Hello BTers –

I have a kind of funny, accidental crawlspace that was created by the previous owner many years ago when the mudroom was extended between the house and garage. Its sketched out in the attached diagram.

There is a crawlspace under half the mudroom. The crawlspace has a concrete slab but there’s a gap of a couple inches on each side between the slab and house or garage. The gap is just dirt as far as I can tell. I’ve never seen water in the crawlspace. I know there have been critters in there.

The crawlspace opens only into an alcove. The alcove has doors to the outside and we keep our trash in there. So its pretty unvented but also unconditioned. I’m going to redo the mudroom and thought I should insulate the floor better (just old fiberglass batts between the joists now).

I’m thinking I’ll either insulate with XPS nailed to the underside of the joists and leave the space unvented and unconditioned, or close off the crawlspace to the trash alcove and insulate with XPS on the slab and up the walls which will somewhat condition the crawlspace despite there being no air exchange with the mudroom or basement. I guess it would be insulated but not really conditioned.

What’s the proper way to do this and is there another option? We don’t store anything in the crawlspace, but there is one hot water pipe that runs through it. How do I deal with the dirt gaps/control joints in the slab? Is there an XPS that bugs and mice don’t like?

Thanks.
Brian