Great information as always! I think a diagram of a recommended wall, floor deck, and roof assembly would be really helpful. This is a good example of what not to do, but if you were to design the whole structure from scratch, how should it be done? Thanks again.
Funny you should bring that up :) It occurred to me as well after we shot that video but at that point it was already released into the wild. So on our short list is to get a video out on cathedral roof designs that work well in cold climates - and are actually a little easier to build. I'm considering mocking up the options in the shop to scale for clarity. Some of the design decisions depend on whether the roof requires a structural ridge beam or not. In the case of this video, the rafter pairs went straight down to the loft floor forming a triangle. It was also a full loft (not a partial loft open to below) so no structural ridge was required as the continuous loft floor created a structural diaphragm that kept the rafters from pushing outwards under load. That said, for this design my personal go-to would be to platform frame the loft floor on top of the walls (I'd prefer that over hanging the floor inside the walls off ledger boards -but it could be done either way). Platform framing would give the builder square uncomplicated joist bays that could much more easily be insulated and air sealed with foam board or spray foam. The rafters would then rest on wood pitch blocks that are structurally screwed to the top of the subfloor. With this method, the rafters maintain full depth as no birds mouth cuts are required and the vapor retarder can be readily sealed to the subfloor where it meets the face of the pitch block. Our intent is to cover exactly this in a video complete with 3D drawings. Another cathedral roof video we filmed about 10 years ago covers the same concept but using a structural ridge and cutting the eave wall studs at the roof pitch to avoid the birdsmouths in the rafters thereby maintaining full insulation depth over the walls. th-cam.com/video/mOK-D3rsomw/w-d-xo.html
Just wanted to say I love your videos and have watched all of them! Just great building science with practical application. The information you provide is useful for people living in warmer climates as well. Maybe someday you could do a collaboration with Matt Risinger or one of the big TH-cam builder personalities to boost the channel's exposure.
Thanks for the feedback. I agree on the collaboration for the exposure. I think with a little heads up we could come up with some interesting content as the custom home builders in particular here in Alaska do some really interesting stuff worth documenting. Interior Alaska and Coastal Alaska are some of the most challenging environments to build durable energy efficient structures in.
@@ilyabenesch9469 Would love to watch that. Maybe you could do a tour of a mid-construction home and point out all the stuff that's relevant/interesting? Something like that would be rad!
Great advice - I wish I'd seen this a year ago. I built an A frame (2x12 rafters) in climate zone 7, with a loft sistered to roof joists. I spent probably 3 full days taping around joists, cutting strips, and generally fussing around, and I'm still not super confident in the end result
If you had the budget, would full closed cell spray be better? Would you then eliminate the air space? I believe open cell would be incorrect due to humidity transfer to the exterior.
For us, the quantity of closed cell spray foam needed to fill the bays would be incredibly expensive - and in theory with 2x10 rafters, that should be enough insulation at R-50+ to avoid ice dams. The wrench in the works is the rafters are still at some risk for heat losses through thermal bridging and whether those heat losses are enough to create hot spots on the roof deck would require a thermal analysis - particularly since the roof is framed 16" on center and the rafters are solid wood not I joists. That's a decent amount of wood in the assembly at ~ R-9. In the video, that 2" of foam board across the bottom of that roof effectively doubled the r-value of the rafter locations in the roof. As an aside, I'd still want to see a 6 mil polyethylene vapor retarder across the bottoms of the rafters as relying on spray foam to provide the air sealing (pressure boundary) long term has a mixed history here - particularly on structures like this one that could experience ground movement. We recommend the vapor retarder with SIP roofs as well for the same reasons. If the joints in the SIPS ever fail then indoor air leakage into the cracks can transport a lot of moisture into the roof assembly.
Great information as always! I think a diagram of a recommended wall, floor deck, and roof assembly would be really helpful. This is a good example of what not to do, but if you were to design the whole structure from scratch, how should it be done? Thanks again.
Funny you should bring that up :) It occurred to me as well after we shot that video but at that point it was already released into the wild. So on our short list is to get a video out on cathedral roof designs that work well in cold climates - and are actually a little easier to build. I'm considering mocking up the options in the shop to scale for clarity. Some of the design decisions depend on whether the roof requires a structural ridge beam or not. In the case of this video, the rafter pairs went straight down to the loft floor forming a triangle. It was also a full loft (not a partial loft open to below) so no structural ridge was required as the continuous loft floor created a structural diaphragm that kept the rafters from pushing outwards under load. That said, for this design my personal go-to would be to platform frame the loft floor on top of the walls (I'd prefer that over hanging the floor inside the walls off ledger boards -but it could be done either way). Platform framing would give the builder square uncomplicated joist bays that could much more easily be insulated and air sealed with foam board or spray foam. The rafters would then rest on wood pitch blocks that are structurally screwed to the top of the subfloor. With this method, the rafters maintain full depth as no birds mouth cuts are required and the vapor retarder can be readily sealed to the subfloor where it meets the face of the pitch block. Our intent is to cover exactly this in a video complete with 3D drawings. Another cathedral roof video we filmed about 10 years ago covers the same concept but using a structural ridge and cutting the eave wall studs at the roof pitch to avoid the birdsmouths in the rafters thereby maintaining full insulation depth over the walls. th-cam.com/video/mOK-D3rsomw/w-d-xo.html
Just wanted to say I love your videos and have watched all of them! Just great building science with practical application. The information you provide is useful for people living in warmer climates as well. Maybe someday you could do a collaboration with Matt Risinger or one of the big TH-cam builder personalities to boost the channel's exposure.
Thanks for the feedback. I agree on the collaboration for the exposure. I think with a little heads up we could come up with some interesting content as the custom home builders in particular here in Alaska do some really interesting stuff worth documenting. Interior Alaska and Coastal Alaska are some of the most challenging environments to build durable energy efficient structures in.
@@ilyabenesch9469 Would love to watch that. Maybe you could do a tour of a mid-construction home and point out all the stuff that's relevant/interesting? Something like that would be rad!
Great advice - I wish I'd seen this a year ago. I built an A frame (2x12 rafters) in climate zone 7, with a loft sistered to roof joists. I spent probably 3 full days taping around joists, cutting strips, and generally fussing around, and I'm still not super confident in the end result
Yep it's a very very strong roof.....but the details are hard to air seal. Good on you for giving it your best effort as that WILL make a difference.
If you had the budget, would full closed cell spray be better? Would you then eliminate the air space? I believe open cell would be incorrect due to humidity transfer to the exterior.
For us, the quantity of closed cell spray foam needed to fill the bays would be incredibly expensive - and in theory with 2x10 rafters, that should be enough insulation at R-50+ to avoid ice dams. The wrench in the works is the rafters are still at some risk for heat losses through thermal bridging and whether those heat losses are enough to create hot spots on the roof deck would require a thermal analysis - particularly since the roof is framed 16" on center and the rafters are solid wood not I joists. That's a decent amount of wood in the assembly at ~ R-9. In the video, that 2" of foam board across the bottom of that roof effectively doubled the r-value of the rafter locations in the roof. As an aside, I'd still want to see a 6 mil polyethylene vapor retarder across the bottoms of the rafters as relying on spray foam to provide the air sealing (pressure boundary) long term has a mixed history here - particularly on structures like this one that could experience ground movement. We recommend the vapor retarder with SIP roofs as well for the same reasons. If the joints in the SIPS ever fail then indoor air leakage into the cracks can transport a lot of moisture into the roof assembly.