Hip Roof Cathedral Ceiling – Insulation Help
I’ve scoured the internet and these forums looking for the answer(s) to my question(s), but am having some trouble and would appreciate any help you can offer.
I’ve just purchased 2 small cabins by the lake (500 sq ft)- they are in need of a total rehab. I’m starting on the first one which is completely gutted to the framing inside. The outside walls are 7′ and I have a hip roof. I don’t want to install a flat 7′ ceiling so I’ve decided on a cathedral ceiling – using the existing 2×4 roof. I’m lucky in that I don’t have to meet any codes or restrictions – not that I shouldn’t try, but I’m willing to compromise some for anything that greatly affects cost. With all that said – here are my questions.
1. It being a hip roof I don’t see any simple way for ventilation. Therefore, I’ve decided on closed-cell spray foam insulation and no ventilation. Is this the right choice?
2. On that same note, the house was built in the 40’s – so I’m working with 2×4 rafters. is 3.5″ of CC spray foam sufficient for air and vapor barrier? I’m in zone 4 – South West Missouri. I understand I would be under the recommended R-Value, but do you see this as an issue given I don’t have to meet code? If so, would flash-and-batt be a good idea? I would have to scab on 2×6 or bigger lumber to the existing rafters.
3. In terms of vapor/air barrier, how does this relate to the walls? I had planned on using regular insulation in the walls, but seeing how the ceiling is inherently connected to the walls, this is going to cause an issue with air leakage into the ceiling from the walls? Or is this a non-issue?
I have about 1000 sq ft of roof and as you know spray foam is extremely expensive. I’m trying to find the best way to work out this specific project. Thanks for any insight you may have!
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