I am planning to insulate the inside of my basement. I have a concrete block foundation. I am planning on going XPS board route, applied directly to wall with foam/tape on all the seams to make it nice and tight.

One complication is at the top of the top block of the wall, it slopes outwards, up to the sill / bottom plate. If I end the XPS foam board at the top of the wall where it begins to taper towards the sill, how should I insulate the remaining top 6-10" of the block wall?

Two ideas...

  1. Spray foam the inside of the rim joist all the way down to the top of the vertical portion of the wall.
  2. Cut small pieces of foam board, with mitred edges, to attach at the top of the wall, taping all of the seams. Could also spray foam the rim joist areas, but would not be as much area to cover with this method.

Are there other options to consider to insulate the top of the block? What are the pros and cons?

Edit: Picture...

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  • What about putting mineral wool and covering it with some boards attached to the floor joists?
    – sharptooth
    Commented Jan 9, 2013 at 5:28

1 Answer 1


I've had an almost exact scenario.

you basically have 3 options 1) spray foam the entire system. Expensive for small areas. 2) 1-2 inch foam board that's cut out exactly. tuck tape the seams to the joists/ceiling. This would be a pain to mitre. (I don't think they're meant for this) 3) batt insulation with 6ml vapor barrier over, then tuck tape the seams to joists/ceiling.

Each have pro's cons. spray will be best seal. Batt insulation would be easiest to cut/work with and seal.

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