R49 – R60 in 2×6 common rafter vaulted ceiling
I am looking for an insulation style to get me the required R-value in Zone 6. The current idea is to use something like a Rockwool system on the cold side of the roof along with plenty of closed-cell spray insulation in between the rafters. The entire roof is a vaulted ceiling and keeps with a 2×6 rafter, I really don’t want to go to a truss if I don’t have to. Would something like Rockwool or something similar work?
Also would something like Drip stop on the metal roof help? I know not with the R-value but with condensation control. This might not be needed if I use closed-cell insulation, right?
Extra Info:
Rafters are 2×6 24″ OC
2×4 purlins will be on the outside of the rafters
~21′ house with a pitch of 12.
Metal roof.
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Replies
You can't span very far with a 2x6, so that is going to greatly limit how big of a cathedral ceiling yuo can build. Trusses don't have that limitation, or you can go up to something beefer than a 2x6. TJIs have some advantages here, for example, since you can get them large enough to make long spans, and the relatively thin web reduces the thermal bridging you get from the structure.
If you use enough exterior insulation, you don't need to use spray foam here. That's one of the advantages to exterior insulation. You could use batts on the interior instead to save some money, which would give you R23 if you use mineral wool batts in 2x6 rafter bays. Everything else would have to be on the exterior though, which would be R26 or more of exterior insulation (which would be 4" of polyiso). I would use polyiso here and not rigid mineral wool, since polyiso is much cheaper. You can get "roofing" polyiso that is specifically made for this type of application.
Closed cell spray foam is only needed with cathedral ceilings when you have no exterior insulation, since in that case you need a 'full adhered' insulating material on the interior, which pretty much means spray foam. With sufficient exterior insulation (at least 40%, but I prefer 50% or more of the total R value on the exterior), you can insulate the interior normally with batts and not worry about moisture issues.
Bill