Unconventionally vented roof
I am putting a new roof on a restuarant (wood shakes). Right now the underside is open to the dining space but has fiberglass batting covered by FSK for insulation. The restaurant has to remaian open for business so changing the inside is not an option. During the summer the sun bakes the roof and their AC cost are astronomica. My question is this, can I vent the topside of the roof by putting foam with a radient barrier on the roof deck,Fir up an air space and put OSB, paper and shingle on top of that, and have it vent at the ridge? I can’t change the color or material of the roof to make a cool roof. This project is in Salt Lake and if this is a viable option what should the thickness of the foam be and what size should the air space be? The R value now varies between R-19 & R-30.
Replies
If the roof will support the weight, then yes.
Spacing is a tricky question. On the one hand, theoretically you want the space less then an inch to prevent convection. On the other hand you want the space to max out to reduce radiation from the hot side making it to the interior. I'd bet radiant gain would be orders of magnitude higher than convective gain, so I'd suggest as much space as you can get. 2x4 or 2x6 on edge to get at least some room to fight radiant gain from the hot side. Ideally, even more than that.
Foam thickness -- there is a law of diminishing returns in R value. If you already have R-30, don't know how much more would really buy you much. I'd spend effort on getting it all up to at least R-30.
Whatever you do, do not forget that when you mount this "second skin" to the existing roof with vents at bottom and top, you have made a sort of wind scoop, and the more distance between, the more of a scoop it is. Make sure it is properly designed and connected to resist all anticipated wind loads, which will be substantial.
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