R60 Cathedral Cieling on Timber Frame Cabin
I have been working on the design for a small timber frame cabin I plan to build in the Catskill mountains in upstate New York, climate zone 6. I am trying to get to R49 (current code), or better, R60 on the cathedral ceiling without covering the timber rafters and T & G ceiling. One approach from Rob Myers’ 2016 series “A Timber Frame House for Cold Climate” [https://www.greenbuildingadvisor.com/article/a-timber-frame-house-for-a-cold-climate-part-3] would have me pile 10 inches of XPS on the roof and screw it down with super long and expensive screws. This approach is pricey and I’m a little leery of trying to aim a few hundred 12″ screws through all that foam.
My research on GBA led me to the article “How to Build an Insulated Cathedral Ceiling” [https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling] and the suggestion to deepen rafters by suspending the ceiling below the rafters from plywood gussets. This got me thinking maybe I could do something similar above the rafters. Since this structure would be holding up the roof with a 50 psf snow load, it would have to be stronger than the plywood gussets, so I thought I might build 18″ trusses out of 2x4s (as shown in the attached image) or use I-joists.
I’ve sketched out the assembly and it seems to me like it would work, but I’m no expert, so have at it. Beyond general thoughts on the assembly, I have a few specific questions:
1. Assuming the seams and corners of the rigid foam are properly taped and sealed, does the assembly as drawn make sense from an air sealing perspective. It seems a little weird that the roof air-barrier is inside the cavity insulation while the wall air barrier is outside the studs, but I’m still creating a tight foam shell around the entire structure. In the details I’ve seen the air barrier is created at the dry wall, but it’s tough to create a good tight air barrier with the drywall since there are big timbers in the way.
2. The timber frame structure only requires big rafters 36″ o.c. as this truss is fully supported from underneath could I set these 36″ o.c.? Maybe with 1-1/2″ T & G replacing the sheathing?
3. Truss or I-joist? If I build the trusses myself it will be a whole lot cheaper; I-joists a whole lot easier/faster. Any design/engineering reason to prefer on or the other?
4. The second attached image shows how I could create a 2″ air gap above the dense-packed insulation on the roof. The cavities could then be filled from the top. Does this seem like a reasonable way to do this?
5. Finally, would it be worth the trouble/expense of filling the little triangle above the wall framing with some spray foam? I’m thinking that might be a good fallback for the taped seam where the roof foam meets the wall foam.
Thanks in advance for your thoughts.
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