Cedar Roof Shingles over Rockwool
scottgazelle
| Posted in GBA Pro Help on
We’ve decided to use Rockwool (Comfortboard 80) for exterior roof insulation. The Comfortboard will be placed over the WRB, which is over OSB sheathing. My question is how best to fasten cedar shingles over the Rockwool. Are furring strips sufficient, or would a second layer of sheathing be required. I am concerned about sandwiching the Rockwool between two layers of sheathing, since it seems to defeat part of the benefit of the Rockwool (vapor permeance). Any advice would be very gratefully appreciated.
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Replies
Scott,
Furring strips are sufficient, although depending on the exposure on your shingles you may find that they end up being so close together that the result is an almost solid substrate, and you might as well have used plywood.
I don't know much about how near shingles do in your climate. Here in the PNW you are lucky to get 20 years out of them. and that might be worth considering when deciding which substrate will make replacement easier when it (all to soon) comes around.
Thanks Malcom - if using plywood, would you worry about trapping moisture in the Rockwool? Though tedious to install, I was thinking that furring strips would lead to a more breathable exterior.
Scott,
I don't know enough about it to comment usefully. I suspect the plywood is more than permeable enough to keep the Rockwool dry. The moisture problems with cedar shingles I've had to concern myself with here are keeping en0ugh air-flow around them to stop rot.
The above sheathing Rockwool is the only insulation or there is other insulation below the sheathing?
Possibly applicable: IRC 806.5 says: "Where wood shingles or shakes are used, a minimum 1/4-inch (6.4 mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing."
sorry for the delay - there will be insulation below the sheathing too - probably a combination of spray foam and Rockwool batts