Okay, this current cold snap has my feet freezing, and my (thankfully high-efficency) furnace working overtime. The insulation in this 1960 ranch+1978 addition is sub-par. Eventually, I'll find a contractor to add another six inches in attic(since I don't fit up there), but I also have a lot of exposed foundation in this raised ranch. (I'm too cheap to think about walls and new windows, but might also make plexi interior storms.)
Since I only use the basement for storage at this point, would I gain anything from hanging sheets of foil-face 3/4" or 1" foam from the sill plates in the former garage bay (now buried) in the original part of the house basement, the long basement wall on the prevailing wind side, and in the crawl under the addition? The former garage has a lot of foundation showing above-grade, and it is cold as hell in there, on the far side of an insulated wall. The bedrooms right above have cold floors, in spite of the fiberglass in the floor. The crawl is insulated (badly) in floor, but not on walls, and is unheated except by leakage from the fresh fiberglax flexi ducts that run through there. (I removed wall furnace from addition, and added a duct trunk and return with the new furnace.) I don't wanna do anything permanent at this point, to leave future basement options open, plus there is some history of water seepage, so I don't want to block quick access to the concrete. I was thinking, just hang the styro sheets like a curtain from sill plate, using 3/4x2 lath and drywall or deck screws pasted into sill plate. Sheets would be tightly butted together, and maybe the joints could get taped. In southern MI, how far down the wall would I need to go- 36" below grade, or all the way to floor? (All the way to floor would be least labor, and no more material, unless I could get 2 panels from each sheet.) Note that I don't want to actively heat the old garage, since it is uninsulated to the outside, and gas is expensive. Basement heat comes mainly from the furnace being down there, but stays at least 50 degrees in the rest of the basement.
Is foil-face foam legal for an app like this, w/o a drywall fire break over it? Would it make any difference in heat loss? I know in a perfect world, foundation should be insulated on outside, but that would be way too expensive for this cookie cutter, and wouldn't pay on resale. The inter-joist spaces on the rim joist do appear to be insulated, but I haven't pushed drop ceiling out of the way to check all of them. I already have 3/4 foil-face foam plugs in all the basement windows, which helped some, as did adding a layer of the same foam above the attic hatch.