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Building Science

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

Josh Salinger shows us how to create more space and vent a roof, and do it all with a reduced environmental footprint.

Josh Salinger, owner of Birdsmouth Design-Build, is on-site in Portland, Ore., to discuss the vaulted high-performance and foam-free roof assembly on their latest ADU build.

Vaulted roofs are a popular option because they create usable living space where an attic would usually be. Typically, plastic forms such as spray-foam insulation or rigid foam are used to deter condensation and moisture issues in a roof assembly.

In this video, Josh shares his plastic-free solution to manage moisture and allow for full-depth insulation with proper venting while creating a high-performance vaulted roof assembly.

Read more about all the details of this roof assembly in Josh’s article, “A New Take on Insulating a Roof.”


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11 Comments

  1. Jon R | | #1

    What are your thoughts about the alternative method using "RafterCaps" (or similar) as described here?

    After blowing in insulation, what is the average vent size after accounting for some upward bow to the Tyvek?

    1. Doug White | | #4

      Have to buy and install a lot of rafter caps (proprietary) assume this wont be a cost savings versus shimming out roof CDX with 1 x strips…also this is a very “fussy” install with the sticky peel and stick would be problematic on top of the roof versus just laying out the commercial tyvek and nailing , taping exposed seams with tyvek tape . Most peel and stick is hard to handle even when just laying it out flat in one long sheet ! And the commercial tyvek is going to be yopu send layers of WB as the CDX /ZIP will have to be covered with its own WRB.

      1. Expert Member
        Malcolm Taylor | | #5

        'versus shimming out roof CDX with 1 x strips"

        The material needs to be 2"x to both meet code and be effective.

      2. Jon R | | #6

        No need for peel and stick and if you are duplicating the function of a RafterCap, you can use 1x strips (attached to the upper side of the rafter, not the upper edge). This does leave less room for fluffy insulation. But might be worth it if you wanted a more that 1" vent (which is my guess for the average vent size with above rafter 2x4s after accounting for Tyvek bowing caused by dense pack insulation).

      3. Expert Member
        Josh Salinger | | #8

        The issue with using plywood sheathing and strips above (likely needed 2x's for ventilation) is if you are using plywood in lieu of a highly permeable membrane, it in effect becomes the primary vapor control layer (the least permeable part of the assembly) as the plywood has a perm rating of around 10 perms. The smart membrane below will vacillate between (roughly) .1 and 10. When the plywood becomes the primary vapor control layer, then this part of the assembly becomes the point of concern since its on the cold side of the assembly. It is essentially open to the universe as the vented area above is no different than the area above the roofing-- the same as just stuffing a rafter bay with fluffy insulation.

        If one is using a highly vapor open membrane such as Tyvek, which has a perm rating of 53 perms, the lower membrane on the warm side of the assembly becomes the vapor control layer. The lower membrane controls the vapor diffusion into the assembly and is warm so there is no condensation, the upper membrane is wide open so the vapor that gets in can escape through it into the vented area above (this depends on the season, RH, interior temps, etc., but the principle stands). The key to this detail is that the smart membrane is not just controlling the vapor, but also is the air barrier.

    2. Expert Member
      Josh Salinger | | #7

      One of the issues we were trying to solve with this roof system is how to vent roofs that aren't simple gables. The problem with the 'RafterCaps' or creating a vent chamber below the roof sheathing via baffles or a site made chase is what does one do if one has a valley, hip, dormer or other obstruction to either the bottom or the top of the rafter bay. By bringing the vent chamber over the top of the valleys, hips, etc. it completes the ventilation from the intake to the ridge. It also has the benefit of using this space for insulation instead of air.

      I have gotten the Q about the bowing many times and I don't have an empirical answer to it. I assume it is bowing, especially at 24" OC and a good dense pack install. That being said, I also know that cellulose settles over time and whatever bowing there is will likely be reduced, but as to how much is anyone's guess. It probably has a lot to do with the quality of the install. In a strange twist, a poorly installed cellulose job would likely reduce the bowing issue.

      The only way to really find out would be to peel off the roof after insulation. Unfortunately, I have yet to find a client who is willing to have us rip off the roof of their new home to check on the insulation install...

  2. ScottOHara | | #2

    Thank you Josh. I really like that detail. Have you used it on a lower sloped roof, say 2 in 12, that is common on a more modern home design? I build in the SF Bay Area where the modern design is popular but I've stopped doing it for spec homes because of the amount of foam needed. This seems like a good alternative if the air would move through the vent on a low slope.

    1. Expert Member
      Josh Salinger | | #9

      My hunch is that the lesser sloped roofs wouldn't perform as well. I can't say if it wouldn't perform at all since there are so many variables with not just roof pitch but other factors such as roof color, size of vent cavity, etc. One needs 2 holes and a driving force. The driving force of stack effect would be reduced if the slope is minimized. If I were to do it on a low sloped roof I would make the vent chamber as large as possible and go for a dark colored roof. Peter Yost at his 'Wing Nut' testing blog has some great information on this here:

      https://building-wright.com/2019/03/22/wingnut-testing-soffit-to-ridge-roof-venting/

      1. ScottOHara | | #10

        That was my thought as well. Just about all of the houses I build are in WUI so adding vent area may be a challenge. Thanks for the link, I'll check out Peter's information.

      2. Jon R | | #11

        +1 on lots of variables with no method of accounting for their effects.

        Note that wind is the larger driving force and this doesn't change much with slope. Also that with the high performance Majrex and careful air sealing, venting requirements go down significantly.

        > SF Bay Area ... amount of foam needed

        A huge effect. Given the monthly average outside air temperature of the three coldest months, the IRC allows zero inches, even with no foam. So my guess is that a 1" vent in a 2:12 roof would perform very well.

  3. Doug White | | #3

    Elegant, not expensive, easy to maintain quality control-brilliant.

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