Oregon coast house build
Hi all…I’m getting close to starting my build here on the south Oregon Coast and I’m down to figuring out which membranes to use. The roof: I have an unvented vaulted roof with (from inside to out): wood ceiling on 2x furring; Intello or Siga smart membrane; 2 x 12s with R-38C batts; 5/8” plywood sheathing, self-adhering WRB, 4” of rigid insulation (probably OC Foamular 250); another 5/8” plywood sheathing; roofing underlayment; aluminum standing seam roof. Walls: (from inside to out): 5/8” painted drywall; 2 x 6s with R-21 batts (OC pink stuff); 5/8” plywood sheathing; self-adhering WRB; 3-1/2” of Rockwool (2 staggered layers); 1 x furring; exterior cladding (either standing seam metal or a fiber cement panel). Given my 4C zone with humidity routinely 50% inside and 70-80% outside, I’m not sure how to think about perm ratings on the WRBs and what makes sense for the metal roof underlayment. Also, is the Intello necessary given the R-20 of exterior insulation on the roof? Would really appreciate some advice and smart thinking from this group. Thanks in advance!
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Typical unvented roofs would have painted drywall as a warm side vapor retarder/air barrier. Since you are looking at wood slats, you won't have an air barrier so a membrane of some kind is needed. Any of the variable perm membranes will work.
The membrane over the 1st roof deck does not need to be vapor open. For these types of roofs, I usually use ice and water shield as it serves dual purpose as the main roof air barrier plus it gets the place under roof and dried in before the rigid and roofing goes up.
The permeability of the underlayment under the metal roof panels doesn't matter. My local yard stock a permeable synthetic underlayment which is what I've used.
For the walls, you have more than enough rigid for condensation control, so the permeability of the wall WRB doesn't matter. It is still best to use something permeable though to allow for drying to the exterior for a more robust assembly, many options out there for that. Most peel and stick wall WRBs can also be installed vertically which is a simpler install in a lot of cases.
Akos,
Leaving aside any advantages in terms of construction sequencing, if the variable-perm membrane near the bottom of the assembly is detailed as the primary air-barrier, is the membrane over the first roof deck necessary at all?
IMO, the membrane over the first layer of sheathing is not absolutely required for long-term performance, but it's a good idea anyhow. Akos mentioned the benefit of getting the house under a dry roof quicker and that's certainly worth something. I've also had experience with wind damage to a similar roof. The wind damaged the flashings and roof surface at an upper edge transition and the damage was bad enough that water was definitely getting in. We didn't know about it for several weeks until someone spotted it from the ground, since the water was running down the inside layer of Ice&Water and exiting at the eaves. That was worth the price of the product.