We have a small ranch in the Colorado mountains at 8100 ft, CZ 5. The house dates to 2000 and was pretty much built to basic code of the time. The location is very windy during the winter months, often 30-50 mph for long periods. The front great room has a cathedral ceiling framed with 2X12 rafters, which is at the end where we get the greatest percentage of the wind. When the wind blows, the front room requires much more heating and gets drafty. The drywall interior does not have any cracks, the windows are fairly new Pella casements, well foamed in, and the doors seal well. The ceiling only has 7.25-in of standard FG batts (~R25) and the wind gets into the channels to blow through the FG, leaking greatly through the few ceiling electrical boxes.
We are planning to replace the external fastener metal roof, and in the process improve the screw holding ability of the roof deck. The screws loosen in the wind and many have compromised seating, even with new oversize screws. The current sheathing is 7/16 OSB, which was code at the time. Design requirements from the county building department have increased both for wind speed, from 105 mph to 170 mph, and snow load, from 45 lbs/sq ft to 73 lbs/sq ft. We are considering replacing sheathing with 5/8-in OSB, meeting current building code, to improve snow load and screw holding. We also want to improve the insulation in the cathedral ceiling and prevent the wind from getting into the insulation bays. This work would be done from on top. Spray foam has been suggested to accomplish both, and could get the R value up to 49, but may be expensive. An alternative we are considering is layers of mineral wool to build up to 9 1/2 – 10-in (~R 40) with careful air sealing of baffles using caulk and/or spray foam. The fire resistance of mineral wool may help us because we are in a wildfire prone area. I also wonder if I need to lay a smart vapor retarder into the bays before filling them with the insulation, if that works? I am looking for comments, opinions and advice on these options or other options.
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