Foam boards to air seal attic floor
JohanH
| Posted in General Questions on
Anything wrong with using rigid foam panels taped and spray foamed to air seal a true attic floor? Additional insulation on top. It will be unused space.
Zone 5. Our 2×8 attic joists from bottom to top are ceiling plaster, wood lathe, 4-6″ loose mineral wool, air gap from the joist bay not being fully filled, 3/4″ T&G pine floor. Instead of cutting the floor to air seal at the ceiling, I thought this might be an easier solution. Thoughts?
Applying this article but using less expensive method… https://basc.pnnl.gov/resource-guides/spray-foam-insulation-applied-existing-attic-floor
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Replies
I'd just do the insulation. You want the assembly to be able to dry toward the exterior. You don't want the existing rockwool to be trapped between two vapor barriers.
DC,
I don't think there is an existing vapour-barrier.
If painting the ceiling plaster is an option there are some paints marketed as a vapor barrier. After that as DCcontrarian mentioned just do the insulation.
dolores_county_co,
You are primarily trying to seal against air-leakage, not vapour movement. If you can effectively air-seal the ceiling, then yes you can just use permeable insulation. But Johan's plan comes from not having access to do that.
There is no vapor barrier currently. And correct, while I can cut into the floor to get to penetrations, partition walls, etc., I wondered if rigid foam on top of the flooring might be easier. This would put the air and vapor barrier on the top of the attic floor.
Would this create moisture problems? Other issues to consider with this assembly?
JohanH,
You can use any relatively permeable material as the air-barrier over your floor (plywood, OSB, a membrane, a WRB, etc). If you use something impermeable ( a true vapour-barrier) like foam, you need to make sure you have either thick enough foam relative to the insulation below, or enough permeable insulation over the foam, to make sure the lower surface of the foam stays warm enough to not become a condensing surface.
To avoid worrying about that, I would use a robust house-wrap like Tyvek Commercial instead, and just add batts or cellulose over it.
As the others have said, you should only use this vapour-closed assembly if it's the right move for your climate area, and you've thought about it from the moisture-and-mold perspective.
I'm working on a similar project and thinking about using a fluid-applied, vapour-open product to air-seal the attic decking, especially around lights and plumbing/electrical penetrations. It's thinner than a 1" layer of spray foam, but that that might allow you better ability to place batts in the joist cavities.
scottperexfox,
"that that might allow you better ability to place batts in the joist cavities."
He doesn't have access to the join cavities - hence his question.