Closed cell spray foam under roofing
Can one apply closed cell spray on insulation directly to the underside of roofing sheeting to which asphalt shingles are nailed to create a cathedral ceiling? I am looking to create a heated loft above a garage with a gambrel roof.
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In a word, yes. There are several articles on that topic here on GBA.
Here's a link to an article that will answer your questions: How to Build an Insulated Cathedral Ceiling.