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Community and Q&A

Attic moisture problems

JeffBirn | Posted in GBA Pro Help on

I live in Oakland, CA, near San Francisco. I have a 1920s building with a low-slope roof, 1.5” in 12, and an unvented attic with R-30 cellulose blown in. The ridge is roughly 3 feet high. A new roof was installed 8 years ago.

Recently I noticed mold in the attic on the roof sheathing and sides of the joists. There are no water leaks so I am lead to believe this is a condensation issue. I want to remediate the mold and want to make sure the mold does not come back.

I believe I have 3 options:
1) Re roof with foam board insulation
2) Spray foam insulation to underside of roof or Flash / Batt
3) Seal ceiling airtight and vent attic.

Option #1 is costly as I still have to remediate the mold.

Option #2 encapsulates the mold but I am not sure about access to perform the work and removing the ceiling might be my only option. Are there any other concerns to consider with this option, i.e. creating a “sick house” since roof is now airtight? Would I still need venting?

Option #3 appears to not be as efficient as #2 but may be my only option.

My walls are not insulated, will this affect the airflow into the roof? Do you see any issue venting in a low slope roof? I would be adding soffit vents according to code, but what type and location of exhaust vents would you recommend? Should an exhaust fan be an option?

Thanks for your help

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  1. GBA Editor
    Martin Holladay | | #1

    Most of your questions are answered in this article: Insulating Low-Slope Residential Roofs.

    According to building scientist Joe Lstiburek, it's OK to encapsulate the mold.

    “We fix the problem roofs — the ones that get moldy — one of two ways,” Joe Lstiburek told me. “When the roof sheathing gets moldy, people freak out. The usual way we repair them is from the inside. We take out the gypsum ceiling and the insulation, and we spray 2 or 3 inches of closed-cell spray foam on the inside of the roof sheathing and the inside of the short walls. We encapsulate the mold. Then we repair the ceiling and we blow the space full of cellulose. If the space is too deep to fill with cellulose, we sometimes blow low-density spray foam over the high-density spray foam, because low-density foam is cheaper than high-density.”

  2. JeffBirn | | #2

    Hi Martin,

    Presently my ceiling is not very airtight, unsealed recessed can lights were installed, etc. After spray foam is installed to the proper R value underside the roof I would still most likely have moist air in the ceiling cavity from the living space. Is this OK? Would I need to provide venting?


  3. GBA Editor
    Martin Holladay | | #3

    I suggest that you follow the recommendations in my article. If you decide to install spray foam on the underside of your roof sheathing, that means that you are choosing to build an unvented conditioned attic. It will only work if you seal all vents, and if you spray foam against your short attic walls.

    You need to maintain air barrier continuity and thermal barrier continuity, from your walls to your roof assembly.

    Once you have finished the work, the air in your conditioned attic will be like all the rest of the air in your house -- warm and humid during the winter, and (ideally) cool and dry during the summer. The warm, humid air won't cause any problems in your attic if there are no cold surfaces where moisture can condense, and no holes in your thermal envelope where air can escape.

  4. JeffBirn | | #4

    Thanks Martin. Your responses have been extremely helpful. I have 2 final questions....I hope

    1. Given the attached detail, what is the reason for keeping a void at the soffit by use of a backdam? Can this be filled with spray foam?

    2. My walls are not insulated, other than energy efficiency, are there any other issues due to this that would cause adverse affects in the attic.

    Thanks again for all of your help

  5. GBA Editor
    Martin Holladay | | #5

    There are at least two reasons not to fill your soffit area with spray foam: (1) Spray foam is expensive, and there is no reason to waste it by filling the cornice, and (2) the expanding spray foam might dislodge some soffit components (if poorly fastened) or might come oozing out of the soffit vents (if present).

    In terms of uninsulated walls: are you talking about the walls of your attic, or the walls of the house under the attic?

    If all of your home's walls are uninsulated, then insulating your attic won't make anything worse. But it would still be a good idea to insulate your walls.

    If you are talking about the walls of your attic, then I strongly advise you to insulate these walls at the same time that you insulate the underside of your roof sheathing. If you leave a few uninsulated cold surfaces in your attic, these locations will be cold in winter, encouraging condensation and mold.

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