Roof Retrofit
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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Replies
WMF,
While you are waiting for more input, you might want to read this article on cathedral ceilings: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
If I lived in a fire zone, I would worry about embers entering the ventilated attic. And I would be inclined to seal the entire attic (not just the cathedral section) to reduce that risk.
You could install all the insulation (rockwool or rigid foam) on the exterior or install a portion (rockwool or rigid foam) above the deck with the remainder below the deck.
Flash and batt method (close cell plus air permeable) is another option, but you might have to remove the drywall in the living room to undertake the installation. To learn more about this approach, see https://www.greenbuildingadvisor.com/article/the-flash-and-batt-method.
Hi,
I suggest plywood rather than OSB. The screw pull out values in plywood are much higher than in OSB. Also with the thru fastened roof, look at the mfg spec on where to place the screws. Typically the screw is specified to go on the flats next to the rib. Not on top of the rib.
For the metal roof you need a underlayment like versashield to get a class A fire rating for a metal roof, a change since the 2000 code. If wild fire is a concern then fire rated attic vents should be added - brandguard is one mfg.
You need to air seal around you electrical boxes
I thank you for your comments, but I still am not clear about the approach of adding spray foam from above. I have 11.25-in depth but would only plan for about 8-in of closed cell HFO foam. I should end up with 3-in open space above the top of the foam under the roof sheathing. It would seem to make sense to leave this vented from soffit to ridge? The spray foam seems to be the best option for air sealing to deal with the extreme winds , and we have to do this work from above.