I live in Maine and am planning to insulate my basement. It has fairly smooth poured concrete walls. I've read online that spray foam is one option, and 2" of XPS directly against the wall is another option.
I was at Home Depot today, and a salesperson there told me that he would leave a 1" gap between the wall and the insulation. I don't believe this is the correct thing to do (instead, the foam should be flush against the wall), but my question is: what would happen if one did that?
My guess is that in the summer, moisture would travel from the ground, through the wall, into the air in the gap. Depending on the temperature of the foam, it could condense and create a problem.
But then my follow-up question is: how, exactly, does the foam being flush against the wall prevent that condensation from happening anyway?
I appreciate your insights - this is interesting information, but I am still on the steep part of the learning curve. :)