Poking around the ceiling of my basement in the storage room the insulation up there along the foundation isn't covering things very well. My townhouse is walk out with the storage room on the fully sunken end. That end the wall is covered with R11 insulation wrapped in a vapor barrier (along with about 2ft back on each side, the side walls aren't insulated at all between my house and the neighbor's houses).
The top 10 inches or so is above the foundation wall (basically what the ceiling joists rest on 2x10 joists I believe, maybe 2x12, never bothered to measure). This is still concrete (basically thinner concrete wall on top of the foundation wall)). I'd guess this part of the concrete is maybe 4-6 inches in thickness. There is what looks like R11 or R13 pushed between the joists against this concrete wall. Well it doesn't fill the spaces all that great (fine in some places), but there are several vent/pipe/conduit penetrations through the concrete here and the insulation doesn't seal around them very well.
I was thinking of getting a few cans of Great Stuff big gap insulation and basically spraying a solid (thin) layer against the concrete in each 'void' and then stuffing the fiberglass batt back in there. Its not a huge area, its the 14 inches or so between each joist, about 10 inches high and there are 13 or 14 such voids (Townhouse is 19ft wide). I figure each void is around 1-1.25sq-ft for a total of about 15sq-ft on the high end.
How many cans of big gap filler would I probably need? I am thinking 3 cans should more then cover it. I don't need a thick layer, I figure a solid inch thick when expanded/hardened to form a solid barrier plus the fiber glass bat of R11 or R13 should provide good insulation there.
Thoughts on technique for insulating better there? Also ideas on how many cans I'd probably need?