Building a Budget-Friendly Unvented Cathedral Roof
I have been researching for hours and have ended up with more questions than answers. I am looking for the most efficient to build/least expensive for labor + materials/easy to build way to build an R-60 cathedral roof in zone 6 (coastal Maine waterfront). The house is 2900 sf partial timber frame and will have cathedral ceilings everywhere. I understand from poking around on GBA that the vented roof I wanted is not possible because of the intersecting gables, dormers, and skylights. So that means unvented cathedral roof. I also want to do as much as I can to reduce thermal bridging. And Covid has tripled the cost of wood as I understand, so I want to do this with only 1 layer of OSB for the roof deck. The timber framer usually uses SIPS for the whole building but they quoted over $110k for materials and $50k labor just for the shell (no windows) which is just too much. How can I do this cheaper?? My budget is ..stretched… to say the least.. if not completely impossible.
So, I was thinking parallel chord roof trusses – do those help eliminate thermal bridging at all? I have read they are easier to work with, save labor, don’t know about cost though. Those could be filled with either closed cell spray foam or maybe for something way cheaper, dense packed cellulose with the intello on the inside? Can you make the trusses deep enough to get enough R value though? Then I was wondering if you can put say 2″ EPS directly on the exterior of trusses then put OSB on top of it? Then underlayment and asphalt shingles. Huber R sheathing does it, but I haven’t found another nailbase doing the same thing using EPS. I don’t want to use Huber because it’s Polyiso and pointless for my climate. I don’t want to have to make a foam sandwich with two layers of OSB, might as well use the SIPS in that case. Another consideration is in the great room which is timber framed, I may go with tongue and groove ceiling, but probably just sheetrock elsewhere. I’m not sure how this will work either with having two different types of framing going on. And I gather there are rules on how much exterior foam you have to use to pair with cellulose, but not with spray foam.
For the walls I’m going for R-30 so I was thinking painted sheetrock, 2×6 advanced framing with d.p. cellulose (do i need Intello?), OSB, 3″ EPS in 2 layers taped, (do I need a WRB – what about insects??), battens to vent, then composite siding like Everlast. Or maybe an EPS nailbase product instead? I could also try to find recycled EPS to save money, if there is any around.
Thanks for any info you can help me with!!
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