Building a Vaulted, High-Performance, and Foam-Free Roof Assembly 

Josh, very nice work! There was a brief comment below about a radiant barrier on the top-side membrane. The overall theory I've been contemplating is to add a reflective foil membrane on the top-side, which reflects radiant energy back into that 1 1/2" vent channel. Reflecting that radiant energy back into that vent channel should transfer that heat directly into the air that is moving up, because expanding air has a natural buoyancy which helps that heated air to rise up to the ridge vent. Simple physics, actually (cf. gas laws: PV/T is a constant). I also really like the addition of standard 2x4s to create that air channel on the top-side. Depending on the nailing schedule, it also results in adding much additional strength and stiffness to the rafters. As such, a similar assembly could be very easily adapted to jobs where old roofing is being replaced: pick up at the point where all old shingles are removed, with bare plywood remaining. A 1/2" rigid insulation with foil backing can be quickly ripped on a table saw and placed between those 2x4s, leaving a 1" vent channel. Alternatively, 2 layers of 2x4s create an air channel that is 3" deep, allowing room for rigid insulation that is thicker e.g. 1" or 1 1/2". I'm sure your capable carpenters can work up the soffit intake and ridge vent details: piece o' cake for capable professionals like yourself. KEEP UP THE GOOD WORK!

So glad I saw your video on unvented vaulted ceiling and moisture mitigation. I was going to do Vapor diffusion Port but there is no room in my roof assembly to allow free air flow from soffit to the ridge. I thought I had to use closed-cell foam + mineral wool until I see this. Thank you!

was there any thoughts of using a radiant barrier on top instead or with the tyvek?

I also live in the Portland Oregon area and wonder if Josh would use this assembly for a very low slope roof?

Are you using longer nails (16d instead of 8d or 10d) to attach your roof sheathing?

Interesting. I just purchased a house with vaulted ceilings.. is there anything I can do to better insulate it. Or would I basically need to tear it all apart and rebuild it?

The low-slope side of your roof may have more issues with venting (less stack effect) did you change the assembly on the low slope side or do you have a working minimum slope for the 1.5" vent cavity to work? Also be careful not to use pva primer on your drywall!

"We're using no plastics!" First product used is Tyvek wrap

It seems that thermal bridging is not considered in any way with this strategy though?? Also, is the water control layer going to be on the roof decking? Will it be taped at the joints to also become another air control layer effectively having 2 air control layers?

How many of these have you built to date?

I’m confused as to why the interior membrane “closes down” with high humidity going from interior to exterior, yet “open up” to let humidity escape going from exterior to interior to dry the insulation cavity. How can you have it both ways?

Nice, but this system still has thermal bridging through the joists. That could be minimized with TJIs or with a rigid foam on the interior under your nailers, but I understand you are trying to avoid foam.

curious could you not just use 1X strapping instead of the 2x4??

You now have a thermal bridge every 16 or 24 inch's. Could be done better

$399 for one roll? Siga must be very proud of this product. Finehomebuilding should also highlight less expensive alternatives to solve problems like this. With my build I'd have to purchase three rolls for around $1200 bucks. That ain't in my budget. I guess "Finehomebuilding" means expensive, uppercrust homebuilding instead of middle-class budget minded homebuilding. Thanks for the vid, though, because it has given me an idea on how to insulate my attic space.

Origami?!!! Lol. I laughed. Cellulose compacts down I have read. So???? Blown out n loose fibreglass or rock wool is expensive. Maybe I could do this for my house next year. Post frame q 8’ 60’x40, maybe use floor trusses instead of roof trusses. Somehow make a vaulted ceilings out of them then use your idea mirpex bottom insulation then tyvek then maybe 2x4 on end with 5” Perlins screws for the air space and steel panels

Hold on. There is nothing rigid against the membrane?? I think you just made the world’s biggest harmonica. Every time there is a wind storm, the homeowner is going to hear that flapping around, no?

Why not just use regular vapor-impermeable membrane (0.2mm polyethylene sheet, for example) on the inside? The "smart pores" cost a lot of money for zero actual benefit.