Is open-cell foam appropriate for spraying the bottom of a roof deck?
We are building in Indianapolis, middle of zone 5. House will be on a slab, under-slab hydronic heat powered by geothermal, 4″ Thermocore SIPS walls with 2″ of rigid foam on outside of that. Truss roof. My homeowner wants the bottom side of the roof deck sprayed. My experience is typically spraying the attic floor, not the roof deck. However, in this scenario, we will have ductwork in the attic, so it does make sense to spray the roof deck, so the ducts are in conditioned space. My insulator is quoting us 5-1/2″ minimum open cell foam.
My understanding is that open cell has an r-value of roughly 3.6/inch. That would only give a total R-value of 19.8. Code minimum is 38, we typically insulate our attics to an R-50 using a combo of 2″ open cell on attic floor covered by 12-14″ of cellulose.
My insulator says that there isn’t really a defined R-number for open cell, and because it stops the heat transference, 5-1/2″ meets code.
I am not interested in “just meeting code”. Knowing the other things we are doing, we want this to be a high-end-performer on the efficiency scale. Can somebody tell me the best methods to use when insulating the bottom of the roof deck? Or even if you think this is a bad idea altogether?
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