Insulating a flat roof: ventilated or unventilated?
I have a (very sligthly sloped) flat roof deck held up by 2×8’s to which the ceiling for the upstairs room is attached. Therefore, the space between the ceiling and the roof deck is about 7 inches – much like a cathedral ceiling, except that it’s basically flat.
In each 16″ wide channel there is a pitiful layer of R13 fiberglass batt attached to vapor barrier paper laid against the ceiling, with cold air blowing continuously over and arround it from openings at either end (“vents”).
Because taking down the ceiling is not an option and I’m not keen to re-roof, I need to work with the 7″ cavity. However, I’m finding strongly differing opinions about whether the solution should (a) retain the ventilation under the roof deck, or (b) seal it off, making the roof assembly unventilated.
For ventilated roof: the best option appears to be blown-in loose fill fiberglass on top of the fiberglass batt, which will cut down on the air infiltration but (apparently?) still allow the roof to ‘dry out’. This would be a marginal energy efficiency improvement.
For unventilated roof: (1) dense-packed cellulose on top of the fiberglass batt would create an air barrier; (2) injection foam (tripolymer or light weight closed-cell foam) to fill up the space completely, which would be impermeable and it would also have the highest R-value.)
One contractor in my area strongly advises against injection foam for this application “because the roof has to breathe.” He also says that the injection foam application makes it impossible to trace roof leaks for repair.
Question 1: Is it really inadvisable to convert the 7″ cavity in the roof assembly to an unventilated space by filling it up completely with insulation? (Any experience out there with this?)
Question 2: Is it appropriate to blow-in dense-packed cellulose in this application?
Thank you for any insight from experience.