Insulating flat roof deck
What I have is a flat roof section, originally a built up roof (tar and gravel). I have had two roofers over, both of whom recommended putting insulating roof board on top of the existing deck and new roof on top.
The wrinkle is that the roof was built as a cold deck like the illustration. So roof board on top, without blocking the soffit vents does nothing for you as far as adding insulation value. Rafters are 2x10.
I have looked at the links from this thread, and it mentions two ways to proceed, roughly--
(1) rigid insulation above the roof deck + permeable insulation below, and no venting of the joist cavities;
(2)spray foam on the underside of the roof deck and ends of joist bays, permeable insulation between this and the ceiling drywall.
To do either well would involve sacrificing the plaster ceilings.
Both versions eliminate the venting as we have it now, with continuous soffit vents and free flow of air between the top of the insulation and roof deck---almost like a mini-attic.
Does a closed/no vent flat roof make sense, or is it worth trying to preserve our current system, using only enough roof board to reslope the roof?
(I've cross-posted this in the roofing section)
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