great stuff should not be used.
I've seen it pick up boxes off the attic floor 1 1/2"
and it shrinks. little control with foam.
off gassing & leakage are my concerns.
this is how I seal supply boxes.
hardcast #1402 mastic tape only
I buy 3" rolls & split width to 1 1/2"
it only needs to go from sheetrock
ceiling (within area covered by grill)
to metal lip inside supply box.
this is the only tape I've found that is
strong enough not to come lose when
you replace the grill.
caulk doesn't stick to the inside of the
sheetrock (dust) and if you put the
grill back up before caulk dries..it sticks
to the grill. I've taken many a supply
grill off that the caulk was on the grill
and not on the gap.
trying to seal from inside the attic..
what about the side of the box attached to
just my experience.
heres my pic.
best of luck.
The cure of the part should not be attempted without the cure of the whole. ~Plato