How to insulate underside of roof deck in 1927 conditioned space
I have lived here for 18 months. First thing I did was re shingle to roof. Roofer is a pro and said decking was in great shape. Shingles are mid-dark gray in color. This is a 1.5 story home in central MN with the second story “finished” at some point with Homasote on the walls and ceilings. Homasote is a brand name associated with the product generically known as cellulose based fiber wall board, which is similar in composition to papier-mÃ¢ché. It is in bad condition and needs to come down. There are two unfinished spaces behind knee walls that are accessible. There is a little bit of fiberglass batting under the roof deck but mostly newspaper from 1933.
The roof rafters in the exposed areas are 3.5 inches deep. There is no ridge vent. There is a single vent in both unfinished spaces. There are vent holes in the aluminum wrap on the soffit but I have no idea if they are actually functional. Contractor says that in a house this old they are typically blocked and useless. He thinks the best solution is to spray 3″ of closed cell foam on the complete underside of the roof deck making it a “hot roof”.
My original idea was to use 2″ rigid polyiso foam between rafters then another layer perpendicular to that taping all the seams. This seems a much more reasonable DIY project. Contractor says I would be asking for trouble. I can’t get anywhere near the code requirement on R value unless I reduce headroom in this finished space by 6-8″ with rigid foam “under” the rafters in addition to the spray foam. between rafters. Mostly I want to seal leaks and insulate better than the existing newspaper.
I am doing a lot of reading and my head is spinning. There are 4 windows, 1 heat vent and no cold air return. Co-worker is telling me I need to extend a cold air return up there from the basement ductwork. The house exterior is stucco.
Thanks for reading all this. I wanted to be as complete as possible. What would you do ?