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Radiant barrier on the attic floor, on top of the insulation?

mrworth | Posted in General Questions on

I received the question below from a friend of mine living in the Central valley of California. Living and working in the Pacific Northwest I don’t have any real world experience with radiant barriers. Any thoughts…?

What is your opinion regarding radiant barriers in residential attics? We have a quote from a contractor for 80 cents/sq ft. – which is approximately $2,500 for our house. His system involves laying rolls of the radiant barrier directly on top of the attic insulation. We have a whole house fan, so would a radiant barrier provide significant additional savings? Your thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    Installing a horizontal radiant barrier on your attic floor is a bad idea, for three reasons:

    1. The radiant barrier can trap moisture, even if the material is perforated.

    2. The performance of the radiant barrier will begin to degrade as soon as the radiant barrier gets dusty -- an inevitable development if the radiant barrier is installed on your attic floor.

    3. For the same performance improvement, radiant barriers always cost more than insulation. In many areas of the country, you can get R-38 of cellulose installed on your attic floor for $1 a square foot, and R-38 of blown-in fiberglass can be as inexpensive as $0.60 a square foot. R-38 is a lot of insulation.

    For the most bang for your buck, the first step is to verify that there are no air leaks through the attic floor. Once you've done that, the next best thing to do is to increase the depth of the insulation.

    For more information on radiant barriers, see Radiant Barriers: A Solution in Search of a Problem.

  2. Expert Member
    Dana Dorsett | | #2

    #1 isn't really much of an issue in the CA central valley type climate.

    #2 is a bit exaggerated- RB continues to have nearly the same performance when dusty as when they're clean. (It's when they corrode that they really lose ground.) If installed as described, more important than slowly accumulating dirt is the performance hit taken by not having large air gaps on both sides of the RB.

    If you're going to use them at all, hanging them on the rafters is far superior to putting them on the attic floor. Installled on the rafters no dust can accumulate on the bottom side, and it accumulates more slowly on the top side. And with inches or feet of free air between the low-E surfaces and the roof deck or attic floor, you get the full benefit of the air films, and low-emissivity of the shiny stuff. Even when the RB is 140F, shiny aluminum can't radiate much heat to the attic floor, and most of the heat transfer is from convection to the attic air, which is very slow across the large gap. But if it's in contact or very close proximity to the attic floor it will both conduct and convect quite efficiently.

    #3 is dead-right. For 80 cents/foot you could blow ~8" of cellulose in there (R27-ish) and yield far better year-round performance.

    Under a 140F roof deck cellulose is superior to fiberglass, since fiberglass is translucent to a relevant portion of the infra-red, whereas cellulose is not. The translucency factor means that the radiating heat penetrates a few inches into the top layers of the fiberglass where it is absorbed, raising the temperature of the fiberglass 1-2" below the top surface well above the attic air temp, which means you're insulating against an even higher temperature with 1-2" less insulation. With cellulose the absorbed radiant heat from the roof deck is re-radiated back, and the surface of the cellulose is convection cooled by the attic air. The hottest layer of cellulose is exposed to the attic air, rather than insulated from it by an inch or two of fiber, as in the fiberglass case.

    While radiant barrier will take a bigger chunk off the peak cooling load than a few additional inches of cellulose, the difference on that peak isn't very much. But the few inches of cellulose will take a bigger bite out of the AVERAGE cooling load of the roof, and take some off the heating load as well.

    Under CA Title 24 RB is a code-approved alternative to going with the prescribed "cool roof" shingle for your CA climate zone, as is adding more insulation. Only if you don[t have sufficient space to get there with the insulation would RB be needed to meet code.with a non-compliant roof finish.

    Once you're over R30 in the attic, even with dark shingles the bigger cooling loads are likely to be from the windows, particularly the west facing windows. Spending the $2500 on exterior shades for both the east and west facing windows would likely be a better cooling season comfort and energy-use investment than RB. But if you don't have at least R38 up there (with at least a 3" cellulose top-coat, if it's R38 fiberglass), that would be better overall.

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