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My 1930 house has a large room ~(25x30) that was added on in the early 1980's without a basement nor slab. It is on top of thick concrete piers, pinned to ledge rock. On very cold days < 15f or very windy cold days, the floors are quite cold. Under the floors are some fiberglass insulation then plywood that seems to have some air gaps, allowing animals and wind to infiltrate the insulation.

I recently had a home energy audit, and one of the recommendations was to rip out the existing fiberglass insulation and have 3 inches of spray foam underneath instead. I agree that this would work, but the cost is $3500.

Would applying house wrap under the existing plywood/fiberglass (gaps and all) be enough to stop the air infiltration and possibly allow me to get away with not having to spray foam? Would there be any other reasons NOT to do this?

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  • The gaps are the seams between the plywood sheets. I honestly don't know what shape the fiberglass is in – Kyle J V Feb 9 at 16:42
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Yes it would. I've built a number of "four season porches" in Minnesota (for high-end clients who would've certainly complained if their floor was cold). They're essentially rooms on posts. Our standard approach is as you describe--fiberglass batts with house wrap below, then treated plywood. As long as the house wrap is securely pinned to the floor framing you'll effectively block wind.

Section view:

================================== <-- floor sheathing
~~| |~~~~~~~~~~~~~~~~~~| |~~~~~~~~
~~| |~~~~~~~~~~~~~~~~~~| |~~~~~~~~
~~| |~~~~~~~~~~~~~~~~~~| |~~~~~~~~ <-- fiberglass insulation
~~| |~~~~~~~~~~~~~~~~~~| |~~~~~~~~
~~| |~~~~~~~~~~~~~~~~~~| |~~~~~~~~
~~| |~~~~~~~~~~~~~~~~~~| |~~~~~~~~
---------------------------------- <-- house wrap
================================== <-- treated plywood

Since you're reversing the order of the outer two layers, I'd maybe use 1x2 battens and screws around the edges to really pin down the house wrap and create a seal.

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  • You mentioned that the treated plywood is under the wrap. Would it be ok to leave the house wrap exposed, under the existing plywood? There wouldn't be any direct sunlight as it's under the floor. – Kyle J V Feb 9 at 16:16
  • It's probably ok, but you may not block pests as well without plywood. – isherwood Feb 9 at 16:17

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