Cathedral Ceiling Insulation – Unvented – Zone 6
I know there are hundreds of discussions about insulating cathedral ceilings, and I’ve enjoyed reading many of them, but I have a day or two to pull the trigger on my choice (with contractor’s advice), and would appreciate your advice. My first choice is closed cell spray foam, 7″, but it is now super expensive. I reread some articles and am concerned of sandwiching the sheathing, decreasing the life of the asphalt shingle, etc, but maybe with Zip-R everything will be ok?
New construction
Climate Zone 6 (Maine) – Need R49, happy to have more
Existing assembly – exterior to interior:
Asphalt shinglesÂ
Possibly ice/water the last few feet or maybe higher toward the ridge, wasn’t there at the roof build
Zip-R 1/2″ sheathing
12″ roof rafters
HVAC is air source heat pump
Thanks
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Replies
Since the place is already roofed, your only other option of installing exterior rigid is no longer possible. Spray foam is something you need to design out at the design stage, it is hard to eliminate down the road.
Unless you can figure out how to properly vent the roof, your only unvented option is spray foam.
I would look at a hybrid assembly with only enough closed cell spray foam for condensation control, so in this case R25 worth. The rest of the cavity can be filled with either open cell foam or batts. Since this assembly fills more of the rafter space than the 7" of all ccSPF (calling ccSPF R7 is pushing it) you'll actually get a slightly higher effective R value assembly.
There are a lot of spray foam roofs around me and they have been built for a long time now and they seem to be holding up just fine.
The sheathing will be fine as long as you keep out bulk water. This means get your flashing details right, which you should be doing anyway regardless of the spray foam. I've heard that "the shingles will bake!" concern too, but it doesn't make sense to me -- shingles get crazy hot on top of vented roofs too, and vented roofs tend to be over crazy hot attics. I don't really think it makes much difference, and it hasn't on my own roof or any others I'm aware of.
Regarding the use of spray foam, I completely agree with Akos here -- you're pretty much stuck at this point. If you had rigid foam over the top of the sheathing, you could have used batts or other "fluffy stuff" inside, but with the roof on, you have to use closed cell spray foam on the underside of the sheathing to be safe -- there really isn't any other reliable option here. I would look into using enough closed cell spray foam for moisture control, then the rest open cell, but in my own experience, there is often little if any cost difference doing this compared with going with all closed cell. I'm not a fan of stuffing in batts under closed cell spray foam either, because the spray foam is rarely flat and even so the batts tend to be difficult to fit in place properly.
Bill