Minimal insulation rigid foam and batts for Cape Cod cathedral ceiling
Hi all, My name is Jacob, and I am thinking about remodeling my Ohio Zone 5, 1.5 cape cod and am trying to figure out the best way forward.
After reading some of the articles here and elsewhere I have narrowed it down to creating an unvented conditioned area under the roof that brings everything behind the kneewall into the conditioned space. I currently have 2×6 rafters with only 5.5″ of usable space, no soffits, a recently redone roof, and currently R13 fiberglass stuffed into the cathedral ceiling.
The article about hybrid roofs from the Building Science Institute attached photo and table I am referencing. https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies#t01 Makes me think that my best bang for the buck option is to install a 2″ foam board sealed against the roof cladding in-between each joist, then install R15 Mineral Wool directly underneath followed by drywall to finish.
That only gets me up to R-25 total but I don’t have to rebuild the roof, fur joists, or spend a lot of money to significantly improve my current situation. I have gotten a couple of quotes for Open and Closed Spray Foam and they are about 2-3 times more expensive for a very similar R-22.
The second best option in $ and insulation is to furr out the rafters and install additional layers of Foam Board and Mineral Wool until I hit the recommended R-20 air-impermeable insulation and R-49 overall insulation. I lose a little space and gain “proper” insulation.
I have two main questions.
1. The tables I referenced show that R-10 Foam Board, and R-13 Insulation is acceptable for walls to prevent condensation. Is that enough for a Zone 5 cathedral ceiling?
2. If this basic install configuration works, Do I need a vapor barrier on the drywall side? I have found a couple of references that a vapor barrier is needed in Zone 5+ but I believe the intention of this design is to have moisture moving towards the interior.
Longer term we are going to have a mini-split installed in the second story does this affect our plans?
Thanks so much for any advice. Like many of us, I am stuck with a lot of time to think up house projects that need doing. It’s possible I have missed some articles directly dealing with these questions on this site as there is just so much to dig through. A helpful direction to search may be all I need.
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