spray foam on the underside of the roof
We have a large attic area with aircrete insulation in the attic. We also have about 50 can lights in the ceiling. Whenever the wind blows hard (Montana it is always blowing hard), we get this sugar like material sifting out around the can lights from the insulation. We also have all the AC ducts and a heat recovery system in the attic. When we get drastic tempature swings we get water dripping out of the heat recovery vents. What i would like to do, is to spray 2 to 3 inches of closed cell foam on the underside of the roof with another 10 to 12 inches of open cell foam on that. It would make the attic a conditioned space. We would not have to worry about heat and AC losses. Would I still need to seal around the can lights or would it not make a difference any more. The garage is also insulated so it shoul not affect heat loss there. The roof has concrete tiles on it.
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In theory, if the attic is conditioned you don't need to seal around the cans, although it's possible you could still get some of the material sifting out due to vibration and drafts between the spaces.
Is it an acceptable practice to insulate the underside of the roof. Do you see any problems with expansion and contraction of the roof sheeting. I know there is some air gap between the sheeting and the concrete shingles. Just dont want to cause any problems.
It's done all the time, and closed cell spray foam is allowed to be used directly against the roof sheathing by the IRC. Couple of caveats, though. If you ever develop a roof leak, the foam may mask it long enough for serious rot damage to occur. And your shingle warranty, for what that's usually worth, may be voided.
Yeah, it's mildly controversial. But I haven't heard of anyone being struck dead for doing it, nor (so far) has the world ended as a result.