paint") to insulation, be my guest.

    Here's a story about someone else who didn't trust in insulation: An ‘Insulating’ Paint Salesman Is Tripped Up By His Own Product.

  98. cpfWzwrPKu | | #98

    I didnt ever say I used nor recommended radiant barrier or insulating paint.
    If I did please point out in my posts where I said that.
    I will say this; We meet up at a house that does not have radiant barrier (With fiberglass in the ceiling joists). We take the temp in the attic.
    We have someone (Maybe Ed Gilson would like to participate) install radiant barrier on the inside of the roof rafters.
    We each put $5000,00 in cash in an independent party's hand.
    If the temp in the attic is not reduced then you take all, if the temp is reduced then I take all.
    You dont seem to understand that fiberglass insulation in the ceiling joists or roof rafters ABSORBS HEAT!
    When the space below that is air conditioned is cooler than the heat absorbed by the fiberglass insulation, the heat from the fiberglass moves into the conditioned space downward through the fiberglass bats and is removed by the air conditioning system at great cost to the homeowner.
    If you check the temp of a fiberglass bat in an attic with no A/C running the temp will be static. Or, it will be the same on the bottom of the bat as it is on the top.
    The only way to keep heat out of a structure is to put all the insulation on the OUTSIDE!
    I've built 3500 ftsq homess with YEARLY utility bills under $800.00.
    I also have 50 years experience doing this.
    What is the extent of your experience? Tell us how many years you have been building/insulating homes please.

  99. GBA Editor
    Martin Holladay | | #99

    Response to Samuel Hay
    Writing in capital letters will not change the laws of physics.

    1. I certainly agree that if you install a radiant barrier under the roof sheathing, you will lower the air temperature in the attic on a hot, sunny day. That's why, in my blog, I wrote, "Radiant-barrier roof sheathing is effective at lowering attic temperatures. Since it doesn't cost much more than ordinary roof sheathing, it makes sense to install it on new hot-climate homes." So we're not having an argument there -- on that point, you seem to be agreeing with me.

    2. My statements about heat flow through fiberglass batts (and other types of insulation) were correct, and I'm not going to change them, even if you write in all capitals. If the attic air is hot, and the drywall ceiling under the insulation layer is cool, there will be a temperature gradient through the insulation. The top of the insulation will be close to the attic air temperature, and the bottom of the insulation will be close to the drywall temperature. The temperatures of the intervening layers of insulation will display a gradient between the two extreme temperatures. That's a fact, and that's why insulation insulates. It slows down heat flow.

    Of course heat moves through the insulation. Heat always moves from hot to cold. It's impossible for any type of insulation to stop heat flow. All you can do is slow the rate of heat flow down.

    What matters here is the rate of heat flow -- how fast or slow it moves. The thicker the insulation, the slower the flow.

    Concerning my experience: I have been building houses since 1974. I have been writing technical articles about energy and heat flow for at least 11 years.

  100. wjrobinson | | #100

    Maybe with hvac off, the
    Maybe with hvac off, the drywall on the ceiling warms up much more than when hvac is on and moving air which in turn would be cooling drywall causing a perfect Martin temp gradient. Hvac off and the intense radiant heat energy warms up glass and drywall and close stagnant air below the drywall which then insulates well enough for much less gradient in the glass due to the rest of the gradient to room temp occurs in the air below the drywall.

    I think this is the case and if so both of you gents could move toward agreement of what really is being observed and how the observed also equates with scientific analysis.

  101. user-942951 | | #101


    Can you take a look at Looks like some suspect information on radiant barrier r-values has been inserted there.

    "Radiant barriers
    Material Value (Min) Value (Max) Reference
    Reflective insulation R-1 [27] (For assembly without adjacent air space.) R-10.7 (heat transfer down), R-6.7 (heat transfer horizontal), R-5 (heat transfer up)"

  102. u6arfoXzdy | | #102

    Radiant barriers
    It is Thanksgiving and I stumbled on the discussion of Radiant Barriers. Very interesting all the way through. Toward the end, Sam and Martin have at it concerning insulation actually insulating and what that means. I hope I can add to the conversation.

    To determine R-values you need 4 temperatures: the surface temperature of both sides of the assembly, and the air temperature on both sides of the assemble. The discussion between S+M centered on the attic temperature and the drywall temperature - but these two numbers don't tell us anything.

    Remember that the drywall is part of the assembly - it has R-value that is added to the R-value of the insulation, which may be a couple of different types of material installed over the years at different thicknesses, but we are concerned with the total R-value of the assembly. So if I have R-30 of blown material of any kind or thickness making good contact with the drywall at R-1, my assembly has an R-31.

    If the attic air is at 125° and the room air is at 75° and I have tons of insulation, there will be little energy movement through the insulation and when things stabilize, the surface of the drywall (the bottom surface of the assembly) might be at 76°, and if it is, the top of the insulation (the top surface of the assembly) will be at about 124°, with a gradient in between. This means that the drywall surface is having little energy affect on the air or objects in the room. and that I am not spending any air conditioning money to cool that surface

    However, if I have a lower R-value, the drywall might be at 90° and the the top of the insulation might be at 110°. The warmer drywall surface will heat the air and objects below the three ways we all know and love - conduction , convection, and radiation. Now I have to spend some money to keep the air in the room cool because the warmer surface keeps trying to heat it up.

    So two things: one is I don't care what the temperature of the air in the attic is (for my energy bill) - I only care what the temperature of the bottom surface of the assembly (the drywall) is, because that temperature is what costs me money given a desired interior temperature. Yes, a radiant barrier will cool off that surface because the attic will be a little cooler, and yes, more insulation will cool it off because, even though the attic is still hot, not as much energy is getting through the thicker layer. So the question is, what is the cheapest way to keep my drywall cool.

    The second is that insulation only insulates when there is a difference in temperature. So as long as there is a hot attic and the air conditioning is running, there will be a temperature difference through the thickness of the assembly. If all I have is 1/2" of drywall as my assembly, the top surface and the bottom surface of that drywall will be different, but close, and about half way between the two air temperatures - and cost me bundles to to try to pump "coolth" into. If there is more insulation, the surfaces will get closer to the temperature of the air adjoining them. The only way the assembly (not the just the insulation) can be the same temperature on both side is if the air temperature is the same.

    That's my story and I'm stickin' with it.
    Ed Minch

  103. GBA Editor
    Martin Holladay | | #103

    Response to Ed Minch
    Your analysis is correct, and we agree. As you pointed out, "So the question is, what is the cheapest way to keep my drywall cool?"

    The answer is simple: to achieve the energy savings desired by homeowners, insulation is cheaper and more effective than a radiant barrier.

  104. ZNHHcRVjbY | | #104

    Case Study: Dallas TX
    I've always been skeptical of radiant barrier ads on the radio, especially when most end with: "And we'll also blow in 12" of insulation--for FREE!" Now, where does the savings really come from?!?

    However, after doing some additional study and finding a cheap local source ($0.10/sf for "seconds"), I decided to run some tests in my attic, specifically because my A/C unit and ducts are in the attic. My findings both from research and testing largely confirms the conclusions of this article.

    The house is 1400sf and roof slope is 5.5/12; but the house has a significant portion of vaulted ceilings, so the ceiling area with attic above is only 1000sf. But with the roof slope plus gables, the radiant barrier needed for stapling to the bottom of the rafters was closer to 1250sf. I spent $147 for the material and had some left over. Installation was in the early mornings at the end of July. (I had installed a ridge vent previously, and the cost of that was about $250 including ridge singles.)

    The immediate impact on attic temperature was dramatic (data below), extending the time that could be spent sealing ducts, attic air leaks, etc. Even more significantly, when the A/C first came on, in the afternoon, it no longer felt like a blast furnace, but almost immediately cool, confirming this as the greatest potential source of savings from the radiant barrier. So comfort in the house was immediately improved. (Note that for summer I had programmed the thermostat for extended run times for greater overall efficiency, so the unit would be off from 1pm to 230pm and from 4pm through 6pm; this of course accentuated the blast furnace effect of the A/C coming on before I put up the radiant barrier.)

    Now for some preliminary data (not totally analyzed yet). I monitored temperatures daily just above the attic floor/insulation, and at about 6' high, plus the ambient outside temperature in well ventilated shade. The results of the temperature differentials are:

    attic floor 6' high
    before ridge vent 12F 24F
    after ridge vent 11F 22F
    after radiant barrier 1F 8F

    The 6' high differential applies to the upper portions of the air handling unit and the return air duct. When the temperature outside is 101F, the ducts are now dealing with 109F air rather than 125F air, and very little radiant energy. When the A/C first starts up, instead of bringing the air handling unit down from 125F down to 60F, it now starts from 109F, or maybe lower.

    The second potential component of savings is the ceiling heat gain. When it's 101F outside, the floor level attic temp was previously about 112F but is now about 102F. With the thermostat at 82F, this temperature differential has been reduced from 30F to 20F, a 33% reduction in instantaneous heat gain through the ceiling. This translates directly to a 33% savings in the ceiling component of the heat load at this peak time of day. Of course, if the ambient temperature is lower than 101F, the percentage savings is higher while the actual heat gain savings is relatively constant, while at higher temperatures (and we had many this past summer), the percentage savings is lower. This rough 33% savings falls within the range stated in your article of 16% to 42%.

    Now, how would a 33% savings in ceiling heat gain be achieved through added insulation? By increasing the R-value of the insulation by 50%. Currently I have about 8" of blown fiberglass, for an R-value of maybe 25. So if I want to increase the R-value on the attic floor by R-13 with blown cellulose, the GreenFiber chart says to add 18 bags for 1000sf, coming to a cost of $147. I plan to do this myself, and add quite a bit more than that while I'm at it--another project for the year future.

    So in this case, the cost of radiant barrier vs. cellulose insulation was the same to achieve the same difference in ceiling heat gains--unless I have to count the cost of the ridge vent as well. However, the radiant barrier gives additional savings with respect to the duct and air handling unit savings, plus it should in theory extend the life of these A/C system components for an additional green savings.

    Finally, for data on daily energy consumption comparing between 2010 and 2011, I have realized savings, though the savings come from multiple components. My baseline usage is about 20kWh per day, so this has been subtracted out of total readings leaving only the A/C component of daily load as:

    2010 2011
    May 10 13
    June 19 13
    July 24 23
    Aug 38 31 * radiant barrier installed = 20% savings???
    Sept 24 27
    Oct 12 14
    Aug-Oct: 24.89 23.71 5% savings (the 3 months during which radiant barrier was in place)
    (A graph of my energy usage over 7 years can be found as the "Office" graph at

    Clearly there are other factors going on here causing aberrations that do not facilitate the comparison. I've already mentioned some (thermostat programmed for longer run times, sealing ducts and ceiling leaks). Additional perturbations include the presence of students coming in during Sept and Oct to use our workshop power tools, lights, etc. Also, our record-smashing hottest-summer-on-record extended well into Sept.

    Nevertheless, August really does stand out as a remarkable reduction in usage--especially in light of the nearly every day reaching around 105F and nights remaining above 80F, whereas 2010 was much cooler. I would argue that a significant portion of that 20% savings has to be attributed to the radiant barrier. If I waited through this next summer before adding more insulation, I would have additional data points to analyze, but alas, I don't think this or other improvements can or should wait.

    Still, can I guestimate the payback period? Let me calculate the annual savings of different percentage energy savings:
    20%: 21.25kWh/day x 180days x 20% x $0.10/kWh = $76.50 per year = 2-year payback
    10%: $38.25 per year = 4-year payback
    5%: $19.13 per year = 8-year payback
    I suspect the actual payback is towards the 8-year end of this scale. Bear in mind, this was a DIY project with cheap material costs, with no anticipation of sweat-equity being paid back.

    Working to be a good steward of God's green earth!

  105. GBA Editor
    Martin Holladay | | #105

    Response to Joe Friberg
    It doesn't surprise me that the installation of a radiant barrier in your attic lowered your attic temperature by about 10 degrees. That sounds about right. Before, it was 110°F; now, it's closer to 100°F. That is, indeed, cooler. Radiant barriers work -- they are just not quite as effective or as cheap as insulation.

    Your understanding of the relationship between heat and temperature, however, is faulty. You wrote, "With the thermostat at 82°F, this temperature differential has been reduced from 30°F to 20°F, a 33% reduction in instantaneous heat gain through the ceiling. This translates directly to a 33% savings in the ceiling component of the heat load at this peak time of day." Absolutely untrue.

    The Fahrenheit temperature is completely arbitrary. A change from 30°F to 20°F is not a 33% reduction in anything -- not temperature, and not heat.

    Many scientists prefer the Kelvin temperature scale. Let's convert your temperatures to degrees Kelvin (°K), OK?
    30°F = 272°K
    20°F = 266°K

    So, using your method of mathematical calculation, this difference in temperature represents a 4% "reduction in instantaneous heat gain." So how did we go from a 33% reduction to a 4% reduction, just by switching from Fahrernheit to Kelvin? After all, these are the same temperatures -- we're just using a different scale to describe them.

    The answer to your conundrum is that a degree Fahrenheit is not a dollar. You can't compare 20°F to 30°F and describe the difference in temperature a 33% reduction in anything.

  106. ZNHHcRVjbY | | #106

    R-value and U-value calculations
    Martin, I don't think I misstated this, though it's been years since I was actually employed running load calculations for sizing HVAC systems.

    R-value in SI units is ft-squared x degree-F-difference x hour / Btu, and inversely, the U-value is measured in Btu per hour per degree-F-difference per square-foot of surface area.

    Sure you can measure R-values in metric units, but our standard in the US reflects degrees-F. And the multiplier of degrees F is not to be measured on an absolute scale, but the difference between the temperatures of the two surfaces.

    So the heat gain in summer between 102F and 82F with a 20F differential is in theory the same as the heat loss in winter between 68F inside and 48F outside since the differential is the same.

    Therefore R-25 insulation in the ceiling of a 10ft x 10ft room with 20F differential passes 80 Btu/hour (= 20F x 100sf / 25). This is the result achieved in my house by the added radiant barrier.

    If you increase the temperature differential to 30F, the ceiling transfers 120 Btu/hour (= 30F x 100sf / 25). This is back to the original configuration of the attic.

    However, if you have a 30F temperature differential and increase the R-value to 38, you bring the heat transfer back down to 79 Bth/hour (= 30F x 100sf / 38). This is the addition of 3.5" of insulation solution.

    The one caveat to be added is that R-values for a material or a system do not in fact always remain static, but may change as the temperature differential increases. For example, fiberglass inherently develops convection as the temperature differential increases, thereby lowering it's effective "R-value".

  107. GBA Editor
    Martin Holladay | | #107

    Response to Joe Friberg
    OK -- your current explanation is clearer. Your point is that stapling a radiant barrier to the underside of your rafters is about equivalent, in your case, to adding 3.5 inches of insulation. I think that sounds about right.

    If your ducts were inside your house, it would certainly be easier (and much more effective) to blow an extra 6 inches or 10 inches of cellulose on your attic floor than to staple up a radiant barrier that might eventually get dusty. For those with ducts in the attic, I have consistently advised that radiant barriers often make sense.

  108. tomsweets | | #108

    good article
    I found this useful

  109. 4Vift9c6K3 | | #109

    Radiant Barriers Do Work but only in certain conditons
    I am an adovocate for radiant barriers. I live in Fort Worth, Texas and with the summers we have I can tell you the difference it makes. Specifically as a house wrap is about the best application. I covered the outside of my shop with the preforated radiant barrier and in the middle of the summer as I was installing it if I put my hand on the 3/4 inch plywood sheeting where the radiant barrier was versus where it wasn't there was a mind blowing difference of heat transfer. So if you put it under siding and create even a half inch air gap as I did you will notice a huge difference. The problem is in attics and crawl spaces. As was said by the writer of the article in attics the dust renders the barrier less effective over time and a royal pain to install but since they have went away from the aluminum and to the reflective film the dust is less of an issue because the reflectivity has increased 100 fold. The chip idea is good because it creates its own air gap but one would have to shread there own barrier to make it cost effective and figure out how to blow it maybe with a cellulose blower. The problem with attics and insulation if you live in a warm climate is that by night fall the insulation has absorbed the days heat and then starts puttign it into the inside of the house so the radiant barrier does make the insulation for more effective even if that means figuing out a way to once a year dust off the barrier. Under the floor is an opposite problem is most climates the floor and crawl space is always cool and so in the winter the cold seems to be the biggest problem and if you put the radiant barrier on inch below the sub floor and follow up with insulation from below touching the barrier it does keep the floors warmer. A couple other important things it seems to be most effective with cellulose insulation and yes it is important that you do it yourself as it is not expensive for the product its the cost of labor, etc that makes it expensive. Far better product than tyvex as a house wrap at the same cost. This is my real world experience with the product.


  110. rocko62580 | | #110

    Radiant Barrier Warming Attic
    I installed a foil bubble double sided radiant barrier over the attic rafters in my townhouse. I didn't measure temperature in the attic prior to installing, however, since the radiant barrier was installed it seems to get very hot in the attic during the day, and that heat seems to be keeping the house warmer.
    Today the temperature outside is 85 and my attic is measuring 103. Would it be possible that heat from the outside, other townhouse units and/or the house is getting trapped on the ceiling side of the vapor barrier? At night the attic cools off nicely and by morning it is the same temperature as the outdoors, so I do not believe I have a ventilation issue. Soffits are clear and ridge vent is open.
    Is this daytime heating common? Is there a solution to the excessive heating going on during the day? Would an attic fan help? Should I just add more insulation? Thank you!

  111. GBA Editor
    Martin Holladay | | #111

    Response to Rocky Pembroke
    1. Don't worry about your attic temperature. Your attic temperature is irrelevant. You don't live in your attic.

    2. If you have enough insulation on your attic floor, it doesn't matter what your attic temperature is.

    3. If your ceiling is too hot on sunny days, it means you don't have enough insulation on your attic floor. So add more insulation!

    Q. "Would an attic fan help?"
    A. No -- an attic fan will just add to your electricity bill, and may suck air-conditioned air out of your house, making things worse. More information here: Are Solar-Powered Attic Ventilators Green? and here: Martin’s Useless Products List.

  112. b1tbuck3t | | #112

    Finishing Attic
    I live in Virginia which has fairly hot summers. I am about to finish my attic and I am primarily concerned about comfort but am also of course interested in lowering energy usage to cool or heat my home.

    From both reading this discussion and other research I definately undestand that sealing my ductwork as well as the barrier between my conditioned and unconditioned space is most important.

    In the finished acctic we plan on having many access doors to the space behind the kneewalls to allow for storage.

    What would be the most effect approach to insulate the entire attic space including behind the kneewalls? This would eliminate having to worry about the access doors and would put the HVAC into the insulated space.

    Would an air gap, then radiant barrier, then an R30-R38 insulation, which is then possibly sealed with an insulating foam board product be best or would it be better to go with a sprayed foam insulation that would both seal and insulate at the same time.

  113. GBA Editor
    Martin Holladay | | #113

    Response to Scott Powell
    1. The insulation should follow the roof slope (all the way to the eaves). The insulation should not follow the kneewalls.

    2. Adequate insulation with a decent R-value is far more important than whether or not you include a radiant barrier.

    3. More information here: Creating a Conditioned Attic.

  114. ZephyrRae | | #114

    Can anyone tell me anything about using radiant barriers for insulating Yurts? I am doing a presentation on low-income temporary rural living spaces in cold climates. I lived myself in a lance camper for 3 years and have fallen in love with living in small spaces because I spend so much time outdoors anyhow and I don't see the need for a large house. I also lived in a tent in Antarctica as a field carpenter and was amazed at how well my body heat and sun warmed my small tent at night (daylight all the time) in combination with the igloo stacked snow around my tent. I am particularly interested in the coupling a simple yurt with radiant floor heating generated by solar thermal. I love the idea (perhaps a dream)of the floor holding potential heat through the thermal mass throughout the night, heat that was gathered from the combination on solar thermal and a wood stove heating the floor. I am wondering about the ability of the walls of a yurts to keep this stored reflected heat throughout the night, heat that could be slowly coming from the floor as the fire dies down in the middle of the night.
    I also love the idea of using deciduous trees as an aid for heat management in combination with a yurt...more sun in the winter, less in the summer...and would like to know what anyone knows about Yurt living in cold climates for a hearty outdoors person?

  115. GBA Editor
    Martin Holladay | | #115

    Response to Zephyr McC
    It's time to travel to Mongolia and talk to the experts. They favor felt, not radiant barriers.

    Once you arrive in Ulan Bator to begin your research, remember that the correct term for this type of shelter is ger, not yurt.

  116. ZephyrRae | | #116

    thanks! I've already done all
    thanks! I've already done all the traditional research online I need...felt however is not good in wet climates and instead of traditional structures like the ger or uy (Turkish) I am considering a more modern western Yurt made by companies here in the States and most of these companies advertise using Astro-shield, duro-last, or Reflectix, all radiant forms of insulation. I was just maybe hoping for an equation that could factor in the projected thermal mass of a certain square ft. floor in combination with radiant walls to determine how hard a heat source, (solar and/or wood stoves) would have to work to keep it comfortable inside throughout the night.

    thanks for the response though!

  117. batch55 | | #117

    moisture problems
    i had a liner installed two years ago over existing insulation in attic. was in attic a week ago and noticed that all my insulation under the liner is wet/damp. anyone else have this problem and any suggestions to fix. right now think my only solution is to have it removed. i have not noticed any change in power bills since installing it either. i live in iowa.

  118. GBA Editor
    Martin Holladay | | #118

    Response to Jay Batcheller
    You don't provide any information on what you mean by a "liner." But I am guessing that you (or a contractor) installed a radiant barrier on top of the insulation installed on your attic floor.

    If that's what you did, here is the problem:
    - You have a leaky ceiling that allows warm, moist, interior air to escape from your house into your attic.
    - The radiant barrier is a vapor barrier. (Even if it has perforations, it can still trap moisture.)
    - The moisture in the air escaping through your ceiling is condensing on the cold radiant barrier, and the moisture is saturating your insulation.

    It's always a bad idea to install a radiant barrier on top of your attic insulation. If you paid a contractor to do this work, the contractor is a crook. You can try to get your money back through small claims court, but it is likely to be an uphill battle.

    The solution is to remove the radiant barrier and throw it away. If you want to improve the thermal performance of your attic:
    - Hire a contractor to perform air sealing work to seal the leaks in your ceiling.
    - Install more attic insulation.

  119. batch55 | | #119

    wet insulation.
    thank you for the info. yes it was installed by a professional. can the liner be reused for anything? thanks again for the info.

  120. GBA Editor
    Martin Holladay | | #120

    Second response to Jay
    Anything can be reused, I suppose. You could use it as mulch to kill lawn grass if you want to establish a new vegetable garden -- at least until the sunlight destroys it.

  121. TruthSeekerMom | | #121

    Hyper Humidity in FL Attic
    I see this was originally written 3 years ago, but hopefully someone can help me. In our coastal city, home values have plummeted and contractors are going bankrupt as average homeowners no longer can afford to make upgrades. When forced to, as I was, by circumstances (families of squirrels then rats then raccoons moved into my attic) to remove my insulation and replaced it, I knew nothing about the physics of attic/hvac. I have a gas furnace in my attic so my insulation guy (one of 3 including a mega builders store) suggested treated cellulose on the attic floor and a radiant barrier added to the ceiling of the attic. He put a tent over the stair opening and, once hvac ductwork was cleaned, the house smell clean (finally). The summer electric bill dropped about 25%. I was thrilled.
    However, I started noticing high humidity this fall when the weather got cooler. Once the heating unit was necessary, it wasn't a problem. However, this spring has been dreadful and the ceiling sheetrock is warping in several places. I went to the usual dry attic last week and noticed the roof framing was damp, the boxes of things I normally store up there (books toys, hand-me-down clothes) wasn't fresh, even in the sealed plastic bins.
    BTW, the single-ply radiant barrier foil is run between roof joists, not on top of them to create an air pocket, but directly against the plywood sheathing. I pulled some back and it was definitely damp between the foil and the wood.
    I have a mansard roof with just one vent on the west and one on the opposite east side of the roof, both have been obscured somewhat by animal-proof grills...
    Do I need to run up there with my kids and start cutting this stuff off? Please help me--I can't find anyone to give me a straightforward answer.

  122. GBA Editor
    Martin Holladay | | #122

    Response to C. Zeitouni
    C. Zeitouni,
    There are several possible mechanisms that might explain why your ceiling drywall and roof sheathing are getting damp. It's hard to diagnose the problem from a distance.

    Certainly, it's a terrible idea to put a furnace or ductwork in an unconditioned attic. The best solution to your problem is to create a conditioned attic by spraying closed-cell spray polyurethane foam on the underside of your roof sheathing. More information here: Creating a Conditioned Attic. The solution isn't cheap, unfortunately.

    If your attic is vented, then the source of the moisture is probably exterior air. That's why vented attics don't make sense in hot, humid climates. Even though the arrangement you describe is common, it often leads to problems.

    It's possible that moisture is condensing on some of your cold ducts, and dripping on to the ceiling drywall.

    By the way, I grew up in Beirut, Lebanon, and I'm guessing that your family may have originated in Lebanon or Syria. Wherever your family came from, I send you warm greetings and wish you luck.

  123. TruthSeekerMom | | #123

    Hyper Humidity in FL Attic
    Thank you, Mr. Holladay--my husband came from Lebanon some years ago. Over the years, I have learned a lot about how to live more simply. I wish we built things to last like his parents used to do--or mine, for that matter.
    When we moved here from Atlanta there were very few properties for sale that would accommodate a family of six in a ranch floor plan. My husband was quite taken with the parapet side roof and rising mansard roof. To find competent contractors willing to work on our home has been a challenge.
    Our insulation contractor is one of the few we've met with integrity. Everyone else wanted to spray the foam, as you suggest. But we have a gas furnace that was almost new when we moved here. So our insulation contractor, who specializes in foam btw, suggested we use the cellulose for now. I don't know if we'll ever be able to change to foam, there is wiring of various kinds running through the attic and in order to sell the house insurance companies require aluminum wired homes like ours be rewired. With kids in college and business slow the last couple of years, we are trying to be economical. There is no room whatsoever to move our HVAC out of the 50-year-old attic. A previous owner partitioned the ductwork and added a heatpump to the bedroom/bath side of our home. It seems inefficient at best and requires a lot of energy in the winter. Our HVAC contractor plans to replace it within a couple of years.
    Meanwhile, could we cover the attic vents in summer while not running the gas heat? Or would that system typically need ventilation even when running cool air in the summer?
    Thank you for your prompt reply--it was a relief to hear from you.
    I read the article on useless things--and I see the attic fan won't help us either:) I wish there was better information for consumers...

  124. GBA Editor
    Martin Holladay | | #124

    Response to C. Zeitouni
    C. Zeitouni,
    Frankly, it sounds like you have a mess on your hands, and a small budget. It's hard to know how to go forward if you can't afford to fix the mess in your attic. The wiring is bad; the venting is wrong; the furnace and ductwork are in the wrong place; and you can't afford to install spray foam, by the sounds of it.

    You could try plugging all of the attic vents and installing a dehumidifier up there. But a dehumidifier will be expensive to operate, and the solution is inelegant and clumsy.

    Perhaps you can get a bank loan to properly address all of the issues in your attic. Good luck.

  125. CiaranM | | #125

    What's your opnion
    Hi Martin, I' am a builder and an inventor in Calgary. I am at the planing stage of our next home which we will showcase some of my projects. One of which is a washer and dryer all in one that uses solar thermal hot water to dry the laundry. Needless to say this home will have an abundance of solar panels. It's also an inner city home on a subdivided lot so there is a need to build into the roof. I am using evacuated tubes and I have an idea to also make solar thermal air over the entire surface. So the 7/12 roof will consist of 9.5" manufactured joists 16" o/c filled with cellulose on top of that I would like to install radiant barrier osb with the foil side up on top of that will be foil wrapped 2x4s @ 8" o/c and finally black rolled steel roofing. So now we have 6.5" x 3.5" chambers running the length of the roof. This home will have the capability to pump this hot air off the roof and use it in a multitude of ways.

  126. GBA Editor
    Martin Holladay | | #126

    Response to Ciaran Morris
    My reactions:

    1. Many builders have experimented with elaborate roof-integrated solar air collectors. The general conclusion is that the value of the thermal heat gathered is so low that the solar energy isn't worth the cost of the equipment needed to gather it. Remember, when you need the heat most, the sun isn't shining. When the sun is shining, you really don't need the heat. Moreover, solar hot air collectors deliver air at a relatively low temperature, and the heat is hard to store for more than a day.

    2. If you are packing your rafters solid with cellulose, you need to have a ventilation gap above the cellulose, without any vapor barriers between the top of the cellulose and the air gap. Your proposed radiant barrier sheathing is a vapor barrier. You end up with an unvented ceiling insulated with air-permeable insulation. In the U.S., that is a code violation. Even in Canada, the assembly risks moisture problems.

    3. Wrapping 2x4s in foil is nuts. It's like wrapping them in polyethylene. Foil is a vapor barrier. You are inviting your 2x4s to rot -- especially if they are a little damp on the day you choose to wrap them. Let your lumber breathe, Ciaran.

  127. hnrhnr | | #127

    Multiple foils perform better than batts
    I am not sure why, but I feel that most of the artciles here are biased toward batts and similar insulation solutions. Large number of studies conducted by "small people" have shown beyond doubt that a multiple foils system perfom at least as good (or better) compare to batts insulation in every cliamte zone. The physics of foils can bet twisted (using big funding studies) but, more and more people are voting with their legs. In large number of countries mulri foils systems replacing and pushing out batts solutions, due to actaul results. The concept of adding RBS (Radiant barriers) ontop batts is fundamentally wrong. Most customers in teh USA were brain washed for year that batts are the right solution (funding, lobby and so forth). Yet, there are many far better solutions, which will deliver much higher saving to customers. These solutions are not widely known to the American consumers due to the wrong reasons (batts companies dont like you to know that there are better solutions). For example, in a typical cavity wall of 2x4 or 2x6 we can install a multi foils system which will provide better insulation than batts, for the same thickness...! Such multi foils system (tested and proven for both winter and summer) will provide mutiple benefits, all year round, without the need to combine insulation products, adding vapour barriers, or worry about air leakage. Yet, I bet that most readers have not herd about such solutions... Well, maybe it is about time?

  128. theinsulationguru | | #128

    I agree with last post.
    I test all products that my company uses. radiant barriers in Texas are awesome if applied correctly. There are also newer, stronger, better e-value products on the market. I've watched so-called experts discuss insulation, radiant barrier, weatherization and ventilation.......Well, until some of these experts actually get under the house, in the attic, between the walls (And I mean dripping with sweat and covered in work with aching muscles and bones) then I feel they are just quoting or expressing what they feel based on other peoples (Companies) test and studies.
    I started, The Insulation Guru for a reason. I have a true passion for what I do. radiant barriers are great. I will go toe to toe with anyone on a good debate. Educate me, I'm all ears.

    Chris Burke
    The Insulation Guru
    Houston, Texas

  129. GBA Editor
    Martin Holladay | | #129

    Response to Chris Burke
    You wrote, "Radiant barriers in Texas are awesome if applied correctly."

    I agree. The only thing is, insulation is even more awesome.

  130. GBA Editor
    Martin Holladay | | #130

    Response to Hanan Rotenberg (Comment #128)
    I can assure you that this web site does not push people to use fiberglass batts. If any evidence is required, I suggest that you read my article, Installing Fiberglass Right. Fiberglass batts are the worst performing of all common insulation materials.

    That said, no commercial insulation contractors insulate walls or ceilings by assembling layers of radiant barrier foil with air spaces in between -- for several good reasons. Although it is always possible to get a significant R-value if you stack up a thick stack of R-1 or R-2 pancakes, this is a very expensive and time-consuming way to proceed. Other alternatives like dense-packed cellulose, rigid foam insulation, or even spray foam are both faster and cheaper to install.

    Moreover, the R-value of such an assembly will degrade over time, as dust accumulates on the shiny surfaces of the radiant barriers.

  131. nezbgman | | #131

    Location of HVAC equipment
    I've heard Martin comment many times that HVAC equipment should be located inside the house's conditioned envelope. But, I've never heard him explain his reasoning. I've been considering moving mine from an unconditioned closet (gas furnace) off the attached garage into the attic. My point of view is a California ranch style home (silicon valley) with flex ducts (R-4.2, currently) in a vented attic (R-30 insulation/air barrier on the celling). Also considering a radiant barrier above the new location, attached to the underside of the roof rafters. The new location will reduce the maximum physical length of the ducts from around 73 feet to 45 feet.

  132. GBA Editor
    Martin Holladay | | #132

    Response to Mel Peterson
    Furnaces, air handlers, and ductwork all leak heat during the winter. They also leak cool air (or absorb heat) during the summer. If the equipment is located outdoors, the homeowner ends up paying a huge energy penalty. During the winter, you are paying money to heat the outdoors. During the summer, you are paying money to cool the outdoors.

    For more information on the size of the energy penalty, see these two articles:

    Creating a Conditioned Attic

    Keeping Ducts Indoors

  133. Energy Guru | | #133

    Refelective Insulation
    A friend forwarded the article and I read it. Thank her and the author. I found the article somewhat informative though perhaps not in the represented manner. Please understand I haven’t special interest/agenda with reflective insulations; however, I know this article indifferent to my direct knowledge or experience with reflective insulation. As a qualified scientist in several energy sectors, I have tested many reflective technologies independently, as well as for those supportive techs to other applications. Frankly, I found the article tripe and offensive. However, the author is right with his assumptions, although such are not my direct experiences… E vs. R… what’s R if only the inverse C? E, as in lowering surface temps – That is what any insulation [system] is all about!

    Reflective insulations get a bad rap from prolific half-truths, antiquated research, and improper configurations -- in field research or practical installation. "R", for traditional insulations, infrared energy has a contributory function to arrive at U or C. Reflective insulations are enormously effective in any climate but there is only one way for it to be so per climate. If you think me wrong, try; just try to prove the [finest] window research wrong. What is a window for visible light IR abated as to an IR opaque floor, wall, ceiling, or roof! I admit the author has done a fine job with his references however, even Florida Solar Energy Commission has it wrong. I mean no offense.

    Unfortunately, the industry at large hasn’t yet quite got it right. If the latter is the author’s intent then I offer kudos. I suspect a responsible author might have said so. Know this though, all and author, every sector of all economic markets except banking/finance and the building industry has incorporated reflective products in one or another form to achieve phenomenal results. Why is that? They work!

    It’s all about reducing surface temperature. An effect to which cannot be achieved by any other affordable means. Without PROPER configuration use of reflective insulation cannot be achieved! Also know, remove IR potential, all "R" else works better - prove me wrong! A word of advice: Please, do not write about "E" unless you do not have any iota of that which E effects... how and why.

    BTW: I am very well aware of all to which you write or refer -- to whom and from where info was arrived. One hasn’t any right to promulgate without DIRECT firsthand knowledge KNOWING when out of one’s league. Can I write about turnips only because I do not like the flavor? Reflective Insulations, properly integrated, can reduce one sector consuming 37% of the nation's energy to about 7%. So what this author may suggest is less than accurate in the grandest picture of matters.

  134. GBA Editor
    Martin Holladay | | #134

    Response to Guru Mike
    As I wrote in my article, radiant barriers work. So we agree on that point.

    The only problem is that constructing wall assemblies and roof assemblies with radiant barriers is much more expensive than using equivalent amounts of insulation. So I'm going to continue to use cellulose -- because it is so much cheaper.

  135. DallasDan | | #135

    AC in attic
    Just moved to Dallas. Condo has two AC units. One on first floor ceiling with access panel in kitchen and one in attic (access panel in second floor bath and access in attic). Ductwork for second floor registers is in unconditioned and poorly insulated attic space. From what I've read, I definitely need to have insulation added. Would a radiant barrier be worthwhile since the AC unit (which doubles as the heat unit during the winter) is in the attic? These heating/cooling units are part of a central system. AC is fed by chilled water and is active year-round. Heat is fed by heated water and is usually active Nov. - April. HOA includes all utilities - but still want to be green and a responsible homeowner.

  136. cstand | | #136

    radiant barrier benefit
    Kansas City area resident.
    A few years ago we had hail damage and had to replace our roof. I had the roofer use a sheathing with reflective foil surface on it so the installation cost was no different than traditional sheathing other than the additional cost of the product - about $5/sheet. If I recall it was around $400 extra versus traditional sheathing.
    In the middle of August I can stand in my attic and not feel like I am in the Sahara desert. Have I quantified the change in my attic insulation temperature, the 2nd floor ceiling temperature and the reduction in cooling costs - no as the choice to select the reflective product was made quickly once I found it was available but I do know that my attic is certainly much cooler today which has to lead to lower cooling dollars being spent.

  137. GBA Editor
    Martin Holladay | | #137

    Response to Chris Stand
    Your observations are consistent with the advice I gave in my article. I wrote, "Radiant-barrier roof sheathing is effective at lowering attic temperatures. Since it doesn't cost much more than ordinary roof sheathing, it makes sense to install it on new hot-climate homes."

    A couple of other observations:

    1. While hail can damage roofing, it is rare for hail to damage roof sheathing. Replacing the OSB or plywood roof sheathing after a hail storm is rare.

    2. Unless your attic is a finished room, it is irrelevant whether a person standing in your attic feels comfortable. Even if your attic is 150 degrees, that fact doesn't matter much, as long as (a) you don't have any ducts in your attic, and (b) you have a thick layer of insulation on your attic floor.

    3. If the thickness of your attic floor insulation is inadequate, or if your have ducts located in your attic, there are usually cheaper and better remedies to solve these problems than installing a radiant barrier.

  138. Lariat | | #138

    Elastomeric Reflectance & Radiant Barrier
    South Florida Storage Building Owner - I am pursuing adding insulation or the foil-double bubble-foil to the underside of the roof to lower my electric costs but don't think that either would be cost effective. I installed a a white elastomeric roof coating over my metal Quonset Hut in 2007. My building is approximately 8200 sf with no interior insulation nor radiant barrier. There isn't a ceiling partition separating the conditioned space from the attic (the roof trusses, joists and purlins are exposed. The height of the building is approximately 23'. My electric bills to condition the space are approximately $650/month during the summer months and $250/month during the winter months. My air handlers (2- 5 tons) and insulated ductwork is hung approximately 12' off the finish floor. On either end of the building (north & south side) are 12" masonry walls, each having one closed exhaust vent (approx. 3' x 3') sitting about 16' above the finish floor. Each vent is not tightly sealed as is mechanically closed. My concern: Payback of either application (ROI)? I feel my elastomeric roof is doing such a great job with the approximate 80% supposed solar reflectance that do I really need the expense of a radiant barrier of insulation or otherwise to offset my electric bills. Even the electric company's representative feels the electric bill wouldn't be reduced enough to be effective. Any thoughts here?

  139. GBA Editor
    Martin Holladay | | #139

    Response to Mark Maher
    There is no way that your question can be answered without a site visit; there are too many variables.

    I suggest that you contact a home performance contractor or energy rater certified by RESNET or BPI to conduct an energy audit. That way you will get a customized list of suggested retrofit measures for your building.

  140. Lariat | | #140

    How much radiant heat penetrates the white elastomeric coatings?
    Martin, My concern is how much radiant actually penetrates the 80% white elastomeric coating? If all the other variables were removed and no longer an issue, would one really need insulation in addition to the white elastomeric coating and be cost effective?

  141. GBA Editor
    Martin Holladay | | #141

    Response to Mark Maher
    Q. "How much radiant actually penetrates the 80% white elastomeric coating?"

    A. Membrane roofing (such as white elastomeric roofing) stops virtually 100% of the radiant energy hitting it. The heat that is transferred downward after the roofing heats up is transferred by conduction, a different heat transfer mechanism.

    Q. "Would one really need insulation in addition to the white elastomeric coating and be cost effective?"

    A. If you are interested in reducing heat transfer from hot roofing to the interior of your house, insulation is the most cost-effective material to use. Layers of radiant-barrier foil interspersed with air gaps cost much more than good old-fashioned insulation.

  142. mp06011999 | | #142

    Martin, thanks for the great
    Martin, thanks for the great article on radiant barriers. My home is a 2006 built 1800 sqft ranch in dry hot Vegas. As is normal, the air handler and ducts are in the unconditioned, vented attic. Truss built (no room in attic) with cellulose blown-in on the attic floor. The living area seems well insulated as the thermostat is set to 80 in the summer and the ceiling and walls maintain that quite well. Most of my windows have been covered by some form of insulation and the house is like a cave. I think we are doing a pretty good job controlling cooling costs as our highest ever electric bill is typically half that ($180) of similar folks' homes. Understand that the Sun almost ALWAYS shines and even in 60 degree weather you will get hot standing in the Sun here.

    I've two questions for you with the understanding that as with ALL things there is always effort vs. reward and I tend not to spend $1000s to save $100s:
    1) Would it make any sense to add more attic floor insulation if my living area ceilings are staying room temp and my ducts run in the hot attic above all that insulation? I'm assuming the ducts in the hot attic are the weak link. And without getting up there right now I'm sure the house was insulated to 2006 code.
    2) My attached garage is not insulated or conditioned, but is drywalled. The walls & ceiling are warmer than ambient and I'm sure that's mostly due to the metal garage door really raising the ambient in the garage much higher than ambient outside the garage, in addition to the relentless Sun beating down on us. Would there be any benefit to adding foil faced foam-board to the south and west facing walls, west facing metal garage door and entire ceiling? If so, can I just glue it up to the drywall and garage door with the foil facing in? I understand if it only makes sense to do the garage door since I have no intentions of air conditioning the garage.

    Thank you in advance. Sorry, forgot to add that the ducts are the flexible silver insulated kind, R8 I believe.

    Mark Maher,
    As a teen (many many moons ago) I lived in KY in a mobile home where the attic is inaccessible. I had a roof leak (silver metal roof) and rolled on a white elastomeric coating extremely thick. It stopped the leak and an added unpredicted bonus was it really cut down on the heat I used to feel radiating off the ceiling in the hot humid summer.

  143. GBA Editor
    Martin Holladay | | #143

    Response to M. Scott
    M. Scott,
    It's impossible to provide advice without a site visit. My usual recommendation in a case like yours is for you to hire a home performance contractor or home energy rater who has been certified by RESNET or BPI to perform an energy audit of your home.

    At the end of the audit, you will be given a customized list of retrofit recommendations, with the most cost-effective recommendations at the top of the list.

    My guess is that your home would benefit from work to convert your vented unconditioned attic into an unvented conditioned attic. The following article explains the work: Creating a Conditioned Attic. Whether the work would be cost-effective is an entirely different discussion, but your energy rater can provide advice.

    If you want to lower the temperature of your garage, adding insulation will certainly help. Whether it's worth it depends on whether you value a cooler garage.

  144. mp06011999 | | #144

    Thanks for the fast reply.
    Thanks for the fast reply. "Cost Effective" is always the key. In all the research I have been doing, mostly with adding a radiant barrier product to the attic, the numbers keep coming up with an annual savings of about $50 - and that's from the radiant barrier selling folks themselves! Plus, add to that that we tend to be low consumers where whenever you see anything talking about "average cost" or "average savings" our household is usually 1/2 that. So I concluded that spending $600 to add radiant barrier to my attic would at best take 12 to 24 years to pay for itself. That's about the time it becomes too dusty to work right or we have long moved.

    Anyhow, as for the garage, ABSOLUTELY I wish to have a cooler garage. It was about 100 outside yesterday and 110 in the garage. Walk within 5 feet of the door and you soon know why. The door was reading 117. I want to glue some foam panels to the door this weekend. I saw a video where a guy tried the foil facing the metal door and the foil facing the inside of the building. Foil facing inside was cooler measuring with an IR temp gun. My guess is on the other example he was actually allowing the foil to come into contact with the metal door. Thus eliminating any radiant barrier and increasing conduction via hot metal door to metal foil to foam. And that's why it was cooler mounting it foil facing away from metal door. Hence why I'm asking, would you glue the foil-faced foam board facing into the garage? That wouldn't "trap" heat in the garage in the summer, correct? Thanks again.

  145. GBA Editor
    Martin Holladay | | #145

    Response to M. Scott
    M. Scott,
    If the outdoor temperature is 100 degrees, and the indoor temperature is 110, you have a very small delta-T -- only 10 F degrees. So insulation has only a limited ability to help.

    While it make sense to try to glue or fasten some foil-faced polyisocyanurate to your garage door -- as long as the insulation doesn't interfere with the door's operation, of course -- you'll need to install an air conditioner in your garage to get the temperature lower than 100 degrees when it is 100 degrees outside.

  146. mp06011999 | | #146

    Thanks. Yea I know,

    Thanks. Yea I know, I don't expect to get the garage cooler than outside without adding ac. I just want to get it "not hotter" than outside. I can work in the garage at upto 103 - my cut off. But as is, I can't even get close to the door once the Sun is on that side of the house.

    An interesting thing I noticed since moving to the dry desert from the midwest that I think falls inline with this talk of radiant energy.... You can stand in the shade here in 100 degrees and be fine. Stand in the sunlight in even the 60s and you will heat up quickly. I think the lack of moisture in the air also adds to the intensity of the Sun - nothing to diffuse it - plus it's bouncing off of all this brown landscape and structures, not being absorbed by moist vegetation.

    I could be wrong about the reasons, but an experience I think relates to your talk of radiant barriers and where they may have a purpose. It would be nice if they incorporated radiant barriers into the new construction here, but it would be even nicer if they would stop putting the ac units in the attics and on the roofs.

  147. GBA Editor
    Martin Holladay | | #147

    Response to M. Scott
    M. Scott,
    In fact, many Las Vegas builders have been using radiant-barrier roof sheathing for years. But evidently yours didn't.

  148. user-3549882 | | #148

    Roofs and garages and saving energy
    In case it helps, I've had the following experiences:

    1) We live in the Chicago area. Our attic is well ventilated. When we had a cedar roof, I wanted to reduce the attic temp. I thought about a radiant barrier but held off. First, I added batt insulation between the rafters. The attic became cooler in summer. I then decided to apply some old white latex paint to the roof. The effect was to cool the roof so that water sprayed to the roof no longer flashed to steam.

    Later we changed the roofing to asphalt shingles (light gray). I was disappointed with the reflectivity numbers and so wasn't surprised when a water spray flashed to steam. So I bought a distressed white latex for $5 and diluted it with a hose sprayer. That gallon was enough to cover a 2200 ft2 roof. The roof no longer steams. Unhappily, I don't have attic temperature data to share. As regards the issue of loss of heating in winter, I also have no data. My operating opinion is the summer heat is the main issue. The attic can easily exceed 130F and that influences the a/c load. With insulation and a whiter roof, the attic temperature drops 20-30F. In winter, the attic is nearly the ambient temperature. In addition, the number of sunny days is limited in our area. Snow on the roof means the roof color is largely irrelevant for weeks at a time.

    2) As regards garages, I've found a way to keep it warmer in winter and cooler in summer. I started with 1" polyiso insulation. I formed a sandwich with the reflective surfaces outboard. This was mounted to the garage door segments with light framing and long screws (no glue). I had to reset the torsion springs. Next, for summers, I placed solar grates over the casement windows on the South side. That reduced heating from solar gain. I also tightened up the seal around the garage door. There is no heater for the garage and no a/c. Still, our garage is now warmer than ambient in winter and cooler than ambient during summers. The garage is attached to the house so this benefits the home as well.

  149. markgwoodruff | | #149

    Incredibly biased article
    How many million existing homes are there in Florida with R19 or less insulation? How many thousand well-insulated (R40 roof/R30 walls) homes are there in the same state? Just how difficult is it for the owners of the poorly-insulated existing homes to staple radiant barrier to their rafters? How expensive per square foot if they do the labor themselves?

    Your decision to almost exclusively focus on buildings that could be instead of the millions of buildings that are poorly informs your readers, most of whom (based on the odds) are more likely to retrofit their existing residences than build a new ones in the coming few years.

    Take the same facts. A more accurate headline would be:

    Radiant barrier: Most cost-effective insulation option for older homes in Zones 1-3, but not worthwhile in newer and future homes

    My bias: My friends and I stapled perforated radiant barrier to the rafters of my 1984 code house in Florida a few years ago. My total power bills immediately dropped over 20% the month after we installed it. The installation paid for itself in three months and has been the single most cost-effective energy-saving purchase I have made.

  150. GBA Editor
    Martin Holladay | | #150

    Response to Mark Woodruff
    I'm glad that you found that your retrofit efforts lowered your energy bills.

    Most researchers haven't been able to replicate your results. Other measures, including improved insulation, are usually more cost effective than the installation a retrofit radiant barrier, which tends to have a very long payback period compared to other measures.

  151. build2580 | | #151

    using reflective foil correctly
    If I understood the bulk of this thread correctly, there are some places that using foil makes sense and I think my location may be one of them. Our summers are very hot and dry with roof spaces getting well up in the mid100sF. Additionally, roof sheeting is not generally installed above the rafters and below the roofing material- in my case corrugated iron. The "common knowledge" here is that the reflective sarking that is laid horizontally and lapped from bottom to top acts as a drainage plane in the winter when moisture condenses under the cold roofing metal and drips. Additionally, with the foil facing down and an air gap min of 2in between it and the insulation below minimizes the delta T that the insulation is then dealing with. First question: is that basic argument valid? Second questions: Is there not also an issue with condensation on the underside of the reflective sarking which will then itself drip onto the insulation below (which has less ventilation and air movement than the space between the roof iron and the sarking).

  152. GBA Editor
    Martin Holladay | | #152

    Response to Enga Lokey
    First, I'm going to translate some of your terms for American readers. (I'm guessing that you are from Australia or an African country that was formerly part of the British Empire.)

    "Reflective sarking" is a product that in the U.S. would be described as a type of roofing underlayment with a reflective layer.

    If you are building a new house that includes a type of roofing (for example, steel roofing) that does not require roof sheathing (plywood or OSB), then it probably makes sense to install reflective sarking under your roofing. The cost is low, and the sarking will help keep your attic cool.

    The main disadvantage of this approach is that, as dust accumulates on the sarking, its performance degrades until it eventually becomes worthless.

    If you want to reduce the flow of heat from a hot attic to the interior of your house, the basic tools in your arsenal are: (a) an airtight ceiling, and (b) a thick layer of insulation. The use of reflective sarking should not lull you to sleep. Even with reflective sarking -- a material doomed to get dusty -- you still need insulation, and you still need to pay attention to airtight construction methods.

    And once you have an adequate thickness of insulation, the radiant barrier becomes unnecessary.

    Concerning condensation: you are correct that condensation collects on the underside of steel roofing in some weather conditions. Whatever type of sarking (roofing underlayment) you install, it must be installed with attention to watertightness. That means it should have no holes; it should be strong; it should have adequate overlaps; and there should be no reverse laps. And you need to think about where the condensation will drip when it gets to the eaves; presumably, the condensation can drip from your soffit vents.

    You may want to choose one of the stronger European roofing underlayments (if they are available in your country) to minimize the chance that condensation will find a flaw in the sarking and drip on your insulation.

  153. lillianjane | | #153

    foil bubble insulation
    Trying to decide if there is any benefit to open attic ceiling installation as a heat barrier in a Canadian seasonal unconditioned residence.

    I can purchase 600 sp ft for $40 - obviously it wasn't as effective as planned when installed as it was " removed from new home to strap ceiling in basement for drywall".

    Also, can it be used on the window covers in an Arizona residence to delect the heat?

  154. GBA Editor
    Martin Holladay | | #154

    Response to Lillian MacCannell
    Foil-faced bubble wrap is a waste of money. It has almost no R-value. For more information, see Stay Away from Foil-Faced Bubble Wrap.

  155. Radiant_Barriers | | #155

    This article “Radiant barriers: A solution in search of a problem” is wrong, biased, and I am surprised that the editors of this web site continue to allow its internet publication without a retraction. In this case, the fault of the article lies entirely with the publishers.

    The Florida Solar Energy Center and Oakridge National Laboratory have conducted numerous, reproducible experiments proving the value of radiant barriers (see below) in different parts (climates) of the country, through different seasons (temperatures and humidity), and using various building materials and construction methods.

    These experiments have been done with and without additional insulation, and radiant barriers have proven their place and value in modern construction.

    When problems arise, these usually have to do with installations that do not follow up on guidelines and procedures.. As the name of these products states these are radiant products.

    If they come in contact with a surface, then following well known physical properties they will transfer heat to them. For this reason, their installation should have air gaps on both sides.

    Best building practices are shown in countless Building America Program documents (see below).

    One cannot simply talk about radiant barriers as if they have no value by using logic. This in fact makes it an Aristotelian digression. Today, we learn my experience and experimentation. We do not learn solely by discourse and logic as with Aristotle or Shakespeare — regardless of how amusing they could be.

    1. Expert Member
      MALCOLM TAYLOR | | #156

      I was with you until you brought in Shakespeare. Are you sure you meant Shakespeare?

    2. Expert Member
      Dana Dorsett | | #157

      From one of the referenced linked to by Radiant_Barriers:

      "I live in Orlando, Florida in an 1800 square foot ranch style house. I have R-11 insulation in my attic, and the air-conditioning ducts are in the attic. A contractor has quoted a price for a radiant barrier installed on the bottoms of my rafters and on the gable ends for $400. Would this be a good investment?

      "For this type of radiant barrier, the appropriate table is Table 4. For Orlando with R-11 insulation, the present value savings is listed as $0.32 when the air-conditioning ducts are in the attic. Multiplying this value by 1800 square feet gives a total of $576. This value exceeds the quoted cost of the radiant barrier of $400, and thus this would be a good investment."

      R11 attic insulation?


      So, if you live in a hot location and have attic insulation levels LESS THAN A THIRD current code minimums you might eventually recoup the investment.

      They don't make the comparative present value of applying the radiant barrier money on bringing it up to code min R38 (or even R25.) Even with 2x4 bottom chord trussed ceiling it would be possible to cost-effectively raise the overall performance to better than what you'd get by adding radiant barrier.

  156. GBA Editor
    Martin Holladay | | #158

    Response to Comment #155:

    Mr. Radiant Barriers,
    Like Malcolm Taylor, I was confused by your reference to Shakespeare. But then I thought that you might be making a subtle insult, with the intention of calling me a slut. (I'm thinking, of course, of "Merry Wives," Act 5, Scene 5, when Pistol tells Falstaff, "Our radiant queen hates sluts and sluttery.")

    I find it interesting that you criticize my article without finding a single statement in my article that you are willing to dispute. I can assure you that my article is accurate.

    As I wrote in my article, radiant barriers work. They just aren't very cost-effective -- because the advantages they provide are small compared to their cost.

    And by the way, I have no idea why you have invented a dichotomy that places logic on one side and experimentation on the other, as if logic and experimentation were enemies. I embrace both, as do most scientists.

  157. Radiant_Barriers | | #159

    The best argument you can make is that radiant barrier performance depends where you place it.

    However, the majority of housing stock benefits from it.

    1. Expert Member
      Dana Dorsett | | #161

      What are the units on the horizontal axis on that graph? It's clearly not weeks or centuries, but presumably a time scale. Decades mayhaps? Can you link to the source where that was clipped?

      1. GBA Editor
        Martin Holladay | | #162

        The label says "Climate cooling demand (cooling demand in months per year)."

        In other words, it's not a time scale. It's an indication of geography. The left side of the x axis is Illinois or Michigan; the right side of the x axis is Florida.

        1. Expert Member
          Dana Dorsett | | #163

          After another cuppa joe I managed to parse that as well- thanks! ;-)

          Still no definition of what constitutes the anticipated lifecycle of the house. (My house is 95 years old, the house on the abutting property is 113 years old, my biz-partner lives in a house that's over 150 years old...)

          Rather than simple payback over an undefined building lifespan, a present value analysis is more valid. The R11 attic in the ORNL example I highlighted in response #157 showed a positive net present value IF the air conditioning ducts were in the attic, and the RB could be installed for $400 (22 cents per square foot) but did not compare it directly with a present valuation with simply bringing the R-values up to code or bringing the ducts inside.

          Taking a look at the referenced table 4, bumping the R-value to R19 cuts the present value of the savings in half, bumpting it to R30 reduces it by 2/3:

          In an R19 the present value of the savings drops from $576 in the example to half that, or $288, which is less than than the $400 presumed cost for the RB. (The material cost of 1800 square feet of RB would be nearly $288 in 2018 dollars.).

          In an R30 attic the present value drops to about $192, which means the installed cost of the RB would be roughly twice the benefit, and even the material cost of the RB isn't financially rational even if valuing your DIY time at $0.

  158. GBA Editor
    Martin Holladay | | #160

    Mr. Radiant Barriers,
    You wrote, "the majority of housing stock benefits from it" -- presumably, a radiant barrier.

    Guess what? The majority of housing stock benefits even more -- at a lower cost for the benefit achieved -- from insulation improvements and air sealing measures.

  159. Radiant_Barriers | | #164

    Mark Twain had a wonderful saying.

    “It ain’t what you don’t know that gets you in trouble. It’s what you know for sure that just ain’t so.”

    1. Expert Member
      Dana Dorsett | | #165

      ...including the fact that there is no credible evidence that the above quote was ever uttered or written by Mark Twain (despite frequent attribution thereof.)

      The ORNL / DOE energy use models are fairly well vetted by both laboratory and field data, and show very low energy use reduction benefits for radiant barrier when the ducts either aren't in the attic (or are insulated and reasonably tight) and the insulation levels in the attic are anywhere near current IRC code (or even half that.) See Figure 6:

      Looking at that Figure, with no attic ducts and R19 insulation (which is below current code) even in US climate zone 1 (south Florida) radiant barrier on a 1550' house saves $20/year. If code (=R30) it would only save $15/year. At the very low end (crummiest material, sub-minimum wage undocumented immigrant labor) you might get the installed cost for that house down to $250. A DIY at box-store pricing for the material would be about the same. If it's saving $15-20/year that's an IRR of 6-8%, a rate of return WAY below where typical homeowners would jump at even if planning to stay there for the 12-17 years it would take to "pay off".

      Bumping the R19 to R30 with R11 batts would run about $400-500 and only knock $5/year off the cooling costs, which is an even worse IRR if only cooling costs are considered. Improving it to code min will have heating season financial benefit too, which can be even more valuable in zone 1 if you're heating with electric resistance (like too many Floridians) rather than a heat pump or natural gas. (The annual peak grid loads in FL are during the heating season, all due to resistance heating.)

      With the ducts in the attic even good ducts with would see twice those savings on the R19 or at code-min R30, but with crappy uninsulated leaky ducts it's 6-7x as much. But is the real "solution" radiant barrier, or is it fixing the ducts? (Me, I'd fix the ducts and first, or apply the duct & RB money toward high efficiency ductless mini-split heat pump. YMMV.)

      Outside of zones 1 & 2 even with crappy ducts and sub-code R values there's not a very high IRR to radiant barrier. And in climate zones 3 & up the heating season value of bringing the R-values up to snuff will usually exceed the annual savings of adding RB, and the RB will reject some amount of beneficial solar gain during the spring and shoulder seasons, increasing the annual heating bill.

      In the concluding paragraphs on page 9 it reads:

      "“If you have poorly insulated and leaking ducts
      in the attic in climate zones 1 and 2 (e.g., Florida, southern
      parts of Texas), radiant barriers will save $50 to $150 per year.
      For other conditions and locations, savings will be much
      smaller or negative.”

      The unstated part of that is that if you DON'T have poorly insulated leaking ducts in the attic the savings will be less, even in climate zones 1 & 2. The fact that the energy cost "savings" can be negative due to loss of beneficial gains during the heating season should give one pause- model the energy use numbers carefully before diving in. The real takeaway from the ORNL analysis is that the ducts really shouldn't be shoved up in the attic- even tight well -insulated ducts.

      [edited to attach a copy of the ORNL analysis, lest it's location on the web change.]

  160. Radiant_Barriers | | #166

    As I said before Mark Twain had a wonderful saying.

    “It ain’t what you don’t know that gets you in trouble. It’s what you know for sure that just ain’t so.”

    You would be absolutely correct if everyone lived on the dark side of the moon. Even here on planet Earth you would almost be correct if everyone lived on either the North Pole or the South Pole; however, the majority of the people live somewhere in between these two extremes.

    For those of us living in between we need not only radiant barriers but insulation as well. Without radiant barriers insulation will heat up quickly and release heat at the wrong time. Depending on where one lives on planet Earth more insulation or less might be needed, and the same goes for radiant barriers.

    Location and climate will determine the optimal mix of these two building materials. People living in the desert will prefer to radiate solar energy quickly for a portion of the year whereas people living near the poles will prefer more insulation for part of the year.

    Absolutes in this case make almost no sense (except perhaps, for instance, on the cold, dark side of the moon).

    Solar infrared energy is about 60% of solar energy, and a radiant barrier may stop about 80% of infrared energy from reaching the interior of a home by reflecting it back to the Earth’s atmosphere.

    80% is a conservative figure in this example since as the Florida Solar Energy Center states “[a]n aluminum foil radiant barrier blocks 95 percent of the heat radiated down by the roof so it can't reach the insulation.”

    This means that out of every 1,000 watts of solar irradiance about 570 watts is infrared energy, and stopping 80% of this means that 114 watts gets into the attic. These 114 watts of solar infrared energy can be delayed with insulation.

    In the summer, insulation can delay transferring this heat to the interior of a house for a few hours. Enough insulation, for instance, can release this heat late in the afternoon rather than immediately, and this helps in making air conditioners work with less effort during daylight hours. However, insulation cannot stop heat from advancing towards the interior of a house since energy cannot be created or destroyed.

    Without radiant barriers insulation would need to delay 570 watts of infrared energy rather than 114 watts of energy, and air conditioners would need to provide more cooling energy to remove an additional 456 watts of energy due to the lack of radiant barriers. Therefore, greater air conditioning energy usage may be required when radiant barriers are not installed.

    Please note that most places on Earth receive a multiple of 1,000 watts per square meter every day, and this fact makes the usage of radiant barriers an imperative at some locations.

    For instance, Oak Ridge National Laboratory states that “[t]he tests to date have shown that in attics with R-19 insulation, radiant barriers can reduce summer ceiling heat gains by about 16 to 42 percent compared to an attic with the same insulation level and no radiant barrier.”

    Moreover, “.... 2 to 10 percent reduction in the cooling portion of summer utility bills. However, under some conditions, the percentage reduction of the cooling portion of summer utility bills may be larger, perhaps as large as 17 percent.”

    The Florida Solar Energy Center after much testing concluded “[i]n hot climates, benefits of attic radiant barriers include both dollar savings and increased comfort.”

    Please note the carefully stated remark pointing out hot climates. These are not for all climates. So despite your insistence in calling attention to their lack of function there are people in some parts of the world which benefit from their installation, and this is not based on my opinion.

    Now, a 16- to 42-percent reduction in ceiling heat gain (as stated above) may not seem much according to you, but it is a large enough reduction, in my humble opinion, to make an impact for the better in the life of people living in a building with insulation and radiant barriers during the summer months, and I will end this discussion on this note since I as I stated before I do not enjoy pointless Aristotelian discourse for the purpose of proving science.

    You are free to continue arguing against radiant barriers as much as you wish, and I and a few other folks will continue to install these in homes since we know what we know.

  161. this_page_left_blank | | #167

    You said,

    "For those of us living in between we need not only radiant barriers but insulation as well. Without radiant barriers insulation will heat up quickly and release heat at the wrong time. Depending on where one lives on planet Earth more insulation or less might be needed, and the same goes for radiant barriers. "

    The fact that the vast majority of homes (I think >90% is not an unreasonable estimate) have no radiant barriers, and operate perfectly adequately kind of sinks your premise that radiant barriers are a necessary component of building.

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