Assuming that your cans are AT IC (air tight, insulation contact) rated you just cover ANY POSSIBLE crack, gap or seam in the cans including, in particular, where the can meets the drywall with spray foam. That's about it. If they aren't IC rated then you have to build a box around each individual can. I suppose you could use rigid foam but I usually use drywall. It has to be a couple inches away from any part of the can. So you cut a sheet of drywall which is the width of the joist bays from outer side to outer side plus the height of 2" above the ca from the ceiling x 2 in length and 4" wider than the can. Then you make cuts on one side of the sheetrock at the length of the height measuring in from both ends. Only cut one side. Then break it there. You have created 3 sides of a box. Put it over the can screwing it into the joists from the sides opposing the can. Then you cut a vaguely T shaped piece of sheetrock which is the width width and height of the bay for the bottom part of the T and is wider and taller than the three sided thing you made earlier. Nestle it in the bay and screw it into the earlier three sided thing. Repeat on the other side. Then foam the hell out of any seam, including the seams which are not cut all the way through. Or just tear them out, sell them on craigslist and replace them with IC rated cans. They only cost about $20 each I believe.
Clear as mud?
The one part foam (or two part for that matter) for doors and windows is low expansion. You cover significantly less area with it can for can. I never, ever, use it for any application other than doors and windows. Really, just get a cheap one part gun and get some real gun foam, like Pur or One Touch. It's a LOT cheaper in the long run (ROI is probably only about 20 or so cans of Great Stuff) and it's much, much better. I haven't seen window foam being more flexible. In fact in my experience it's more frothy and friable than regular expansion foam.
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