Crawlspace / basement junction
Zone 4 build going on–the lot is a steep slope. The uphill foundation wall is essentially a giant retaining wall, and the trusses for each floor sit on a ledge in the foundation wall (essentially a reverse brick ledge, of sorts).
There is a 1″ to 2″ gap between the basement floor sheathing and the foundation wall. The insulation plan is to use closed cell foam in the trusses of the crawl space, against the basement floor sheathing, and continue that up the walls of the basement, etc. Is it enough of a seal to rely on this closed cell foam to fill that gap between floor sheathing and foundation wall? The foam will wrap around the bottom side of the floor sheathing and continue up the wall. Is there a better alternative than relying solely on foam? Does closed cell foam stop radon?
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Kevin,
I assume that when you write about "basement floor sheathing," you're referring to what most of us call subflooring -- either OSB or plywood. Is that right?
You wrote, "The insulation plan is to use closed cell foam in the trusses of the crawl space, against the basement floor sheathing, and continue that up the walls of the basement, etc."
This is confusing. Are these trusses floor trusses (in other words, the trusses above the crawl space)? If you plan to install closed-cell spray foam between these trusses, it sounds like the crawl space will be outside of the home's thermal envelope.
I'm not sure what you mean by "closed-cell foam... continue that up the walls of the basement, etc." Does that mean that the basement walls will be insulated on the interior with closed-cell spray foam? If so, does that mean that the crawl space is outside of the home's thermal envelope, but the basement is inside of the home's thermal envelope?
Yes on all questions Martin. The crawl will be encapsulated with plastic and have a dehumidifier. The trusses above the crawl will be sprayed with closed cell. So yes, the crawl will be outside the thermal envelope. The walls of the basement will also be sprayed with closed cell foam. The big gap I was talking about is between the Advantech subflooring and the foundation wall at the crawl/basement junction. Crude representation attached!
Kevin,
Two inches of closed-cell spray foam is an air barrier. When it comes to radon, the only way to know whether you have a radon problem is to test the indoor air of your house for radon. Planning ahead for a possible radon mitigation system (by installing perforated pipe and a riser) is always a good idea.
For more information, see "All About Radon."