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Old 04-30-2012, 08:32 AM   #1
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Air Sealing Subfloor Joist Space - Foam Board?


I'm in the process of air sealing my house before adding to the existing insulation. Among other sources of information, I've found the book Insulate and Weatherize by Bruce Harley to be an excellent reference. In a few spots, he recommends using rigid foam board to effectively air seal larger openings including subfloor joist spaces that are completely open to unconditioned air in the attic over an attached garage. The attached photo shows an example of what I'm trying to block in my attic.

I'm concerned, though, that almost everything I've read about rigid foam board insulation says clearly that it must be covered by a fire barrier such as 1/2" drywall. If I were to install this in the joist space as an air barrier, would I also have to install drywall on both sides of the foam board in order to meet the fire barrier requirement?

Any other recommendations on how to properly seal off the subfloor joist space from the unconditioned attic air to help with my air sealing efforts?

FWIW, I'm in the Dallas area...

Thanks!

-Mason
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Old 05-01-2012, 08:36 AM   #2
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Air Sealing Subfloor Joist Space - Foam Board?


Rigid foam blockers, spray foam and seal edges, cover with some fiberglass.

The plywood servers as your thermal barrier (although you really only require an ignition barrier in this case) and the fiberglass covering the foam will serve as your ignition barrier.

Get the foam tucked right to the edge of the warm wall and seal it there.

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Old 05-06-2012, 03:54 PM   #3
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Air Sealing Subfloor Joist Space - Foam Board?


Thanks, that makes sense!

-Mason
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