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condensation between block wall/insulation board


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In the process of sealing my crawl space and ran the vapor barrier up to about 3" from top of foundation wall. Now I am installing Dow Thermax 1.5" R-10 on the walls.

Since the foundation walls are not perfectly smooth/even, the insulation board will not make full contact with the walls. If not making contact, it is not doing much good.

Most installations I see have spray foam sealing gap between the board and foundation wall around the perimeter of the boards. So the space between the board and wall is still present but air flow has been cut off.

My concern with this is a possible condensation issue due to the cold foundation wall (in winter anyway) and warmer back of insulation board.

What do you think the chances of condensation formation are here?

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What the heck is your foundation made of that you can't get decent contact between it and the foam?

Adhere the foam with silicone caulk. Fill big gaps and errors with spray foam. Tightly fit foam against the band joist, and use fiberglass insulation thick enough to fill the cavity flush with the rigid on the stemwalls. Get out of the crawl and enjoy a beer.

Unless the soil in the crawl is regularly wet, the vast majority of the time the dew point will occur somewhere in the rigid insulation and there will be no condensation.

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