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Attic insulation question

user-431302 | Posted in General Questions on

I’m doing a gut renovation of a 1000 sq ft finished attic in NJ.

I plan to spray 5″ of closed cell foam between 6″ rafters and 3″ of foam on the attic exterior walls and put 3′ x 3′ doors on the side attics for access

Question: In addition to the insulation above, my Architect suggests insulating the knee walls of the side attics because he says the side attics are not heated space, but I don’t think this is a good idea because: 1) the insulation would be redundant 2) it might confuse the installers to where the thermal boundary is: at the kneewalls or at the rafters 3) there might be condensation on the cold surfaces of the unheated side attic .

What makes sense here? should I insulate the knee walls? Also, would you sheet rock the ceiling and behind the knee walls, inside the side attics (used for storage)?

Any input is appreciated. Thanks.

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  1. gigaboy | | #1

    According to both LEED and NAHB Green, attic facing kneewalls must be insulated. You can do it a couple of ways. 1) Use open cell foam / cellulose / fiberglass but then you must install drywall or foamboard on the attic side. Tape/mud or foam all seams so there is no air leakage. 2) The easiest way is to use closed-cell foam, which acts as both a thermal and air barrier.

  2. GBA Editor
    Martin Holladay | | #2

    You are in Climate Zone 4. According to the 2012 IECC, the minimum ceiling R-value in your climate zone is R-49. That means that you need a minimum of 7.5 inches of closed-cell spray foam between your rafters -- not the 5 inches you are planning.

    I don't think that you need to insulate your kneewalls; instead, you need to find a way to get an adequate amount of insulation at your roof plane.

    Mark: If LEED and NAHB Green require that kneewalls must be insulated, I am astonished. For at least 20 years, energy experts have been teaching builders that it makes more sense to insulate the sloped roof over (and behind) a kneewall than it does to insulate the kneewall.

  3. Expert Member
    Dana Dorsett | | #3

    Mind you, the HFC245fa blowing agents used in 5" of ccSPF has a lifecycle greenhouse gas (GHG) potential several times that of the energy use it's offsetting. SFAIK there's only one water-blown closed cell foam on the US market that doesn't have this GHG problem, but it's only ~R5/inch compared to ~R6/inch. (Brand name: Icynene MD-R-200, not to be confused with the MD-C-200 which is another HFC245fa-blown 2lb foam.) If you care about that issue, either limit the closed cell foam to 2" (which is more than enough for the air-seal and moisture control), and fatten out the R values with something else. With the water blown 2lb foam you'd want at least 3" (R15) for moisture control, since it's permeance is about a third that of most HFC blown 2lb foam. In most other respects (including mechanical rigidity and adhesion) it's comparable to the HFC blown goods. (And no, I don't work for the manufacturer.)

    What Martin said about insulating kneewalls. Air-sealing kneewalls is a fools-errand (and I've played that fool on multiple occasions). Insulating & air sealing at the roof deck is much more likely to succeed.

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