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We're looking at insulating the box joists of our concrete foundation. We have 2x3" walls framed which are holding batt insulation, which are 3-4" away from the wall (to accommodate a layer of batt insulation behind the framing). (This all existed when we moved-in - if we were starting from scratch we would have framed right against the wall and spray-foamed the whole thing. Alas.)

We plan on doing 2" of closed-cell spray foam, but what I'm not sure how to go about properly sealing the transition from the wall framing to the foam. Originally I was going to loop the vapour barrier over the top of the framing and behind the insulation, so that any gap left after spraying would still be vapour-impermeable. But then I read that the foam will not stick to the polyethylene sheeting, so that's a no-go.

Should I just spray "down" a little bit at the bottom of the header area, and hope the foam expands to reach (and seal-to) the top plate of the framing wall? Or should I add blocking at the top of the wall first? Anyone have experience with this specific setup?

Box joist

Front-view of the box joist area (yes, that is frost - this is why we're spray-foaming!)

Top view

Top-view of the framing wall and gap

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  • couple of questions. Are the walls covered with drywall or some type of sheathing? How deep are the joist boxes? What do you want the transition to look like? Finished or utility? Is the top plate of the wall even with the top of the concrete wall? I have reasons for all these questions, Trust me. lololol
    – shirlock homes
    Dec 12, 2012 at 21:57
  • Thanks - the walls are covered with 6mm vapour barrier (polyethylene) - seen hanging down from the top in the first photo. The joist boxes are about 6" deep (2.5" of framing, then the gap pictured in the 2nd photo). The top plate of the wall is about 1" below the floor joists - the concrete foundation wall extends above (see first photo). The basement will not be "finished" (too short) - this is all just utility.
    – Derek Hogue
    Dec 13, 2012 at 0:47

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When we insulate the joist boxes in apartment buildings or commercial buildings we are required to close the boxes over the foam with drywall or flame retardant foam board. Since your job is for utility purposes here's my thoughts. After you spray a couple of inches of foam into each box, cut measured pieces of foam board that will snugly fit above your wall, between the joists and up against the 1" rise of the sill plate behind the wall. Put a few dabs of construction adhesive on the edges and fit them into place. This will do a couple of things for you. It will create a larger dead air space in your now insulated boxes, extra R-value and also seal the top of your wall against the foundation. Smaller pieces can be glued in under the joists to complete the wall seal. If you want to further seal and joints in the foam board, use some HVAC aluminum tape.

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  • UPDATE: in the end we went hard and just sprayed 4+ inches of foam in all the joist boxes, which allowed the foam to seal right to the top of the framing wall. A little pricier in terms of materials, but faster, less hassle and a better, tighter seal.
    – Derek Hogue
    Dec 19, 2012 at 19:51

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