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Foam insulation

Building a home soon. My contractor says he uses closed cell foam on the outside of the blocks below and above grade on the walk out basement for insulation and to seal against water penetration into the basement. I,m concerned that the foam will not hold up below grade allowing water into the basement. If I'm wrong, won't I still need french drains around the basement to keep water pressure off the block walls. or should I use Block-Out on the outside and put the foam on the inside between the studs.

Asked by Rus Pearson
Posted Jun 19, 2014 4:15 PM ET

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2 Answers

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1.

Rus,
Footing drains are required by code, and are always a good idea. They should lead to daylight if the house is on a hill.

Closed-cell spray foam is very durable below grade, and is a good product for insulating the exterior of a foundation wall. Of course, the above-grade portion of the wall should also be insulated, and the insulation should be protected from physical damage and sunlight.

Answered by Martin Holladay, GBA Advisor
Posted Jun 19, 2014 7:37 PM ET

2.

What he said.

Furthermore, polyurethane foam is closely related to glues such as Gorilla Glue (tm), and bonds pretty well to just about anything, as long as there isn't a layer of loose grit or something to interfere with it.

And, closed cell foam installed between studs is a waste of good foam from an insulation point of view- the studs are about 6x more thermally conductive than the foam severely undercutting the thermal performance potential. Putting studs up against the foundation would allow moisture wicking from the masonry into the susceptible wood, and the stud edge would run colder in winter, accumulating moisture. When using closed cell foam in conjunction with a studwall on the interior of a basement it's best to put 1-2" of foam in a continuous sheet that extends all the way up & over the foundation sill & band joist, with the studwall on the interior side of the foam. If you want to insulate the stud bays, rock wool or unfaced fiberglass batts is preferred, with NO vapor barriers- the assembly needs to dry toward the interior, since the below grade portion of the foundation has 100% saturated soil air on the other side of it, and the closed cell foam is a vapor retarder even on the above grade portion.

Answered by Dana Dorsett
Posted Jun 20, 2014 2:24 PM ET

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