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Community and Q&A

Air-Sealing Intersection of Roof Sheathing and Sidewall

Jghansen | Posted in General Questions on

I’m using cut and cobble foam board in an unvented, vaulted office shed gable ceiling. I now know this was a mistake based on resources I’ve discovered here, but the ship has sailed. I’ve convinced myself it’ll be ok because it’s an office shed so no major sources of humidity, and we live in a very dry climate (Colorado front range @ 6k ft, climate zone 5), and I’m going to vapor seal this to the best of my abilities to prevent any moisture from reaching the roof decking. But I recognize I’m taking a risk.

My question is this: where the roof sheathing meets the side wall, there is a small gap into the soffit. The soffit is not vented, but it’s not sealed, either, so there are air gaps with the siding where air can get in and out. I originally sprayed these closed with canned spray foam, but I’m now planning to leave these gaps open so that in the event of any moisture between the foam board and the roof decking, the condensation could drain down and have some chance of evaporating into the dry Colorado air. This opening would be above the vapor barrier from the occupied space.

Is this an ok plan or do you recommend sealing those gaps to complete seal in the insulation from any inside and outside air, since there’s not a ridge vent?

Thanks for your input – Jeffrey.

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  1. GBA Editor
    MIKE GUERTIN | | #1

    Seal the joint and don't worry about the moisture. I think your assumptions are correct. Without a moisture source other than a cup of coffee and perspiration there won't be a lot of vapor in the space. And you're in a dry climate. Any vapor that does make it into the cobbled foam rafter spaces can diffuse back into the office space.

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