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I have dramatically improved the ventilation and insulation situation in my house's knee wall attic space, but I have one concern.

I nailed up foam board over the studs and fiberglass batts. Do the seams really need to be sealed with expanding foam or tape? How much more performance would I gain? If so, which is the better option?

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Sealing the seams will not increase the R-value of the insulation. However, sealing the seams is important as an air infiltration/moisture seal, preventing air from infiltrating and, as a result, preventing moisture (condensation) from occurring between the foam board and batt. Typically expanding foam creates a better, longer-lasting seal than tape.

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    Agreed, but my concern would be air infiltration, not moisture. However, sealing it solves both. – Mazura Nov 23 '14 at 0:32
  • Mazura, I agree with you 100%. I really meant moisture as a result of air infiltration (aka condensation). – BigRon Nov 23 '14 at 15:13
  • I'm not too concerned about moisture. I have a vapor barrier on the backside of the drywall. I am more concerned with making sure I get the best insulating performance out of the batts and the foam board. – Evil Elf Nov 26 '14 at 21:06

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