Ensuring Rockwool ComfortBatt Insulation is Tight to Underside of Unvented Roof Assembly
We’ve just completed the exterior work of an unvented roof assembly and are moving on to installing the rafter bay insulation. The house is a 1923 bungalow with the typical 1 1/2 stories. There was an addition from the 80’s with a 2/12 shed roof. We are in zone 4a. Assembly from the outside in: three tab asphalt shingles with standing seam on the shed roof, synthetic underlayment, 1/2 in CDX plywood, 3 inches of XPS (sorry, didn’t understand problem with greenhouse gas potential at the time), another layer of synthetic underlayment, 3/4 inch mix of CDX and solid wood, 2 x6 rafters sistered to actual 7 1/4″, R30 Comfortbatt. The living spaces will be finished with drywall, the shed roof will be exposed ComfortBatt. Rafters are 24″ on center.
While trying to insulate the very hard to access portion of the shed roof during the roofing process we discovered the Comfortbatt insulation did not completely fill the rafter bay and would leave a gap up to 1 inch from the under side of the roof decking. As a result, because we needed to move on with the roofing portion of the work, we removed the furring strips reducing the rafter depth to a nominal 2×6 and used strapping perpendicular to the rafters to hold up the batt. This seems to have derated the R30 insulation to R23 but maybe even worse than that if the material is too dense.
The Rockwool installation instructions are a bit thin with unvented roof assemblies and, despite searching off and on for weeks, I have not found this question answered on GBA.
What is the correct way to support the batt insulation while ensuring the rafter bay is full and there is no gap between the roof decking and the top of the batt insulation? Please, any help will be greatly appreciated.
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