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Spray Foam versus Sheathing

I'm planning to insulate my attic rafters above a living space. several sources show the use of iso board or other sheathing with canned spray foam for joints and other fill-in requirements, in the attached picture for basement rim joists. I am contemplating using this for rafter insulation.

Experimenting with the board, it seems extremely easy to cut and fit, and it appears it would be much cheaper than a full spray foam installation. It might be a little more labor, but other than that what other reasons could there be to use a full spray like tiger foam?

The panel method could also allow us to easily build in a ventilation channel if we want the 'cold roof' option.

Re: Spray Foam versus Sheathing

filling between stud cavitys with foam is done but much more time consuming. however much less messy especially for around plumbing and electrical.

as for rafter bays the problem lies with creating "hot roof", meaning no air circulation underneath of the roof sheathing. there has to be through vents in the bay but its much harder to get a snug fit with the rigid foam so it stays put.. installing the through vents then spray foam is a better method.

with creating a "hot roof" two problems arise, the warrenty on roof shingles becomes void because they become baked. this does happen ive stripped several roofs which were done with 30 yr shingles that lasted merely 5 to 10 years. also with no air circulation the roof deck will rot

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