When putting up XPS insulation in a basement, prior to framing, is there a reason the insulation shouldn't go all the way to the floor? In this guide, they show the XPS resting on top of 2x4s (lying flat) when they glue it to the wall, and later you can see the gap when they are putting down the bottom plate. But they don't explain why they do that. Why wouldn't you want the insulation to go all the way to the floor? Is it a moisture issue? The XPS is already in direct contact with the wall, and part of its purpose is to form a vapor barrier, so I'm a little confused by that. Likewise, I noticed they don't have it cover the sill plate at the top, either. I would think you might as well insulate all the way. Why not?

  • The XPS will not draw moisture. I would plug the hole as tightly as I could. You can even caulk the insulation to the plates. – Paul Logan Apr 30 '18 at 3:52
  • My thoughts say that the gap is for air circulation to help keep the wall dry in areas where moisture may be in issue. If your walls are always dry, then you could probably forgo this. – Micah Montoya May 1 '18 at 13:28

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