Rigid foam on crawlspace floor without concrete slab
brentwilson
| Posted in General Questions on
Is it okay to put 2″ of XPS foam on a crawl space floor that does not have a concrete slab? My thinking would be to have gravel, then the rigid foam, then the vapor barrier on the top of it. It is possible that at some point in the future, a concrete slab could be pumped in. But I would want a solution that was okay even without that.
Climate zone 6B.
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Replies
Brent,
Concrete scratch-coats (rat slabs) are primarily to provide protection from mechanical damage to the vapour-barrier and foam underneath. Everything performs just fine without them, but when you access the crawlspace there is a good chance you will pull the poly away from the walls and any piers. Some Canadian codes require fill as ballast instead.
Levelling the gravel below is time well spent. Securing the poly with the odd CMU helps alleviate any movement.
Brent, the IRC building code that most of the US uses as a reference requires a thermal barrier over any foam, see R316: https://codes.iccsafe.org/content/IRC2015/chapter-3-building-planning.
As you might know, XPS comes with very high embodied carbon--its blowing agents are potent greenhouse gasses. EPS or mineral wool are better choices from an environmental perspective. Or recycled XPS if you can find it. Mineral wool has the advantage of not needed fire protection.