I have a (very sligthly sloped) flat roof deck held up by 2x8'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").
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