Insulating an existing roof
So I’ve been reading the blogs about the best way to insulate a roof, but it always seems that they are dealing with new roofs or gables. We purchased a house a few years back and have been steadily renovating it. The roof was “new” asphalt at the time of purchase – seller added additional layer of asphalt shingles so not a complete tear off. In either event, I dont wish to redo the roof at this time, but i need to insulate it as we intend to begin converting it to bedrooms. The roof is a hip roof with three dormers (one at each cardinal direction except west). From the various posts it seems like i will need to treat it as an unvented cathedral ceiling. What i’m debating is the combination of insulation to employ. Spray foam concerns me because of cost and that it conceals leaks. I am in Zone 5 (connecticut) so all seasons are of concern.
Existing roof construction is as follows: Asphalt shingles, plywood, purlins, and 2×4 rafters (built 1920, lumber is really 4″)
I have two thoughts.
OPTION 1: seal Eave and all joints at the perimeter with spray foam then fill cavity with 3 1/2″ of roxul, vapor permeable air barrier on the inside face of rafters, 2x furring and 1/2″ gypsum. Later, reroof-complete tearoff and put down 3-4inches of polyiso followed by 3/4″ plywood, A/V barrier and standing seam metal roof.
OPTION 2:seal Eave and all joints at the perimeter with spray foam then fill cavity with 3 1/2″ of roxul, 2 layers of 2″ rigid over rafters – staggering joints, Vapor barrier, 2x furring and 1/2″ gypsum.
Both options would be around R41, but head height is a concern so ideally with option 2 i would prefer not to do more than 2 inches of Rigid. I definately do not wish to trap water and by code i should be around an r49. Advice would be appreciated.
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