Ceiling insulation for a room addition to my house
I’m planning a room addition, 16 x 16 with a 6/12 gable roof and cathedral ceiling, zone 6. I’m not yet committed to anything specific so would like to run this scenario by folks for advice and opinion. I’m wanting to avoid the 5-6 inches of exterior foam (R-25) that would be needed if I went that route. And, I would like to go unvented. Would the following scenario work?
A 12″ fiberglass batt in between the rafters. I can arrange for adequate depth by using 2×12 rafters (11.5″ actual so slight compression of the batt), or use a narrower rafter and scab down. Below the rafter I’m thinking 2″ polyiso, (either one layer or two 1″ layers) meticulously taped for air sealing and reduction of thermal bridging thru the rafters. Sheetrock below. I could use strapping over the polyiso, which would be particularly appropriate with foil facing on the polyiso (I also need advice on the facing I should use). Above the rafters could be OSB or plywood decking, or 2×4 purlins and no decking since I will be using metal roofing. This would provide a cold air space but is not “official” venting I don’t think, but might help with drying to the exterior. Or, would a permeable facing on the polyiso allow for drying to the interior and be preferable?
In theory this would have an R value of something around 50, which would satisfy me if other aspects of this arrangement are not in conflict.
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