Why Leave a Gap in Crawlspace Insulation?

July 01, 2007
July/August 2007
A version of this article appears in the July/August 2007 issue of Home Energy Magazine.
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I read your article “Closed Crawlspaces Do Double Duty,” from the 2005 Special Issue (p. 32), with great interest.  The crawlspace in our new home is built over a 4-inch cement floor over plastic and 4 inches of bluestone.

The slab is about 3 inches below grade at some points and level with grade at others.  This winter it never got below 49ºF in the crawlspace, with temps as low as the ’teens outside. I’d like to insulate the crawlspace walls with 2-inch foil-backed rigid foam insulation. Does the foil face the block wall or the inside of the crawlspace? Do I need to keep a gap at the bottom of the foam, or can it be extended to the floor?

Dave Jordan
Arlington, Virginia

I’d recommend using a product called Dow Thermax, which usually has a foil facing on both sides. (Some versions have a white facing on one side, in which case I’d put the white to the block and have the foil face the crawlspace, for no other reason than to take advantage of the potential radiant barrier effect.) Thermax has passed more stringent fire safety tests than any other foam product on the market that I am aware of, and for practical purposes it is waterproof. It shouldn’t be too hard to find, but if you can’t find a source you could contact Tom Evans at Dow: (804)467-4130.

The gap we recommend at the bottom of any wall insulation in a closed crawlspace serves two purposes: it reduces the chance of moisture wicking up into the insulation if there is standing water present, and it allows room for termite inspection (and treatment, if needed) at the base of the foundation wall. I would suggest that you talk with your pest management company and a local home inspector or code official to ensure that what you want to do (putting insulation all the way down to the slab floor) will not jeopardize your ability to get termite protection/warranty, and that it will not be seen as a code violation if you sell the home in the future.

Cyrus Dastur is a building science associate at Advanced Energy, in Raleigh, North Carolina.

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