I’ve recently sold my previous home and purchased a 1960’s Cape here in northern coastal Massachusetts. 

The original home has a poured concrete foundation, but a 25’x15′ addition that was added sometime in the 1990’s (I’m guessing…) has no basement.  The main part of the addition is supported by concrete block piers supporting 2×8 floor joists, but the joists at the extreme front and back of the space (which I suspect were at some point outside porch decks) sit on 4×4 posts supported by 10″ concrete sono-tube footings. This room is drafty and the floor is cold.

The addition has some shoddily built, 3′ tall ‘skirt’ walls between the floor platform and the ground, made of 2×4’s and T-111 sheathing. But there’s no chance of making those walls airtight to create a sealed crawlspace without doing some serious excavating and constructing some real foundation walls. So it’s essentially an open platform with some fiberglass insulation applied to the underside, between the joists.  (That fiberglass is mostly useless, as with no supporting layer beneath it, much of it has fallen away from the floor sheathing over the years, and is now being leased out as “Luxury Condos” by the local Mousing Authority.)

So, my question: What approach do I take to insulate and air-seal this floor system? I’ve done a bit of research, but all of the articles on crawlspaces seem to assume that there are concrete walls, and that the space can be tightly sealed. This space definitely can not be.

My idea is to replace the old insulation with 8″ of mineral wool, then apply a 2″ layer of foam insulation to the underside of the joists, sealing that foam layer’s joints with spray foam / tape to create an air sealing layer.

Is this approach reasonable?  Do I need to apply another fire protection layer under the foam, or can I just leave that exposed to the ground below?  And should I try to apply a layer of polyethylene sheeting along the ground as a vapor barrier, and maybe run it up the inside of those skirt walls (which will be rebuilt…) and somehow tie that sheet into the foam layer?

Any advice on better approaches, or critiques of my intended approach above?  

Thanks!