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Foam Insulation on attic floor?

Jason Crawford| Posted inEnergy Efficiency and Durabilityon

I’m about to insulate my attic. I’ve read a lot about insulating the attic rafters with foam to create an unvented attic. I’ve read about laying down a think layer of loose fill on the attic floor in a vented attic. I’m having difficulty deciding between them. And now am wondering about a third option.

I’ve never read about using foam on the attic floor. It seems like a good compromise solution. Foam is expensive and the surface area to apply the foam to would be less if one is covering the single flat surface instead of gables and inclined surfaces. It should also provide a better air seal than loose fill over (a probably not perfectly sealed attic floor). The dynamics would seem to be similar to the exterior rigid foam walls that we read about and promote. There might be less rodent activity with foam than with 16″ for loose fill. And with reduced thickness (6″ vs 16″) there might be the possibility of keeping the unconditioned attic somewhat more accessible.

我遗漏了什么东西?

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Replies

  1. Charlie Sullivan||#1

    It would work, but it's not actually a good idea.
    1) It's not really much more work to seal your attic floor than to seal all the foam joints.
    2) 6" of foam gives between R-24 and R-36. 16" of cellulose gives about R-60. You'd need a lot more foam to get to R-60 and that would be expensive.

  2. Jason Crawford||#2

    Thanks Charlie. This begs another question. One of the two options the insulation contractor here was offering to put down just 3" of closed cell foam on the rafters + gables. (He wanted to use open cell at first, but I requested only closed cell and he proposed 3" of it.) That sounds like it would have been about R18. His thinking was that one will get ?95?% of the benefit with this level and what is added beyond that doesn't justify the cost. He also said that the 3" would be a better barrier to heat and moisture than the 16" of loose fill and so it would be roughly equivalent. Is he mistaken or unreasonable?

  3. GBA Editor
    Martin Holladay||#3

    Jason,
    Any contractor who tells you that R-18 is just as good as R-60 doesn't understand the laws of physics. For more information on this issue, seeIt’s OK to Skimp On Insulation, Icynene Says.

    It's possible to use a "flash-and-batt" approach to insulating your roof assembly, but you can't install just the flash portion of the insulation without also installing the batt portion of the insulation.

    You didn't tell us your climate zone. If you live in the northern half of the U.S., building codes require at least R-49 of roof insulation. If you want to use the flash-and-batt approach, you can use closed-cell spray foam for a portion of the needed R-49. The foam portion would have to be R-30 in climate zone 7, or R-25 in climate zone 6, or R-20 in climate zone 5. Then you can make up the rest of the needed R-49 with blown-in cellulose, blown-in fiberglass, or fiberglass batts.

    For more information, seeHow to Build an Insulated Cathedral Ceiling.

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