GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Building Science

How Heat Moves Through Homes — Building Science Podcast

Space-age radiant barriers work great in the vacuum of outer space, but here on Earth, heat moves by conduction, convection, and radiation, simultaneously, all the time.

To listen to GBA Radio on your iPod, right click, save to your desktop, drag into iTunes. This podcast series is excerpted from a two-day class called "Building Science Fundamentals" taught by Dr. Joe Lstiburek and Dr. John Straube of Building Science Corporation. For information on attending a live class, go to

In our last episode, Dr. Joe Lstiburek talked about efflorescence and the serious damage that water and salt can do to masonry. This week, Dr. John Straube explains how the three forms of heat flow work, and debunks the claims of a few common insulating materials.

Comfort is the Primary Purpose of Buildings

So, I actually call this “thermal control, insulation, and thermal bridges,” because thermal bridges are important. Let’s go back again and say, “Why do I want to control the heat flow?” Well, there are a whole bunch of reasons, but we often forget that the No. 1 reason we want to insulate is for thermal comfort. So, we want to make sure that exterior walls, roofs and floor slabs — that the temperature of these components stays above 68° in the wintertime and below 78° in summertime.

If the wall or roof or slab gets too far beyond those ranges, we feel discomfort regardless of what the thermostat says. Actually, a lot of interesting buildings that work well that don’t have a lot of insulation — say, the old five-wythes of masonry built in 1864 or something like that — they work not because they’re that well insulated if you put them into our value test; they work because they maintain the surface temperatures at a relatively high level in the winter and are relatively cool in the summer. Even though the air temperature may not be that well controlled, you feel comfortable in them. And so just maintaining comfort is the No. 1 reason why, when we moved to lightweight assemblies, we had to start adding insulation; it was just too darn cold.

Windows are tough to keep warm

Now, we still have that problem in our windows and our doors. So sliding patio doors — large areas of R-2 — well, they’re not very comfortable either in hot weather or in cold weather to sit next to. They just don’t meet the test of providing sufficient thermal control for comfort. We’re not even talking energy. So, they fail the first test.

We also want to control surface and interstitial condensation. Now that we are quite clear on “condensation occurs on cold surfaces,” we can also now look at the psychometric chart and say, “So, how cold is cold?” Well, it depends on interior relative humidity, right? And if we keep the relative humidity in, say, the 30% range in cold weather, that means the surface temperatures have to be above 35 or 40 degrees. If you’re in the 40% relative humidity range, you now have to keep interior surface temperatures above 45 or so degrees.

And again, where do we get that surface condensation? On our windows, because our windows are the least insulated part of the building enclosure. So, we see that first. But we also can get it in corners and at thermal bridges.

R-5 meets most comfort and condensation concerns

The next thing is to save energy. But really, our struggle first and foremost in terms of stopping the biggest problems is: Make sure we get comfort; make sure we don’t get surface condensation. That’s what we need to have minimum levels of insulation for. And actually, R-5 will solve those two problems by and large in most climates. R-10, and you’re a slam dunk — even though we see codes with R-40s and R-20s and whatever. If we could just get R-5 everywhere, we could solve 95% of comfort and condensation problems — and R-10, we’d solve 99% of them.

But we can’t do that because we can’t even achieve R-5 and R-10. Saving energy, reducing operating costs and pollution is what the building codes are worrying about when they specify R-values — and we’ll find out how useless that is as an approach to saving energy as we work through this presentation.

Energy-efficient buildings need smaller heaters

Then we have to save distribution and heating plant costs. Most people seem to forget that if I actually do a good job of controlling heat flow across my building enclosure, it means I can reduce the size of my air conditioner, the size of my furnace, the size of the ducts, the size of diffusers, the size of the fans, the size of the fan motors, the size of the filter, the size of the space it requires to put all of that into a building.

So, we see a lot of life-cycle cost analysis of insulation and air tightness and such, and almost never do they include the escalating cost of energy — and they never include the rather significant savings that can be achieved in terms of downsizing your air conditioning plant, or your furnace or your ductwork. When you start reducing the size of ductwork or even eliminating it almost entirely, well, that opens up architectural opportunities: “Oh, you mean I don’t have to put in a suspended ceiling here? You mean I don’t actually have to take the duct to the outside wall?” Well, of course you don’t. If you insulate your walls sufficiently and provide good windows, you don’t have to provide a duct to the outside wall; you can provide the heat at the inside wall and simply blow the air over there. If you have a piece of junk wall and a piece of junk window, yeah, not only do you have to put the vent right below the window, you’re still going to be uncomfortable. But the duct savings are significant; it’s just that they’re not usually reaped.

Meeting codes doesn’t guarantee good performance

Now, decreasing load diversity is something that matters to commercial buildings; less so to residential. Load diversity is the term used to describe the fact that at one part of the building you may have a significant cooling or heating load, while at the other part of the building you have the opposite. It’s very common in commercial buildings, for example, to have the east side require cooling at 10 o’clock in the morning. The sun is shining through the east windows, everyone has moved into the office, it’s all heated up; whereas on the west side, they’re still running the boiler to keep it hot.

The only way that situation could actually occur is if you have a very bad building enclosure. And yet it’s pretty common; it’s a pretty common scenario. And that’s not just energy wasting; that’s annoying for the controls people have to work out. It’s annoying that you have to have systems that can do cooling and heating at the same time — what a pain in the butt. And yet by building bad building enclosures, we have managed to create that as almost a standard in office buildings and schools, etc., in our part of the world.

And of course there’s the “I don’t wanna go to jail reason” to meet codes and specifications — which I always put at the bottom of the list. But actually, most designers that we work with, they put that as No. 1: “I don’t care if it matters; we’re just going to meet the codes.” And building code officials who say, “Look, just put R-20 in there. I don’t care if it actually does anything; jut put it in there to meet the code.”

Heat and how it moves

The Basics of Heat Flow

As construction methods and materials change, and energy gets more expensive, how and why we insulate our homes become more important.

Why insulate?

  • For thermal comfort
  • To save energy
  • To stop condensation and the potential for mold and rot
  • To reduce the size, cost, and complexity of our HVAC systems

In order to slow the flow of heat through our foundations, walls, windows, and roofs, it helps to understand what heat is and how it moves.

What is heat?

  • Heat is energy in the form of vibrating particles
  • The faster the particles move, the farther they move apart — slow particles make solids, faster ones turn to liquids, and even faster ones become gases

How can it move?

  • Conduction: solid things touching other solid things — drywall touching wall studs touching plywood
  • Convection: fluids, like water or air, moving around in an open space, like a pipe, wall cavity, or room
  • Radiation: heat in the form of electromagnetic energy moves through open space — the less stuff in the way, the better

So, to understand a little of this, we’re going to go through the three modes of heat transfer: conduction, convection and radiation. Those three modes of energy transport are acting all the time and in parallel; they don’t really interact that much. We have to look at each of the three to understand the total picture.

First, we should understand the nature of heat. Heat is vibratory energy in molecules. At any temperature that we’re experiencing, the molecules are vibrating. If you put more heat into them, they vibrate faster. In fact, if I take a piece of stainless steel and I put so much heat into it that I get to about 2,000 degrees, it’ll actually vibrate so much that the whole thing falls apart into a liquid. And if I keep heating that up, it’ll boil and the molecules will fly off into a vapor.

So, whatever the heat is, it’s vibratory molecules, and when we talk about conduction, we have hot and cold as one solid object, and the fast-vibrating hot molecules physically collide with the slower-moving cold molecules and transfer momentum. By transferring some of the speed from the hot to the cold, the hot gets colder, and the cold gets warmer; that’s how heat is transferred by conduction.

Conduction requires contact and low R-values

To have this mechanism work, you need solid material so that the molecules are directly in contact. Even having a gap of an eighth of an inch, you stop conduction. You still have conduction through the air, but the two solids have stopped. Our most common experience with this is metallic materials. You have a frying pan — cast iron — you put your hand on it. If it’s been sitting in the fire too long, the heat will conduct up to the handle and it’ll be hot. What we do to stop that, of course, is we put a piece of wood around the handle of the frying pan because the wood is less conductive.

Now, the way we measure the performance of conduction is something we call conductivity. And conductivity is a material property, like vapor permeability is a material property. The symbol is typically K, but in Japan and Germany, they use the Greek symbol lambda. We can figure out what the resistance is. By figuring for a specific thickness, you get a conductance, and from the conductance, you do an inverse and you get R-value. That measures resistance only.

Unfortunately, if this is the science behind it, the R-values that we see installed on our Styrofoam SM or on fiberglass insulation, they actually also include convection and radiation (we haven’t talked about that yet). So, the R-values we use for insulation products aren’t really scientifically based on conduction.

If air or water move freely you get convection

Convection is the movement of the hot molecule here to over here by physically grabbing the whole bundle of molecules and dragging it across. So, it’s a mass flow movement of grabbing the hot fluids, which could be water from the boiler, and transporting it to the second-floor bedroom; or it could be a hot furnace, and transporting it to the kitchen — but just physically moving the fluid. That’s convection.

Convection, actually, is more important in our building assemblies than most people realize — and it’s because our building assemblies have gaps and openings in them and we use materials that allow air to flow through them. And so when we look at something like a fiberglass batt that’s been installed from the inside, as I push this friction-fit batt, these fibers get pulled backward, right? They have to because of friction. Then that causes the fibers to stick out here, so the installer tucks the batt in like that. That’s your standard thing. As the installer tucks the corner in, it compresses the batt insulation and often causes a wrinkle at the middle.

How do we know this? Well, we get people who install fiberglass batts with Plexiglas sheathing and we look at the far side and can see all these little gaps and openings form. So, this is what it really looks like.

Batt insulation rarely works well in the real world

The question is: What happens at these gaps? Well, this will be filled with hot air because it’s on the warm side; this will be filled with cold air because it’s on the cold side. Hot air rises and cold air falls, and so we have these two micro-ducts on either side of the fiberglass insulation. As hot air rises, it goes through the insulation and comes down and goes around like that. And that transports energy.

How much energy? Well, it depends on how big those gaps are. Twenty, 30, 40 percent of the total heat flow across the wall can be by this mechanism. Which means your R-20 batt will drop to R-12 or R-10.

What’s neat about this mechanism is that as the temperature difference across the wall increases, there’s a greater and greater proportion of heat flow transported by this mechanism. As you need the R-value more and more, it actually drops. Which is why when they test R-value in labs, they never test it like this because then they wouldn’t get R-19. So, what they do is they turn it on its side and they make darn sure there are absolutely no gaps — no studs, no gaps. They also make sure that the temperature difference across the batt is never more than about 30 degrees. That way, all this shit can’t happen and they can just get good R-value numbers.

Now, if you take an R-19 batt and have the audacity to install it in a vertical application, your R-value goes down. If you then have little gaps — like batts are actually installed — well, then your R-value goes down. The bigger the temperature difference, the more the hit is; it’s a nonlinear relationship.

But it gets worse. Not only do we have the gaps and openings around it, we have the studs in between that, and of course the heat flows more easily through the studs than it does through the batt. So, who cares about the R-19 of the batt when the heat’s actually flowing through the triple studs around the window? And the steel studs?

With wood studs, you can argue that if you were a perfect tradesperson, you would install with no gaps at all. If you were given the three minutes per batt to install — most installers of course use at least three minutes to install a batt — you could probably make this work. But if you put it in a steel stud, the lips on the steel stud guarantee that no matter what level of trade quality is provided, you will have gaps around that steel stud. And those gaps will be the size of that lip — both 3/8 of an inch by an inch and a half wide. So, you’re guaranteed to lose between 30 and 40 percent of your R-value just right there. Isn’t that cool? Or hot, depending on the weather really.

This same loopy stuff can go on in wintertime around insulation installed in attics, because there’s nothing stopping the air from looping around up here, and if there are any voids around the rafters or joists, you get loops going on. In some cases, when the temperature drops to 10 degrees, some types of blown-in fiberglass (which mostly aren’t sold anymore) at 20-degree outside temperatures were getting half the rated R-value, and at 0 degrees, which does happen on the coldest day of the year in northern Minnesota, they were getting something like 30% of the minimum rated R-value.

So, you’d blow in R-40 and you’d get R-15 on the coldest day of the year, which of course is precisely when you need the most R-value. By the way, the way they fixed that these days is you go in and blow 4 to 8 inches of cellulose over top to provide a kind of air-impermeable cap on top of the fiberglass. So, if you do have problems, it’s often quite convenient and inexpensive just to blow a foot of cellulose on top and you’ll get a major improvement in performance.

Radiation likes empty space

Then we have radiation. These vibrating molecules create waves in space-time, which we call electromagnetic radiation. At the temperatures that we’re talking about, they’re infrared radiation.

If you were to make these molecules move fast enough, they would eventually glow red-hot; you’d be able to see them. They’d go from infrared to actual red. And if you keep heating them up, they’d get white hot. And if you kept heating them up, they would actually start giving off ultraviolet radiation — and then a nuclear explosion, you’d get gamma radiation, they’re so hot.

But most of our building applications aren’t worried about that. We’re worried about infrared radiation. That’s what infrared cameras look for: They look for the emission of radiation given off by the temperatures that the molecules have. Now, for radiation to be important, it really likes to transfer through — not solids; it likes to go through voids. It doesn’t even like to have gases in the way. That’s why the sun is able to transfer its energy from 93 million miles away to the planet Earth; because basically there’s nothing between us and the sun other than vacuum, except for the last 30 miles or so. Not even — it’s really only about 5 miles of air between us and the sun. So, radiation is quite effective. About 90% of the radiation given off by the sun hits the planet’s surface. We’re trying to change that, of course.

Now, what you need is a gap. If you have aluminum foil, which does not emit radiation very well, it does not change the heat flow across the building assembly unless there’s an air gap. So, you need a gap.

The Thermos bottles — they’re shiny glass on the inside, and that’s so there’s an air gap, and the shininess deals with the radiation. If you filled that void up with foam, the R-value would go down, not up, because you would eliminate any radiation benefit of the shiny metal. So, you have to have the gap; a slightly bigger gap would be good.

Now, within the pores of insulation like fiberglass or rockwool, there are so many voids that radiation actually does play a role in jumping from fiber to fiber inside that product. And in a fiberglass batt, about 40% of the heat transfer at common temperatures is due to radiation. Foam, it’s about 30%. Now, the reason that matters is that as the temperature changes, the contribution of radiation changes.

We’ve all probably been around a fire on a cold night; watch the campfire burn and you can feel the heat radiating to your face. That’s because it’s hot. It really makes a difference whether that fire is hot or cold whether you feel the radiation on your face or not. As you get down to building-related temperatures — 100 degrees, 50 degrees — radiation gets less and less important.

But at high temperatures, radiation is important and it’s a major transfer mechanism; at low temperatures, it doesn’t play as big a role. So, radiant barriers are very good for high temperatures — say, the roof in a sunny climate. They’re less important for cold conditions — say, the underside of a crawlspace; they don’t play as big a role. But in every case they need an air gap.

NASA’s radiant barriers are useless when you pour concrete over them

With radiant floor heat, it is actually kind of misnamed. There is radiation transfer, but actually most of it is by convection. So, convection matters, radiation matters, but more importantly, when I think of radiant floor heat [I think of] snake-oil salesmen who sell these radiant barriers underneath radiant slabs. Radiant-radiant, right? They should go together; they’re both named radiant.

But of course, when you pour concrete on the aluminum foil, there’s no air gap, is there? So, there’s no R-value benefit. The R-value of a piece of aluminum foil underneath a chunk of concrete on top of soil is around 0 — somewhere between 0 and bupkus. However, they don’t test them that way, do they? They test them in horizontal apparatuses with an air gap above and an air gap below with ridiculous temperature differences across them. And then they get, like, R-8. But it’s hard to suspend that slab 4 inches above the radiant foil in most of my radiant slab designs.

So, what they’ve done to address that is they put the little bubble wraps — the radiant foil bubble wraps, and so on — and they have, well, tiny bubbles (Don Ho sang about that until his recent death). These tiny bubbles in between the aluminum foil do help the R-value, and you can get as much as R-1 on some of the bigger bubble products.

Now, the cost of that R-1 bubble wrap per R is about 3 times the cost of buying extruded polystyrene foam, but you can market this stuff as “space age.” Well, NASA uses it. OK, let’s think about this. I’m in outer space. There is no air. So, what are the heat transfer mechanisms? Well, there’s no convection; there’s no air. All I’ve got is conduction and radiation — so, if I’m not touching it, of course there’s only radiation. NASA uses radiant barriers because radiation is the only way they can transfer heat from them to other spatial bodies. It’s the only mechanism that works. ‘

But as long as you’re building your buildings on Earth, in an air-filled environment, there are other mechanisms that are actually more important. But the NASA technology and the “ceramic balls” — it’s all just bullshit.

But somehow they manage to sell this stuff by playing on people’s ignorance. They’re not sure about how all this works.


  1. Doug McEvers | | #1

    Heat through homes
    Great article,

    The forth and equally important energy bandit is infiltration, properly placed air barriers and insulation work in harmony to minimize heat loss.

    Thank you for mentioning the folly in using ceramic paints and foil bubble products, radiant slabs with only foil bubble insulation have proven this product to be completely worthless.

  2. Norman Mancusi | | #2

    Thank you, it was very
    Thank you, it was very informative.

  3. Chris Hunter | | #3

    I find it disturbing that Mr.
    I find it disturbing that Mr. Lstiburek admitted that radiation is a factor in building envelope heat gain,
    yet states that radiant barriers are a joke, all of this based on false word-of-mouth advertising by unscrupulous salespeople. There are some companies exaggerating the truth about radiant barriers, but I would not throw out the baby with the bath water.

    Lstiburek and Marting Hallady are in lock step with their disgust of false claims given by the abovementioned individuals, and rightly so, but again, this does not negate the power of radiant barriers when installed properly. The state of Florida conducted a lengthy study on radiant barriers in attic spaces and found them to be quite effective. Go to and enter 'radiant barrier in attics' for a link to this report, one conducted by a govt entity.

    Lstiburek states sarcastically about radiant barriers being developed for outer space and its empty vacuum, and so isn't it obvious that they will not work here on earth with all the accompanying elements contained therein? Give me a break- if Mylar did not work on earth, then emergency blankets would be useless, which have no R-value, of any meaningful amount.

    I refuse to get carried away in emotion based opinions simply because individual(s) horribly embellish a worthy products capabilities. If a salesman states that his siding will not fade even after 75 years of southwest U.S., exposure, and it fails this test, does this mean his siding is junk? Of course not.

    A little more logical reasoning would be appreciated in a setting such as this.

  4. GBA Editor
    Martin Holladay | | #4

    Response to Chris Hunter
    1. This Podcast is a recording of John Straube, not Joe Lstiburek.

    2. You're right that radiant barriers can work -- as long as they are facing an air space, and not buried under concrete. Their main problem is that they usually cost more than insulation. To put it another way: insulation that costs the same price as a radiant barrier usually does a better job of reducing heat flow.

  5. John Straube | | #5

    Please read the article again.
    Dear Mr Hunter.
    This presentation was given by me, John Straube, not Joe Lstiburek (this information is in the lead in to the article). I dont think you read it very closely. Please read it again.

    In the seminar I did not say all radiant barriers have no resistance to heat. I said that a radiant coating in contact with poured concrete has no R-value. Radiation occurs on earth, and in fact, comprises about 40% of total heat transfer through a batt for example. But it comprises under 1% of the heat transfer through concrete, and 0% of the heat transfer through metallic coatings.

    Mylar emergency blankets provide an R-value of about R2. They are used because this is the best you can do with a tiny volume. If the goal was to use the least volume of material, I would use multiple layers of RB. But if I can use other insulations I will in most applications, because they work better,
    If you want to know what the true value of a radiant barrier system is, their are nice tables in the ASHRAE handbook of fundamentals, or in my building science textbook. A summary: You can get a boost of about R1-4 in a wall depending on the temperature (high temperatures show more benefit) and for a roof you can get R1 to 10+ from a radiant barrier. R1 in Fairbanks on a January night, and R12 under an uninsulated black roof in Florida.

    All of the above is true if the radiant barrier is clean -- no dust or condensation or corrosion.
    But a radiant barrier over which one pours concrete is R0.0 to one decimal place.
    A bubble sheet (small gaps with shiny facings) might get R2 if the floor is run pretty hot, less if it is less.
    This information is also in books, but it is harder to find, so no scientist would ever consider the application: it is pretty absurd.

    If the references and information is not sufficient, we are available as building science consultants and can write a detailed report with calculations, references etc. It would take a day to generate a proper 5 page scientific description.

  6. David Neeley | | #6

    Radiant barriers properly applied
    I know of no credible person in or out of the insulation industry who suggests putting a radiant barrier under concrete in contact with the ground. However, I read a study of very similar houses in Fort Worth, Texas, in which half had radiant barrier installed beneath the roof sheathing (facing the attic airspace) and half did not. On hot afternoons, the attics with the radiant barrier were approximately 40 degrees F. cooler. (I wish I still had the reference to that study, but unfortunately it went with an untimely hard drive crash).

    As someone else said, tests have also been conducted by the Florida Solar Energy Center, probably the premiere institution studying hot weather environments and building technology. The Oak Ridge National Laboratory in Tennessee, too, has done studies on radiant barrier.

    In warm climates, a radiant barrier is quite often more cost-effective for attic installation than relatively small amounts of insulation being added. The U.S. government suggests they be considered as far North as New York City--so it stands to reason that our Canadian friends might consider them not cost-effective.

    For warm climates, too, a cool roof is strongly advisable--with the best being a bright white metal. Were I building back in Dallas, I would use both of these without question--and a relatively high level of conventional insulation in addition, most probably in the form of a foam such as isocyanurate if it is affordably obtained, or blown-in cellulose otherwise.

    There simply aren't many hard and fast rules that work in all climates--but this article said nothing about warm climate strategies.

  7. Tom Magney | | #7

    Radiant Barrier Installations
    I am quite curious as to the proper installation technique for a radiant barrier attic installation. Cost is not a factor as I have access to free commercial recycled radiant barrier material (the "useless" material you mentioned for installing beneath a radiant floor heating system), and my labor is gratis. Our ceiling is insulated with approximately 1 to 1-1/2 feet of cellulose material installed at least 25 years ago. We have about 6 weeks/year of 90+ degree weather. The thing is, I'm doing my damndest to avoid an air conditioning unit at the house - hot weather is typically a noon to 6 pm affair, and then a cooling marine layer comes in.

  8. Kevin Thompson | | #8

    How Heat Moves Through Homes
    I was fascinated by Dr John Straube's analytical discussion of conduction, convection and radiation but nothing prepared me for his absolutely stunning and brilliant conclusion - it's all BS!!! He's absolutely correct! The evidence has been right under our noses (if you live on a farm) all this time. The answer to all of our residential energy problems is to raise cattle! Have the energy companies stop their research on alternative fuels and instead invent a device to harvest the methane gas the cows and bulls produce which we can then use as fuel. Use that fuel to heat up their waste so that we can separate the water from the solids and the water extracted can be used for consumption (give it a French name like "Eau de Elsie" and people will spend a great deal of money to drink it - trust me on that one). Tell them it's filtered through cellulose - it will sound "natural" - but don't tell them you're using the cellulose insulation from old buildings. Finally, take the solid material and pack it into adobe molds to make bricks which you can then use to make insulated walls. And don't let them know that this idea is several centuries old. I would give you a link to the audio version of this concept but the sound quality is poor as my tongue is implanted firmly in my cheek. Thanks for your always enjoyable articles and discussions.

  9. Matt Weaver | | #9

    radiant barrier
    Are the radiant barriers being refered to relate to radiant heating? I am a "simple" homeowner about to build a house in inland Southern California. I feel I have an opportunity and responsiblity to be smart about energy usage. I was going ot use radiant heating ,from a vendor I found in Homebuilding magazine, to warm the concrete floors in winter. Is that recomended?
    I was also going to use the spray foam for roof and insulation and a product called "rafter-mate" to cool the attic.
    What i would really like to do is bring fresh air from under the slab into the house but cannot find a vendor to help with this other than an HRV system.
    Any suggestions, names of vendors, books etc i would appreciate
    Thank You

  10. Isaac Levy | | #10

    Energy Transfer
    Finally some sanity in an otherwise crazy quilt that is called energy efficiency. Thanks you Dr. Straube.
    I notice in the comments, especially by Mr. Neeley that practical applications of radiant barriers exist. It appears that they may be best applied in hot climates, as in Phoenix and throughout the soutwest.
    My concern would be secondaryt effects, such as overheating of the roof components, like tarpaper/barriers etc., or even the impact of a radiant barrier on the wood structure over time. It's not uncommon to have 115-120 degree days here in Phoenix. That puts the attic airspace at well over 150 degrees, even with adequate ventilation. Do you have any wild idea of what the temperature would be of the underside of the roof in the presence of a radiant barrier??
    It's been too long to haul out Bird, Stuart, and Lightfoot and try to calculate this myself.... :(

  11. geezer2 | | #11

    Would be great to see the lecturer's visuals slides. How can we do that?

  12. JohnEd | | #12

    heat gain
    Thanks for the common sense approach to your writing, now I can really appreciate my yard full of towering oak trees all the more!! Shade is the ultimate radiant barrier!

  13. tesxasjack | | #13

    Southern A/C systems & radiant barrier pay off
    Several have given adequate support for using radiant barriers in coastal attics based on reducing the attic temp. Not mentioned was the fact that most homes in the gulf states and other southern areas are built with duct work and air handlers in the attic. The result is the heated air lying in idle ducts is blown into the living space on each A/C cycle. So the payback on radiant barrier us multiplied for these home, well beyond on the benefit of reducing heat moving through the ceiling into the living space.

  14. JohnEd | | #14

    attic HVAC
    I asked an AC installer friend why is the... and he finished the question and then answered, don't make much sense at all, does it. Folks just want it cheep and fast!!

  15. Mac | | #15

    Well researched subject matter.
    I built an 1250 sq ft energy efficient home in Minnesota in 1962 that cost me less than $100.00 a year for all of my energy usages. I did all of the work my self. triple glazed windows etc long b4 all of this energy efficient talk... I put so much insulation in the walls that I had to have help to force thesheet rock onto the studs. (no air gaps.)
    Today at age 73 I am again building a 3000 sq ft home and hope to keep my energy consumption below $1000.00 a year.which I think is achievable.
    We shall see. ( I am again doing all of the work myself.
    I am combining double Radiant barrier on the attic rafters wit air gaps ,
    blown in fiber glass over polyisocyanurate solid foam with foil facings.with air gap underneath..
    I perhaps will blow some cellulose on top of the fiberglass.
    I have polyisocyanurate in the walls and 1 1/2 inches of ridgid foam on exterior walls over 3/4
    sheathing. I did not use foam except arounfd windows and in ;hard to insulate areas.due to cost.
    I anticipate using full thick fiberglas bats inside of the polyisocyinurate and perhaps radiant barrier additive to the wall paints.
    I welcome comments on my methods.
    Oh I might mention that I am heating the home with in floor radiant (convection) heat using a ground water heat pump aided by Solar panels with wood and gas furnace as a backup. I have an Air to air heat pump/ Air conditioner as an option
    .It has a 16 SEER rating.
    I can switch from one form of heating to another depending on costs and comfort level or my mood.

  16. Stan | | #16

    Insulaing a Yurt
    This is a Question for Dr. John Straube. What is the best way to insulate a Yurt? Most insulation packs are silver bubble wrap is there a better way given the limited space?
    Thanks Stan

  17. Crispn in Waterloo | | #17

    Insulating a Yurt
    There is a yurt (ger) insulation project running right now in Ulaanbaatar, Mongolia which is worth looking up. It is funded by the World Bank among others including the Asian Development Bank so you can look online for their project papers. The Ger Insulation Project involves putting additional layers of fairly air tight materials in layers over the frame. A typical poor person's ger has 1 or 2 layers only. People with a bit of money buy additional layers that look rather like the tarps that go over trucks, but are a kind of quilting material. The materials types and their insulation values are discussed in the papers.

    You can contact me at crispin at newdawn dot sz if you don't get lucky. Quite a lot of work has been done on improving the thermal performance of ger coverings. There is even a computer model of them for calculating the heat losses and heat demand based on the materials available done by a certain Randy at the University of Waterloo, Ontario. Four other students worked with me on getting the coal stove performance integrated into that model. That work continues (air quality improvement project).

    Randy says the air turnover in a ger is terribly high so sealing the perimeter in a traditional fashion is recommended. There are large heat losses through the skylight/stove pipe through the glass area in the center.

    Good luck

  18. Kenneth Davis | | #18

    So what to do now?
    Very interesting article, but given all this information about what doesn't work and how inefficient most insulation is, one is left with the burning question of what does work? In my case I have a steel building with a layer of the bubble insulation with white plastic on one side and metal foil on the other next to the steel sheathing I would like to know what I should do to insulate it.

  19. Barbara | | #19

    Uncomfortable cold floor
    I used $50 worth of that shiny bubble wrap under my 6'x12' radiant heated bathroom floor. My house is about 3 feet off the ground, open all around. It's in the deep south where heat and humidity and pests are our biggest problems. Not all radiant floors are on a slab. I think this installation is where it's supposed to work.

    The rest of my floor is not insulated at all. It's about 2" of solid maple with beams on 4' centers. I just tough it out with rugs in the winter. In the summer it's fine. I'm worried about condensation and termites more than heating costs. My highest power bill was $70 this winter. I could shove some solid styrofoam up in the three gaps between the beams. It's only 12'x12' so the cost would be modest. I just don't like to provide privacy for pests and fungus between wood and plastic. I like to allow wasps to hunt freely under there and keep spiders from coming up through the few gaps where the tongues and grooves aren't as tight as they should be.

    Am I right to put moisture and pest concerns above warm feet issues in this case? At any rate, I'm pretty sure a radiant barrier would be stupid under the wood floor. The conductive barrier of anti-fatigue mats and a cotton rug on the top side just seems more logical.

    (Background info: I used spray foam in the rest of the house so it's like an ice chest. Does an excellent job keeping bugs and sound out. It also dampens the ringing of rain on the white metal roof but doesn't do much for the heat of the sun.)

  20. Bob | | #20

    Fun with Insulation
    Apr 20, 2010
    11:56 AM EDT Fun with insulation
    by Bob

    In my opinion, you can write hundreds of thousands of words about insulation, and as we can see thansands of people do write them. The bottom line is what does it cost and what does it save.. You can build a house able to withstand the blast of an atomic bomb, but should you? , do you need to? You can build a house and heat it for $100 a year, but should you if it costs you $20,000 more to build it? Many people today EMOTIONALLY answer that question with a yes. It is not rational, but people emotionally like the idea, and I think some just like the challenge of the problem. And that is ok for those people. We have all heard people say "solar and wind are the way to go, who does not like FREE energy." Well, wind and solar are hardly free, they costs on an investment basis five to fifteen times that of conventional energy when the amortization of the capital investment is factored in. . But people are emotionally attached to the futuristic fantasy the technology.. As for houses what we need is a reasonable and rational approach. Who cares if there is convection in my walls if the cost benefit of a sytem without convection gives me a say 1% return on investment vs a say a 10% return with cnvection?. And yes it is ture that energy costs are rising and we do not know by how much or how fast. But in financial terms distant future years matter economically less than current years. And secondly the law of deminishing returns always come into play. Both of these can be factored into economic calculations as well. As you approach a point where you can heat a home for $500, it costs every more to get the next $100 savings. . Spray foam is not cost effective. It is fun and exciting, but economically it makes no sense. It is fun for people to say my heating bill is $100, and I say knock yourself out, everyone needs a hobby. Bottomline, is that articules such as the one above are intellectiually interesting, but worthless unless detailed cost benefiit analyses are included. Every time I ask for such anaylsis, I receive angry responses which is actually proof of the emotional response to the issue and proof of the indefensible economic realities of the proposed systems.

  21. John Straube | | #21

    Costs of Insulation
    The question of cost is a very valid one, and you should not get angry responses. There is no possible way to answer your question however because although we can guess at the cost of adding insulation, we dont know what the cost of energy will be in the future.
    What will the cost of energy be in 10, 25, and 50 years? In the past 30 years, the average increase in a mix of retail energy costs across America has been around 7%/annum, that is it double every 7 years. If you assume a 30 year mortgage and a 30 year timeline (you may not care beyond 5 years, but society pays the costs for much more than 30) then you get a certain answer for how much you should insulate.
    How much it costs to insulate is also highly variable. When we recommend R35, as an example, some builder say "that will cost an arm and a leg". Actually it depends on where you are, who is building it, and HOW you are insulating.
    We do this type of analysis for most of our projects.
    If you assume a lifetime of the house as the mortgage, the cost of energy to increase at long-term historical rates, one comes up with true (including thermal bridiging) R-values for walls in the low to mid 30s for houses in the 5000-6000 HDD range. Vented attics with blown cellulose would be insulated to between R60 and 75. Basements walls to R20 and slabs to around 5 - 7.5 depending on soils.
    If you build the house to be economical for 5 years, and do not care much about either comfort or moisture durability, then you would insulate to slightly less than code or to code.
    Some people have higher goals: they wish to damage the planet as little as possible and choose to trade-off granite countertops for insulation. Such people often don't do Rate of Return calculations for the skylight over the Jacuzzi, the comfort of heated bathroom floors, the luxury of $8/sf porcelain tiles in the kitchen vs the $4/sf tiles. Most people choose quality and performance in consumer goods like cars, houses, and laptops for reasons that have nothing to do with financial calculations.
    Finally, we try and talk people out of more insulation well before we reach the point at which it costs as much to generate energy with an unsubsidized PV array and use a heat pump. This is the current maximum sensible insulation cost, but this is just an opinion, and you might use other upper lines.
    Hope that helps answer the question.

  22. John Straube | | #22

    Oops. 7% per annum is a doubling every 10 years.

  23. J. Parks Hammond | | #23

    Podcast Content and Radiant Barrier Use and Misuse
    1) I read this as an article in my 04/19/10 Fine Homebuilding Magazine "Are Radiant Barriers a Waste of Time? (Learn why radiant barriers don't work everywhere, and get a handle on how heat moves through your home.)"

    I was vastly disappointed in the content and grasp of the author(s). For example, the statement,

    ". . . Then we have radiation. These vibrating molecules create waves in space-time, which we call electromagnetic radiation. At the temperatures that we’re talking about, they’re infrared radiation.

    If you were to make these molecules move fast enough, they would eventually glow red-hot; you’d be able to see them. They’d go from infrared to actual red. And if you keep heating them up, they’d get white hot. And if you kept heating them up, they would actually start giving off ultraviolet radiation — and then a nuclear explosion, you’d get gamma radiation, they’re so hot. . . ." shows a highly deficient grasp of physics, radiation, and radiant energy transfer. If you don't understand basic physics and radiation, then you should limit themselves to empirical descriptions of heat transfer in building assemblies instead of raising doubts about the veracity and concept of your article by padding the article with discussions about principles of physics and radiation that even a freshman college physics student would recognize as deficient and/or misinformation (for a quick correction see Wikipedia-Radiation-Thermal Radiation, there are many other sources)

    2) The core of the useful information in the article was in one paragraph ". . . But at high temperatures, radiation is important and it’s a major transfer mechanism; at low temperatures, it doesn’t play as big a role. So, radiant barriers are very good for high temperatures — say, the roof in a sunny climate. They’re less important for cold conditions — say, the underside of a crawlspace; they don’t play as big a role. But in every case they need an air gap. . . ."

    I would have preferred more qualitative and quantitative information on effectively utilizing radiant barriers as part of a building insulation system, but after reading the article, I recognized the "gossipy, colloquial conversation" form of a "green" article undisturbed by a lack of research or facts, in my opinion.

    However, a more factual and knowledgeable approach to building energy transfer retardant assemblies and their interaction would be much more useful for both building professionals and hobbyists.

    I don't believe this article is up to the usual content, factual information and research of a Fine Home Building (FHB) Article. Yes, I recognize that it is a Green Building Advisor (GBA) article, but both are Taunton Publications and should have equally professional content and editing, in my opinion, as a subscriber and professional. This article needs conceptual, factual and research upgrades.

  24. John Brooks | | #24

    Excellent Podcast
    Dr John,
    I thought your Podcast and your follow up remarks were excellent
    As usual
    I look forward to the next Podcast.
    Thanks for taking time to respond to some of the comments.

    GBA Rocks

  25. Anonymous | | #25

    Four-letter words
    It has been said that when a writer lacks command of his native language, said writer will fall back on four-letter words.
    Except for poor use of English, this is a fairly well written article.

  26. peter pornchanan | | #26

    radiant barier
    Living in Thailand, hot humid weather most of the year with tropical downpours depending on season.
    Am using aradiant barier onder the roof tiles, complete with airgap and vented ridge, this is working very well since this barier has 2 functions, 1 as barier 2 as rainwater protection which may come through the tiles by heavy downpour and strong winds.
    Now about 2 years later the effect of the barier is much less, because when installed the reflection rate was much higher than it is today, there is no way of cleaning up the deposits than to take of the tiles which of course will not be done. conclusion: as always there are benefits and draw backs!
    A complete coverage of the roof with for instance with osb plates or other solid material is not an option due to the humid weather which willl create lots of funges especially if the plates are covered with a radiant or bitumen foil.
    When looking here to old buildings which actually have no insulation at all but have a pleasant climate we see that the trick is natural ventilation and evaporation !
    But in our (modern?) age, building with concrete and stone there is a very effective way of keeping your house cool in the summer , mist your roof with water alternative make a kind of roof garden, water is evaporating and keeps your roof very cool, like the old buidings with tatched roofs.
    Afterall generally the roof is the largest area receiving the most radiation.
    Interesting and also very useful info was written by Larry Hartweg, zero energy house, basically discussing radiation barier, ventilation and evaporation of the building enveloppe.

  27. Susan Crook | | #27

    Little things make a big difference
    Maybe we can't install a windmill, benefit from solar panels, or rebuild an entirely new green house, but there are a few simple things we can do that make a big difference in energy usage. For example, if you take a look at the flex duct in your attic, it's probably kinked and preventing efficient airflow (like a garden hose). There's a product called SMART Flow Elbow you can snap on (like a brace) that helps - 80% more efficient airflow.

    Also something else we added was a "FilterLOCK" on the furnace filter to cover/seal the filter slot. It also protects allergens and dangerous pollutants from entering the duct system.

    Just doing these two things reduced our gas bill over $10 a month this winter. We were surprised with the results.

  28. user-757117 | | #28

    A matter of priorities.

    The bottom line is what does it cost and what does it save...

    you assume the only factors to consider are financial. This is wishful thinking.

  29. user-757117 | | #29

    Peanut gallery

    even a freshman college physics student would recognize as deficient and/or misinformation (for a quick correction see Wikipedia-Radiation-Thermal Radiation, there are many other sources)

    Does this mean you are a college freshman then? That would explain why I had to read a Wikipedia article on Thermal Radiation rather than benefit from the wisdom of your obviously superior awareness.

    I would have preferred more qualitative and quantitative information...

    Check your definitions. Dr Straube's article is essentially all qualitative in nature. Here's some Wiki-reading for you:

  30. jayne | | #30

    Little things make a difference
    Susan you are so right. We thought we needed to get better bathroom fans because they were noisy and never worked well. Instead we replaced the flexible duct in the attic with smooth aluminum duct. We used adjustable angle elbows for the angles and taped all joints with alum tape to make it air tight. The difference was amazing. It just makes sense that the smooth duct will have much less air resistance. Sealing things up helped too. (the flexible duct was loosely stuffed into the roof vent). The same fans work great now and are much quieter. Added timer switches so they run for several minutes after leaving the room and all moisture and mold problems have been eliminated.

  31. Cheri | | #31

    Advice? Rad. Barrier vs. OR w/ extra insulation, & ventilation
    Caveat--this is long.
    I have an 850sf addition going on, though stalled at present, at 6400' on an exposed mountainside on Colorado's Front Range. We get violent wind in winter and unmitigated blistering sun in summer, with temperatures often in the 90s.The existing part of the house-unfortunately-with its 20' ceilings and exterior with NO HOUSE WRAP (what were they thinking!!??) makes for what I've been told is called the Stack Effect with convection currents and a lot of heat loss.

    There's 14' tall glass across much of the south side, which makes for impressive views but is VERY inefficient in both winter and summer. I have to pull down the tinted see-through shades and still vent the house on sunny winter days in December and January when it overheats(!!) and they put in no thermal mass to hold heat. Windows are not low-e glass and money is not there to replace them all. Another loan is out of the question at this point. I'm working on making pull down shades out of Prodex radiant barrier for them all for winter nights before next winter, and would LOVE feedback about doing this with such a material. It seems like it has to somehow attach firmly to the walls at the sides and bottom or cold convection currents will just flow down the glass behind the roll down blinds and into the house itself.

    The addition: It's 2x6 walls, new low-e wood windows all from Craigslist for a fraction of buying-new cost. The roof framing is low-pitch scissors trusses and plywood sheathing with Prodex radiant barrier over it (not bubble wrap), and a standing seam metal roof raised up 3/4 inch off the sheathing with 1x2 cedar battens every 7 inches (so many battens in order to withstand occasional heavy snowloads in winter).

    We have yet to install vents to vent the hot air between the radiant barrier on the rood deck and the underside of the metal roof. The roofer turned out to be a loser and it is now sitting 90% finished, so I need to find a new roofer asap. I am intending to have continuous venting soffits but I am unclear how often, just below the ridge, to install these vertical stack-like vents. The cedar battens have a few inches of breaks betwen them end to end and they end altogether just before the ridge so there can be airflow and we don't need a vent every 7 inches--the distance between battens.

    If I recall correctly we chose these vents over a ridge vent in part to not lose the envelope of beneficial warmer attic air in winter. The radiant barrier on the roof deck and solar-run gable vents will do the job in summer for that tiny attic space. Given the low pitch difference between the roof (4/12) and the cathedral ceiling of the inside, there is not much space inside the trusses at their outer edges for insulation.

    I am imagining about an inch of rigid poly-iso, which I can find on Craigslist affordably, on the underside of the trusses before applying the drywal. I'm thinking this will both help the overall R-value of the ceiling and will prevent much heat loss into the attic and make the ceiling surface warmer. Is 1 inch appropriate or would more actually make sense, and in what ways? OR....?

    I am intending to wrap the addition walls in Prodex silver radiant barrier too, as I REALLY want this addition to need ZERO cooling in summer. The existing house overheats by 2pm. There will be a whole house fan to bring the inside of both the existing house (1340sf) and the addition down into the 60s every night and the house will be buttoned up tightly every morning. I intend to do the same gap measure over the Prodex on the exterior wall sheathing with cedar battens, and apply non-flammable Hardie board lap siding over the battens for vertical air flow. I AM concerned about pests and bats getting under this though, if there is a gap at bottom and top for vertical air flow. The windws have already been set in, so wrapping the house further in rigid insulation (type?) would be awkward wouldn't it?

    Then I can't have an interior vapor barrier either, as the Prodex is already going to function as a vapor barrier on the outside. Humidity is usually very low here though, and I fight static indoors in winter.

    I am confused about the implications of insulating outside of the stud walls to make all the framing part of the constant-temperature thermal mass of the interior(how much insulation?), VS. superinsulating inside walls or using rigid insulation on the inside. We'll have a nearly-3 inch concrete poured floor in the addition for "radiant' heat, with hopes of doing solar thermal one day if it becomes affordable.

    At present I have a high-efficiency propane boiler with hot water baseboard heat in the existing house. The guy who put it in a few years ago for the previous owner says that this boiler has the capacity to heat the floor of the addition and there will be some passive solar gain into this slab with south windows more appropriately sized in the addition, but they are only about .35 SHGC.

    The concrete floor that will happen in the addition is suspended over an unheated garage. There is a massive steel beam running the length of the garage ceiling, holding up this yet-to-be-poured floor. The 2x12 floor joists are hung from the steel beam to optimize ceiling height in the garage. I need to insulate the concrete floor of the addition from the steel beam and the cold garage, but the beam is a real obstacle. The floor deck above the beam is 3/4 plywood.

    It has always seemed clear that a radiant barrier there, without an air space, would be a joke, regardless of the assertions of folks selling the NASA products.. How to insulate there? I will super-wrap the beam below in the otherwise cold garage. Should I super-insulate the whole underside so that the framing AND the steel beam all share in being the thermal mass to heat the space above them? OR?

    I am rethinking so much that was intended, and has changed since inception last year. At this point construction is on hold due to finances. I would love some opinions, including using this Prodex radiant barrier in places beyond the roof deck in these particular circumstances. Also, what to best use to insulate the garage slab that will be poured as soon as I can afford it. Thanks for patience if you read this far! I can be reached at disposable at q (dot) com.

  32. GBA Editor
    Martin Holladay | | #32

    Too many problems to address here
    Your house has too many problems to address in this forum. You need a knowledgeable energy consultant.

    1. Your obsession with radiant barriers is leading you astray. You don't want an exterior vapor barrier on your walls.

    2. One inch of polyiso provides R-6.5. That's not much. You need to focus on higher R-values and a reduction in air leakage, and spend less time thinking about radiant barriers.

    3. It sounds like you have far too much south-facing glass. You need to reduce the glazing area or improve the shading.

    There are many more possible reactions to your long narrative. Suffice it to say that you appear to be floundering, spending money on irrelevant materials and failing to invest in measures that matter.

    Good luck.

  33. Brian O' Hanlon | | #33

    Economics and Insulation
    I was reading comments above by Mr. Staube, in relation to the economics of insulation in buildings. I dug up an old article to re-read today. It was something by Tom Dunne of the school of real estate at Dublin Institute of Technology in Ireland, where I studied myself for many a year. Dunne's article is a little bit slow to start off with, but is worth reading to the end, as he does convey a certain point. You can access the article here:

    Mr. Straube has offered some analogy between people buying homes and buying other consumer items, which I did find interesting. Of course, Mr. Dunne of DIT makes the point that housing stock in urban areas is existing, and is rarely supplemented by new housing stock, except in times of exceptional economic prosperity. In real life, the price of a house is based on a whole plethora of issues other than cost of construction.

    We witnessed a housing boom in Ireland, especially in the 2000's, and towards the end of that era the new European Directives on energy conservation were coming into effect in the market. The market for housing in Ireland at the moment has disappeared. There is very little market for new or second hand housing units. Even the rental market has quietened down a lot. I have done some shopping around myself in recent times - and something interesting that I found was - even though the law in Ireland now dictates, that a renter, or buyer should have access to the energy performance rating for the dwelling, in practice I have not been able to get the information. What I find is that, most sellers of housing in Ireland wait until you are almost committed to the purchase of the property, before they will invest the couple of hundred euros in getting the energy rating. This of course, is too late in the process of buying a house, to allow energy ratings to influence the buyer. By the time, you see the energy rating, you have already made up your mind and are not going to change it. My suspicion is, is that real estate agents are worried that a 'G' rating of a dwelling, will chase away potential purchasers of properties in a very flat property market.

    I have passed the energy rating exam myself, so I am deeply aware of the technicalities myself. I think the implementation and quality control of the European Energy Performance in Buildings directive in Ireland has been good. But more work needs to be done at that interface between the real estate agents and the buyer or renter. I have spoken to a lot of people who have rented/buyed in the past year and none have come across an energy rating certificate. Which is strictly against the law in 2010 in Ireland. I do gain enormous benefit from GBA and Building Science websites. When I have the time, I hope to study some of the excellent publications available from Building Science Corporation. Keep up the great work.

  34. Brian O' Hanlon | | #34

    Foil Vapour Control
    The use of shiny materials to stop radiant heat loss has featured a little bit, I notice in the United Kingdom building materials scene. Glidevale being one particularly well known supplier of membrane products to the roofing and timber framed housing industry. This article at their website from Nov '08 for instance.

    I need to look at Glidevale's product information in more detail, but I have a feeling the foil based products have a good deal more to do with blocking of vapour, than prevention of heat loss through radiation. For sure, we have no climate in the British Isles which remotely compares to Florida or the south western states. At the moment, in this part of the world, we are starting to wake up to the realisation of intelligent membranes, which can allow the building to dry out in both directions, all year around. I know for instance, that realisation wasn't there in timber frame building 10 years ago - when timber frame really emerged for the first time, as an alternative to the traditional masonry building techniques we had used.

  35. user-1124864 | | #35

    ASHRAE on foil
    I remember reading in an ASHRAE journal from the 70's or early 80's that the most effective wall insulation (in tests) was several layers of foil. In the real world this didn't work, since the foil was quickly torn or perforated and convection took over. I don't remember the test conditions and haven't had access to ASHRAE journals to review that article, but if it said what I remember, then radiant barriers can be quite effective. I assume, based on John Straube's analysis, that the temperature differences must have been quite high for the test.

    I have been an advocate of radiant barriers; I'll have to rethink. In the mean time, can anyone prove that I wasn't dreaming about that article?

  36. GBA Editor
    Martin Holladay | | #36

    Response to Tim Slager
    Building scientists have known for years that a 3/4-inch air space with aluminum foil on one side has an R-value that varies from about R-1.6 to R-3.4. The R-value varies, depending on whether the heat is flowing upwards, downwards, or sideways.

    If you want to make a building assembly consisting of a stack of pancakes -- a series of 3/4-inch air spaces separated by aluminum foil -- you can do that. You will find, however, that such assemblies:
    - Cost more than ordinary insulation for the R-value you get.
    - Are tricky to build
    - Have deteriorating performance over time due to dust accumulation.

    So -- you can do it if you want. It's just expensive, tricky, and stupid. But it works (at least until it gets dusty).

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |