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Radiant Barriers: A Solution in Search of a Problem

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

Posted on Sep 24 2010 by Martin Holladay

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 emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. (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 ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. Fundamentals, a vertical 3/4-inch air space has an R-value of about R-1 — assuming that the heat-emitting surface adjacent to the air space has an emissivity of 0.82. If the same air space is faced with a radiant barrier with a emissivity of 0.05, the R-value of the air space is boosted from R-1 to about R-3.

Radiant barriers make sense for uninsulated barns

The effect of a radiant barrier on a building assembly's R-value may be significant or insignificant, depending on whether the assembly is well insulated or poorly insulated. Radiant barriers do not significantly benefit well-insulated assemblies.

For example, consider drywall installed on a SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. wall. If the SIP has an R-value of R-30, the emissivity of the drywall hardly matters. Since the drywall is at room temperature, it's at thermal equilibrium with the other objects in the room, so radiant heat transfer isn’t a significant heat-transfer mechanism for people or objects in the room. (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.)

A poorly insulated assembly, however, will benefit from a radiant barrier. For example, consider an uninsulated barn with galvanized steel panels for roofing and siding. On a hot sunny day, the steel panels are warmer than the interior of the barn, so they radiate a lot of heat. If a radiant barrier is installed on the interior of the wall, it cuts down on the transfer of radiant heat from the steel panels to the interior surfaces.

Because building codes require the walls and ceilings of new homes to be insulated, there isn’t any need to install a radiant barrier in a well-designed home.

Building a stack of 1-inch air spaces

A few “religious believers” in radiant barriers have experimented with building wall or ceiling assemblies consisting of multiple 1-inch air spaces separated by aluminum foil. While these assemblies work — after all, if you put together a thick enough pile of R-3 pancakes, you can eventually achieve a reasonable R-value — they cost far more to build than ordinary walls with conventional insulation.

Radiant barrier fanatics have also experimented with horizontal radiant barriers on the top side of attic floor insulation. There are two problems with such radiant barriers:

  • Once the radiant barrier gets dusty, it’s no longer a low-e surface. Radiant barriers have to stay shiny to work.
  • Unless the radiant barrier is perforated, it acts as a vapor barrier. During the winter, condensation will form on the underside of the radiant barrier.

Don’t be tempted to install foil-faced bubble pack under a concrete slab. The R-value of foil-faced bubble pack — generally between R-1 and R-2 — is far too low for such an application.

Radiant-barrier roof sheathing

The most common type of radiant barrier used in new-home construction is radiant-barrier roof sheathing — that is, plywood or OSB with a radiant barrier on one side of the panel. These panels are installed over unconditioned attics, with the shiny side facing down. Radiant-barrier roof sheathing only makes sense in hot-climate homes that have HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment or ductwork installed in an unconditioned attic.

Here’s the logic: the builder knows that the HVAC equipment and ductwork will get very hot in the summer. The builder doesn’t want to move the HVAC equipment and ductwork where they belong — inside the home’s conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. — because it’s cheaper to install everything in the hot attic. So the builder installs radiant-barrier sheathing to keep the attic a little cooler. At best, it’s a halfway solution to a basic design problem.

That said, 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.

New homes in a cold climate, on the other hand, shouldn't use radiant-barrier roof sheathing. Up north, a sun-warmed attic helps lower heating bills.

What about energy savings?

Installers of attic radiant barriers often make exaggerated energy-savings claims. What do the experts say?

According to a research report published by Oak Ridge National Laboratory, “The 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. These figures are for the average reduction in heat flow through the insulation path. They do not include effects of heat flow through the framing members. … THIS DOES NOT MEAN THAT A 16 TO 42 PERCENT SAVINGS IN UTILITY BILLS CAN BE EXPECTED. Since the ceiling heat gains represent about 15 to 25 percent of the total cooling load on the house, a radiant barrier would be expected to reduce the space cooling portion of summer utility bills by less than 15 to 25 percent. Multiplying this percentage (15 to 25 percent) by the percentage reduction in ceiling heat flow (16 to 42 percent) would result in a 2 to 10 percent reduction in the cooling portion of summer utility bills.”

That’s the bottom line — a 2% to 10% reduction in the cooling portion of your summer electric bill. (Obviously, that’s less than a 2% to 10% reduction in your summer electric bill.)

However, notice that these figures were calculated for a very poorly insulated attic — one with R-19 fiberglass batts. Even in Florida, it’s now illegal to build a house with such a poorly insulated attic. If you have a code-compliant home in Florida, you should have R-30 attic insulation.

If you have properly installed R-30 or R-38 attic insulation, don't expect a radiant barrier to lower your cooling bills — unless, of course, your builder installed ductwork in your attic.

Meanwhile, up in Minnesota ...

If you live in a cold climate, radiant barriers make even less sense than they do in Florida. According to the U.S. Department of Energy’s Energy Efficiency and Renewable Energy Clearinghouse, “Two field tests, one in Minnesota and one in Canada, both found that a radiant barrier placed over R-19 attic floor insulation (which is less than half the DOE minimum recommendation for those climates), found that the radiant barrier contributed to less than a 1% reduction in energy consumption for heating and cooling.”

As with the Florida example, it should be pointed out that even these meager savings are associated with attics that are poorly insulated. If the attic has code-minimum insulation, the savings disappear.

A retrofit study in Nevada

In 2001, researchers from the National Association of Home Builders (NAHBNational Association of Home Builders, which awards a Model Green Home Certification.) Research Center evaluated several energy-retrofit measures in a 1,270-square-foot ranch house in Henderson, Nevada. The researchers focused on measures appropriate for hot climates.

At a cost of $650, the researchers installed a radiant barrier on the underside of the attic rafters. The energy savings attributable to the radiant barrier were calculated at $11 per year, meaning that the simple payback period for the radiant barrier was 59 years.

After I wrote a report on the research for Energy Design Update, several radiant-barrier dealers wrote letters complaining that the numbers couldn’t be right. However, Danny Parker, a senior researcher at the Florida Solar Energy Center, came to my defense. Comparing the NAHB researchers’ findings with the results of his own Florida research, Parker wrote, “I looked over the NAHB report and think it’s pretty good. … In the section on economics in our detailed comparison with ceiling insulation, we show a radiant barrier system to be more expensive than added ceiling insulation. And our retrofit experience with radiant barriers shows them to be very expensive to put in (high labor costs).”

Less effective — and more costly

Even radiant barrier products that provide some benefit — for example, foil-faced bubble wrap or “radiant barrier chips” — have an Achilles’ heel: they cost more than conventional insulation. When I wrote a report on foil-faced bubble wrap in 2003, I found that the product was selling for between $.38 and $.50 per square foot. In other words, the R-1 bubble-wrap costs more than R-5 extruded polystyrene.

In 2004, I looked into the cost of radiant barrier chips — a product designed for horizontal application on attic floors. At that time, the developer of the product claimed that radiant barrier chips could be installed for $1.50 per square foot. In other words, it cost more to install a thin layer of radiant barrier chips than 12 inches of cellulose.

Avis à nos lecteurs francophones: Cet article a été traduit en français par André Fauteux («Membranes réfléchissantes : une solution en quête d'un problème»)

Last week’s blog: “Are Dew-Point Calculations Really Necessary?”

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Sep 24, 2010 8:56 AM ET

Great article Martin. Around
by Jamie Kaye

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.


Sep 24, 2010 1:31 PM ET

Radiant Barriers article
by Curtis Dean

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.

Sep 24, 2010 3:36 PM ET

Another great report!
by Kris Knutson

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.

Sep 24, 2010 4:57 PM ET

A couple of years ago I was
by Robert Hronek

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.

Sep 24, 2010 10:33 PM ET

Hemp Houses
by Pam

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....

Sep 26, 2010 5:56 PM ET

Back to an unshiny radiant barrier idea
by Kevin Dickson, MSME

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?

Sep 27, 2010 5:27 AM ET

Response to Kevin Dickson
by Martin Holladay

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.

Sep 27, 2010 6:25 PM ET

Foil on Polyiso Insulation
by Brad Buser

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?

Sep 27, 2010 9:19 PM ET

Response to Brad
by Martin Holladay

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.

Sep 28, 2010 12:19 AM ET

Nonshiny radiant barriers
by Kevin Dickson


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.

Sep 28, 2010 5:56 AM ET

Response to Kevin
by Martin Holladay

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.

Sep 29, 2010 3:25 PM ET

by Greg Follet

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.

Sep 29, 2010 4:02 PM ET

Response to Greg Follet
by Martin Holladay

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.

Sep 29, 2010 7:59 PM ET

The Performance Value is in the Application of a Radiant Barrier
by Rod Brewer

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.

Sep 29, 2010 9:39 PM ET

window applications
by Sally Leong


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.

Sep 29, 2010 9:39 PM ET

Response to Rod Brewer
by Martin Holladay

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.

Sep 29, 2010 9:53 PM ET

Response to Sally Leong
by Martin Holladay

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.

Sep 29, 2010 10:20 PM ET


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...

Sep 29, 2010 10:43 PM ET

Worked for me
by Lee

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.

Sep 29, 2010 11:32 PM ET

radiant barriers
by gred

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.

Sep 30, 2010 6:06 AM ET

Response to Ed Gilson
by Martin Holladay

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.

Sep 30, 2010 6:11 AM ET

Response to Gred
by Martin Holladay

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.

Sep 30, 2010 12:41 PM ET

Radiant Barrier in Houston
by Glen


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:)

Sep 30, 2010 12:55 PM ET

Response to Glen
by Martin Holladay

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.

Sep 30, 2010 3:05 PM ET

Radiant barriers
by Don

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.

Sep 30, 2010 3:24 PM ET

Response to Don
by Martin Holladay

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.

Oct 1, 2010 5:37 PM ET

OK, I get it
by Wayne Jones

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.

Oct 2, 2010 6:45 AM ET

Burying ducts in blown-in fiberglass
by Martin Holladay

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.

Oct 2, 2010 8:30 PM ET

"Up North a sun warmed attic helps lower heating bills"
by Ivan Druker

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.

Oct 3, 2010 4:04 AM ET

Response to Ivan Druker
by Martin Holladay

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.

Oct 4, 2010 4:17 PM ET

The biggest problem with
by Chris

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.

Oct 4, 2010 4:24 PM ET

Minor nit-pick
by Gavin Farrell

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.

Oct 4, 2010 4:28 PM ET

Response to Gavin Farrell
by Martin Holladay

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.

Oct 5, 2010 8:58 PM ET

aarg, sorry to nit-pick.
by Gavin Farrell

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"

Oct 6, 2010 3:01 PM ET

Sorry Gavin
by Riversong

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.

Oct 6, 2010 3:40 PM ET

Now for Martin...
by Riversong

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.

Oct 6, 2010 3:46 PM ET

Radiant Barrier-High Cost, Low Effectiveness
by Curtis Dean

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.

Oct 6, 2010 3:55 PM ET

Response to Robert
by Martin Holladay

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.

Oct 6, 2010 4:49 PM ET

Reporting or Advocacy
by Riversong

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.

Oct 6, 2010 11:30 PM ET

using the wrong standard.
by Believer in Alaska

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.

Oct 7, 2010 4:57 AM ET

Response to Believer
by Martin Holladay

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.

Oct 8, 2010 9:48 AM ET

Sorry Gavin - Response to Riversong
by Gavin Farrell

'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.

Oct 12, 2010 11:26 AM ET

ISO order
by chris

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?

Oct 12, 2010 11:50 AM ET

Response to Chris
by Martin Holladay

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?

Oct 12, 2010 12:08 PM ET

Zone 6 have 2x4 rafters. ISO
by chris

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

Oct 12, 2010 12:29 PM ET

Second response to Chris
by Martin Holladay

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.

Oct 13, 2010 10:24 PM ET

Interesting Martin .......I'm
by Chris Sanders

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?

Oct 14, 2010 4:13 AM ET

Response to Chris
by Martin Holladay

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.

Oct 15, 2010 8:07 PM ET

Order when layering
by Chris Sanders

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.

Oct 16, 2010 4:32 AM ET

Fourth response to Chris
by Martin Holladay

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.

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