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Community and Q&A

New roof over spray foamed attic

Ilmari Polojarvi | Posted in General Questions on

Hello,

I’m putting a new roof on my 1850s Maine farmhouse. I had the ceilings in the attics sprayfoamed to the floor of the attic. The roof continues over kneewalls in the second story, and does not have spray foam there. I have a mildew smell coming from that area on one side of this house, and trapped condensation may be the cause (we’ll know better when we get to that area in the roof replacement).

I want to make sure the new roof does not have any condensation issues. The roofers mentioned that in a previous situation like mine that they had reroofed, condensation had formed between the roof shingles/sheathing and the foam, and the water had no place to escape because of the spray foam, causing mold and rot. I’d like to make sure the new roof has all the precautions possible.

Any advice for my situation?

Thanks!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    How much foam, and of what type? (Open cell? Closed cell?)

  2. Ilmari Polojarvi | | #2

    Foam was appropriate for Maine winters, which I believe is closed-cell (worked under the advice of Efficiency Maine experts) and done to the depth of rafters 4-5".

  3. Expert Member
    Dana Dorsett | | #3

    At 4-5" of closed cell you're looking at R25-R30, and a vapor permeance of 0.20-0.25 perms, give or take. The low permeance limits the rate of drying toward the interior by quite a bit, so any leakage of the roofing stays in the roof deck for months or even years, but it also blocks wintertime vapor diffusion of indoor moisture into the cold roof deck.

    If there's a budget for it, installing R30 of rigid foam over the exterior, and installing the new roofing on a nailer deck would keep the average winter temperature at the structural roof deck warm enough to not collect wintertime moisture, allowing it to dry slowly toward the interior. A self-healing fully adhered vapor impermeable membrane such as Grace Ice & Water Shield between the structural roof deck and exterior insulation would be a good idea. Using 5" - 5.5" of rigid polyisocycnurate and a 5/8" nailer deck would add about 6" to the overall assembly which would require working out the fascia board trim details, etc. If using reclaimed roofing polyiso (about 1/4 the cost of virgin-stock goods) derate it a bit and go with at least 6" of foam.

    If a foam-over is going to be too expensive, the vapor-impermeable membrane will still be worthwhile leak prevention, and the assembly can still dry toward the interior, but the drying season is shortened considerably. At 0.25 perms on the interior foam the roof deck can't accumulate enough moisture over a winter to matter, and the moisture content of the roof deck will eventually stabilize at a comfortably low level. It's worth measuring the moisture content of the roof deck with a 1-pronged wood moisture meter in several places. Ideally it will all be well under 20%. If it's in the 20% range or higher it may be worth giving it several days to dry out a bit (you have to pick the weather) before laying down the membrane. (Ideally moisture content would have been assessed prior to installing the spray foam too, which may have been part of the problem.) Below 20% moisture content the risk of dry falls away quickly, and even it it takes years to stabilize, the roof deck won't be further compromised.

    See also:

    https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf

  4. GBA Editor
    Peter Yost | | #4

    Standard asphalt roofing shingles have an in-situ (as-installed) vapor permeance of about 0.2 - 0.3. That means little drying potential to the exterior; little to none on roof facing north but pumped up drying potential to the south courtesy of the sun and season.

    I would not take the roofers assessment of where a moisture problem came from (condensation) on another project as gospel. Could have been a bulk water leak; the pattern of leakage is often a really big clue to the source of the moisture problem.

    Sandwiching your structural roof sheathing between asphalt shingles and closed-cell spray foam is not ideal, but if you are careful with your bulk water management on the roof (made easier by fewer and simpler penetrations; a simple rather than a complex roof configuration), thousands of said roofs have done just fine.

    Peter

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