Hello,

We were anticipating using 4″ of EPS rigid foam around the interior perimeter of our basement wall and then framing an empty 2×4 wall in front of that.  Turns out 4″ of EPS does not (quite) meet the R-20 continuous exterior insulation code (I should connected those dots but didn’t — too many balls in the air), so we are now looking into other options.  The complicating factor is that our contractor put 4″ of foam around our concrete slab, and it sticks up above the concrete in various heights around the perimeter (we were going to foam that gap where the foam pieces came together).   We do not have exterior insulation.

If we go with 2″ of XPS up against the concrete, and then frame our 2×4 wall in the expected location (which also conveniently lines up with the underside of the floor joists) we meet the code a different way.  However, we then have a continuous 2″ gap between our insulated framed walls and the rigid foam board.  I’ve done a lot of looking and most air gap questions are for small sections of wall, so this is a bit different.

One concern I can think of is that the tape for the perimeter foam board might come undone over time, allowing moisture to seep through.  We could push the Roxul batts back up against the rigid foam, leaving only 2″ gaps behind the studs themselves.

I would appreciate any thoughts you might all have about this approach, or if this is setting us up for badness in the future.

We can go with 4″ XPS around the outside, it just is a couple thousand extra dollars which we would like to use elsewhere, but not at the expense of moisture concerns down the road.

Thanks so much!!!