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?
Front-view of the box joist area (yes, that is frost - this is why we're spray-foaming!)
Top-view of the framing wall and gap