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We bought an older home that we use more as an office. The basement is totally uninsulated including the rim joists. Last winter, it got really cold down there to the point that the floors upstairs also were cold. So I know the rim joists need to be insulated cause they're exposed to the outside with only the siding and what looks like no insulation behind the siding (in the basement only).

If I only installed rigid foam on the rim board, would that make a substantial dent in reducing the cold or would that only be a drop in the bucket? I'm not interested in insulating the basement walls cause it's more an office than our home and it doesn't get down to zero every year but if it would make the basement just cool/cold instead of frigid, I'd be happy with that.

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Nothing below grade needs to be insulated in most regions. If you are in Canada or very north part of the world insulating the whole wall is advised but not for those below the border.

I have done a lot of basements. I insulate walls a foot below grade - to create a pure stop of airflow down.

With no wall we stuff as much rockwool in the rim joists as it will hold and sometimes throw a strap or two in to help. Two sheets of rockwool is the norm (depth-wise). You will notice a dramatic difference within a day. Spray foam will provide the same results (maybe slightly better) but if I need to move an electrical/plumbing/gas line it is very easy to move the rockwool comparatively.

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