I am planning on installing 2" rigid foam on my basement walls to act as a vapour barrier, before framing in the 2x4 walls (AB, Canada). The rigid foam comes in 8x4 foot lengths, but my ceiling height is only 7 feet. Sorry for my plethora of questions:

  1. Do I cut grooves for the joists, or simply cut these to be 7'? I am thinking cut to 7' and address the joists afterwards (maybe after framing)?
  2. Also, should I simply stuff the small rim joist space with fiberglass or rockwool?
  3. Should I do subfloor before insulation?

Dimensions of basement wall

  • 2
    Usually the space between those joists are the most important/biggest bang to insulate for cold.
    – crip659
    Commented Mar 12 at 17:50
  • I would imagine so! Would it suffice to just stuff the gaps with insulation (and then more once I frame up the walls)?
    – Shinobii
    Commented Mar 12 at 17:56
  • 1
    You will want a vapour barrier if using other types(like glass wool). The main thing is have tight to the ceiling and joists. More insulation is usually good, but only use a vapour barrier once on the warm side(if insulation needs/does not have vapour barrier).
    – crip659
    Commented Mar 12 at 18:03

1 Answer 1

  1. Stuff the void with as much fiberglass as practical
  2. Cut the rigid insulation to match the height to the underside of the joists.
  3. Use the cutoffs to fill the space between the joists. Ideally, these would be tight to the underside of the subfloor. This means that you will need an extra rigid board to complete the job.
  4. Use expanding foam to seal the gaps between the joists.
  5. Install your 2 x 4 framing.
  6. Install your subfloor using the recommended gap around the perimeter.
  7. Use joint backer rod to fill the gap between the framing and the subfloor.
  • Okay, makes sense and this is what I was thinking, but you laid out the steps so clearly. If I may ask, I should not require vapour barrier if I do it this way correct? Or should I do a vapour barrier anyway (or would that be detrimental)? Thanks!
    – Shinobii
    Commented Mar 14 at 13:40
  • 1
    If you apply something like sheathing tape to the joints of the rigid insulation and used the expanded foam as detailed, you do not need a vapor barrier. Commented Mar 15 at 13:08

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