Should fiberglas insulation be removed before spraying foam?
The house in question is 1950’s flat-roofed split-level house, with a built-up asphalt roof.
The roof leaks and the contractor plans to remove all the asphalt and the sheathing and spray foam to increase the insulation value. Presently there is a layer of fiberglass in the 2×10 cavity.
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I'd be inclined to bypass the tear out and look at options to add an extra layer of rigid foam insulation and sheathing on top of what's there. This would also give the opportunity to ensure the final roof finish (white, reflective of course) has proper falls of at least 1/8" per foot - a flat roof should never be actually flat or it will pond as the structure deflects, leading to premature failure. Detailing the additional layer at the roof edge should be considered - an additional overhang of 12" or so may be appropriate.
Part of my question is that spray foam will be used as the fiberglass is some what useless except that it could act as a release between the sticky foam and the drywall ceiling in place; that is unless that creates problem as the foam provides vapor barrier above the fiberglass.