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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. :)

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It doesn't make sense to me at all. Yes adding a one inch gap does add R value as does any dead space - but then why not a 20 inch gap?

Also I am not sure how you install xps with a one inch gap. Of course your could add furring strips and then you still have to fire block. Wow that is just tons of work.

Why would he say that? I guess he doesn't want the XPS to get wet if you have moisture problems. Well xps handles moisture just fine and the reason it is often put in basements is that it can get wet, retains very little water, and does not mold (to a point).

The XPS should be on the wall. You want a "bigger" gap for airflow for the drywall. Your drywall will mold faster than anything in a basement. So you want as much space for air to circulate on both sides of this. Using 2x4 framing gives ample space for small amounts of moisture to dry before it attaches to the drywall and molds. Would one more inch between XPS and framing help? I am sure but not sure it is worth the returns which will be very small.

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Adding a 1" gap behind the XPS creates a dead air space that adds to the R value of the overall insulation. This is a standard recommendation with sheet foam insulation. It does assume that you have a good seal at all joints.

If you have a moisture seepage problem, this doesn't add to it. if the walls are damp, they are damp. You should be considering moving moisture away from the foundation by grading, gutters, etc.

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Most sites I've read online mention installing the XPS directly to the concrete, often using an adhesive. Though I can see your point that, provided it's a good seal, it wouldn't seem to cause additional problems to have it an inch off the wall. –  susie derkins Apr 6 at 0:25
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The reason to attach it directly to the wall is to try and make sure the surface of the wall doesn't have air contact for condensation. This works perfectly with spray foam (one of the big advantages) but of course, this is impossible to do with sheet foam as no concrete wall is 100% flat. So whether you have a 1mm gap or 1" gap, it doesn't really matter. However, it likely is EASIER to have it up against the wall, though.

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