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I'm finishing my basement in southeastern Wisconsin. This is a poured full basement below grade. We are ready to insulate the walls and are going to use closed cell spray foam.

I've had two different contractors come out and recommend different thicknesses for the foam. One said I only need 1" against the walls, and 3" in the sills. The other said 2" against the walls and 3" in the sills.

What kind of performance gain would I be looking at between 1" and 2" sprayfoam?

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up vote 7 down vote accepted

Closed cell foam typically has an R6 value (that's R6 per inch)

http://www.fomo.com/resources/technical-bulletins/opencellvsclosed.aspx

So 1" of foam would be R6. 2" of foam would be R12

I've lived near you - Wisconsin gets cold and stays cold - and in a basement the longer the ground stays cold the more thermal break you want. so frankly in your climate, I'd be looking for foam to completely fill between the joists with foam - 3.5" if you're using typical 2x4's - giving you closer to an R20.

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We ended up going to 2". The rationale is that the permeability of the foam is so little that there is no air/moisture getting through so you don't need as high of an R-Value. –  Paul Lemke Nov 4 '11 at 16:38
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Can't say I agree with the reasoning, because the two issues - vapor barrier vs. thermal break - are completely different. The vapor barrier can be as thin as 6Mil plastic - the R value has to do with temperature conductivity - and more is always better (but to diminishing degrees). –  The Evil Greebo Nov 4 '11 at 18:11
    
After a whole year with the foam I have to say it's working great. The ambient temperature is a little lower than we'd like so 2" might have been a little thin, so in the future, look into going thicker. –  Paul Lemke Aug 15 '13 at 15:03
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