Insulating an historic house’s attic
I live in Montgomery County, PA, which is at the northern edge of the 4-A climate zone according to the map on this site.
I am turning the attic of my historic house into a bedroom and wish to install insulation followed by sheetrock on the roofline. The roof is a 10-year-old cedar shake backed with tar paper, there are 1″ furring strips between the tar paper and the rafters that act as lathing for the roof, the rafters a a bit irregular and are 6″ deep by about 3.5″ wide. There is a ridge vent but no soffit vents. Spray foam has been recommended to me by a contractor, but in addition to being expensive they can only get up to R-28 given the depth of the rafters. I am also unsure whether spray foam applied directly to the lathing is a safe option for a cedar shake roof.
My tentative plan is to extend their depth with reclaimed boards, then install a rigid board with a radiant barrier followed by mineral wool to bring it up to R-38 (recommended minimum for my climate zone according to energy.gov), then finally install sheetrock into the spaces between the extended rafters, leaving the ends of them exposed for effect.
In addition, I think but am not certain that I need to install soffit vents and leave a channel for air to pass to the ridge vent. This is based on my reading of the 2015 International Residential Code.
I also have a supply of cafco-board at my disposal, I was thinking about using this as the rigid board, maybe as all of the insulation (in layers) if I have enough.
I have attached a picture of the inside of the roof to give a better idea of what I am working with.
This is all new to me, thank you very much for any and all advice!!!
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