I have an addition foundation poured in my backyard that hasn’t been backfilled yet. I’ve sealed it with unfibered roofing/foundation sealer and now want to do something about the insulation. I was planning on using the rigid foam board on the outside, but am not sure how to fasten it to the wall or how much of the wall needs to be covered. My foundation rises about 18″ above grade so I was thinking that the insulation would stop somewhere below grade for visual reasons, but I didn’t know if that made sense.
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You can pop some 2" rigid foam board on it, either on the inside, or the outside. If you are in a climate where winter temps drop well below freezing, you might do two 2-foot courses below the rim, otherwise do one course. Slam it to the foundation walls with Hilti nails and big sheetmetal discs for washers. You should be able to rent the gun and buy the fasteners at the rental yard. Here, my local lumberyard has the sheetmetal discs, about 2" diameter. We us 'em for this purpose, and for nailing on felt paper.