How thick can you go on roof deck insulation?
Hi again. You may recall that I live in Baltimore, Maryland, and am planning an addition to my home. I live in a brick masonry 2 wythe load bearing house built in 1938. I see this also as an opportunity to retrofit my house, that would keep the house as an integrated system. In have been thinking about doing a “chainsaw retrofit”/PERSIST, except there is no chainsaw because my existing roof has no overhang or soffit, with only gable end window ventilation. After removing the existing roof (which needs replacement) I would wrap the entire house (existing and addition) (i.e., all my roof sheathing, 2-wythe brick walls and masonry – perhaps AAC – addition) with a class I (yes, perm<0.1) vapor retarder (in keeping with Joe Lstiburek’s “Perfect Wall” and recommendations for vapor control in the humid south), to convert the small attic space into a conditioned space, and put exclusively external insulation (I’m thinking mineral wool because of its good water repellence and termite-free and off-gassing free properties) over top, then a rainscreen gap by furring, then the cladding (shingle for existing roof, perhaps metal cool roof for south-facing addition, lath and stucco for walls – with brick slips “lumpy stucco” over the existing house to maintain current appearance). For external roof insulation, how many inches of insulation can you logistically put up there? I see Joe Lstiburek and Betsy Pettit point out that 6″ of polyisocyanurate (faced R 6.5) can get you to R-40, which passes the R-38 for code. I also see that the REMOTE manual uses 6″ of foam insulation. But I’m not thrilled by its gradual loss of R-value and that the pentane that is off-gassed might contribute to smog in my city. But I see that Martin in his blog writes that the PERSIST system has been used up to 8″ of external roof deck foam with a further 2″ thick sleepers. This would mean 10″ below the shingle nailing bed. And I can use R-4.2/inch mineral wool or rigid board fiberglass for 9″ and have the 1″ drainage gap and so pass code with the same 10″. I like to be able to see the underside of my roof sheathing from the attic space, and feel for moisture, as well as to keep mineral fibers as much out of the conditioned space as possible. Mineral wool would also give better vapor permeability compared to foam, to allow drying to the outside. If I found appropriate structural screws to go through the plywood/OSB and then 10” of drainage space and rigid board insulation, could this work? Are there such screws? Is there a limit to the above deck thickness for any other reasons? I would of course pass this by my structural engineer, but I am interested to know the experience in the green building community.
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