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Spray-foaming a Roof?

spclark's picture

Under conventional OSB/plywood sheathing under asphalt shingles, is it common practice (I'd certainly think so but that's why I'm asking!) to leave a ventilated air space of 1" or so between sheathing underside and top of foam?

Would there be instances (roof design) where an airspace would not be recommended?

either or (post #213812, reply #1 of 4)

You can go either way. I'd recommend you look at IRC regarding foam and roof sheathing. I want to say R804 or R805, somewhere around those sections IRC addresses foam, sheathing, vented or hot, and vapor retarders and their location.

There are 10 kinds of people in this world; those who understand binary and those who do not.

Hi there,       I (post #213812, reply #2 of 4)

Hi there,

      I believe the main/only reason to leave the airspace is to preserve your roof shingle warranty which may require a "vented roof assembly".  I don't think any roof warranty is worth much of anything to be honest.  I would eliminate the air space and foam the underside of the roof sheathing for an unvented assembly.  This is not only common, but I would say, it is now the most common way roofs are insulated in my climate zone 5.  The point of a vented roof is to allow cold winter air in to keep the underside of the roof sheathing cold so snow on the roof doesn't melt, then re-freeze at the (cold) eaves overhang causing ice dams.  A properly foamed unvented roof will prevent warm inside air from ever reaching the underside of the roof sheathing, so there is little chance of ice dams.  

Thanks FF! (post #213812, reply #3 of 4)

Your logic follows my thinking too. Mongo I'll look into those relevant IRC sections as you suggest.

From a practical standpoint, (post #213812, reply #4 of 4)

From a practical standpoint, a 1-inch "ventillated" air space might as well not be there.  It serves as a mouse run but not much else, since very little air will flow (especially after leaves clog the vents).

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville