Using rigid foam insulation in an unvented roof
Hello, I would like advice on insulating my garage. It has cinderblock walls and an unvented hip roof with no ridge or soffit. Rafters and sheathing are exposed and the outside is metal panels. I can’t really alter the outside appearance of the home due to my local historic society (home built in 1842). My question is, can I glue/screw rigid foam board directly to the underside of the roof sheathing, in between rafter bays, then finish by screwing plywood or drywall directly to the foam board. Is there risk of moisture build up (between foam board and sheathing) if I do this? Can I insulate/finish the walls directly up to meet the roof, and leave the soffit closed off and empty?
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Dan,
The method you describe is called "cut-and-cobble." The method is never used by insulation contractors, and it is somewhat risky (although less risky for a garage than for a home, since garages don't usually have people living in them who generate moisture).
For more information on the cut-and-cobble method, including warnings about its use between rafters, see Cut-and-Cobble Insulation.
For more information on insulating sloped roof assemblies, see How to Build an Insulated Cathedral Ceiling.
There will always be a risk of moisture build up and mold if your home generates enough humidity to cause the condition. I can only assume your concerns are generated from a known problem already occurring in your home. A dehumidifier can help reduce the concern but source control is best. Remember mold begins to grow at 50% RH and rapidly spreads above 65% RH. Installing a monitor will help assist you in monitoring the problem before it haunts you. There are articles published illustrating spray foam and sheathing rot when the products are not installed correctly or improperly specified.