Insulating Dirt Crawlspace Floor with XPS
I’ve got a crawlspace that has a dirt floor that is a little uneven. The cinder block walls and rim joists of the crawlspace has 2 inch XPS foam insulation applied. Could I, without any negative effect, apply a few inches of pea gravel on top of the dirt to make the crawlspace floor flat, and then apply a 2 inch layer of XPS foam on top of the pea gravel? I live in climate zone 3, on the edge of climate zone 4, and would like to have my crawlspace completely encapsulated and insulated. I think the thicker XPS is rigid enough to be crawled on quite a lot without destroying it, and from what I understand about XPS, I believe it would suffice as a vapor barrier, too, preventing moisture from the dirt below reaching the open air of the crawlspace.
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You get most of the benefits of insulating a crawlspace from simply insulating the walls -- insulating the floor is much less common. I would just put a crawlspace liner on the floor and not bother insulating. If you do want to insulate the floor, I don't see a problem using XPS, but it needs a flat, even surface. If you have little hills and things like that, the XPS will have a tendency to break/split. It would also be a good idea to put a layer of plywood on top of the XPS to act as a subfloor to protect the XPS for the same reason.
I have ocassionally used small scrap pieces of XPS as kneeling pads. It works for a while, but the force of your kneee will eventually dent and crack the XPS. over time, the XPS will break apart from repeated stress cycles from crawling.
You could but according to the IRC building code it needs to be protected against fire. Same for the walls. XPS has extremely potent, persistent greenhouse gas emissions, 1400 times worse than carbon dioxide, so EPS is a better choice for green builders. There is a new XPS formulation that's about 25% better than the regular stuff, but still worse than most other options. It's important to install a vapor retarder, with or without insulation.