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Community and Q&A

Transitioning Control Layers from “Perfect Wall” to Roof

TateL | Posted in Green Building Techniques on

I have been doing a lot of research in preparation for building my own house on the perfect wall assembly. This has involved watching a lot of Matt Risingers’ videos, as far as I can tell he has shown several different versions of the “perfect wall”. But the part I am getting hung up on is how to transition the 4 layers (rain, air, vapour, thermal) of protection from walls to the roof. On Matt’s personal house build, he did the “monopoly” framing by adding eaves after making the protection transitions with extending cut LVL boards off the roof deck. In another video, he shows an affordable version of the perfect wall by Scott ( they show Zip sheathing running up the wall, then under and around the roof truss tails, creating the eaves and carrying the protective barriers up to the roof deck. Unfortunately, he didn’t go into further detail about the exterior insulation.

Therefore my question is, this second method seems more affordable to do and easier, but wouldn’t you have to carry the exterior insulation up and around with the exterior sheathing? If not and you have exterior insulation on the walls and roof, the soffit and fascia areas would be exposed creating a potential for condensation on the sheathing.

I have depicted these details with the attached photos, would like to hear how people have completed this detail of transitioning the 4 layers of protection from wall to roof.

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  1. Expert Member
    DCcontrarian | | #1

    Generally a vented roof is cheaper than an insulated roof.

    Generally it's cheaper to build a wall for your climate rather than a wall that works in any climate. What climate are you in?

    Generally it's cheaper to use common building materials rather than exotic ones.

    1. Expert Member
      Akos | | #3

      "Generally it's cheaper to use common building materials" +1

      By plus one there, I mean simply use more of common building materials to get better performance.

      Anything exterior rigid only makes sense if you are tight on space otherwise stick to thicker walls and simple vented roof with trusses and lots of loose fill attic insulation. 2x8 24" OC wall with R30 batts is good enough in most climates and can be built for much less money than anything with exterior rigid.

      With a bit of planning, some attention to details and elbow grease, there is no reason why you can't get this bellow 1ACH without any of the complications of non-standard construction.

      1. matthew25 | | #5

        I agree that exterior insulation can add complications and cost. But I wonder what your thoughts are on the more traditional vented attic assembly.

        The air-tight drywall ceiling approach is certainly mentioned a lot among people in this community but I wonder how "standard" it is in reality among common builders? For example, the Jake Bruton or Steve Baczek detail of having a piece of horizontal zip sheathing above the top plate as an attachment point for your air tight drywall ceiling definitely seems a little complicated and non-standard. The easiest, most continuous, air barrier would definitely be the monopoly framing style where the WRB can easily stretch directly from the walls onto the roof sheathing without interruption. I'm not suggesting the OP should use this method, especially now that I know he is in CZ5. I think the vented attic would be a good economical choice, but curious on your thoughts on this. Getting an air tight drywall ceiling with all of the mechanical and lighting penetrations just doesn't seem like a no-brainer "standard" detail to me.

        1. Expert Member
          BILL WICHERS | | #6

          You don't need anything fancy for the airtight drywall to attach to above the top plate -- the air barrier is the DRYWALL here, and the air seal to the wall is the mud and tape in the corner, not the attachment point for the drywall to the framing. If you use sealant for the drywall on the walls, the ceiling drywall doesn't really need extra sealant, since it only needs to "seal" the drywall on the wall, not the framing in the wall.

          Sometimes people overthink air sealing details. I like the way of describing the building envelope as "something you can draw without pickup up your pen". Keeping joints simple, so only two things that are easy to connect are involved in making the seal, also helps. When doing things this way, the "complicated" part is down around the mudsill/rimjoist, not up in the ceiling. This does assume the top level ceiling though, intermediate ceilings betwen first and second floors are a little more complex, since those need to connect the upper and lower wall air barriers together, with the ceiling essentially irrelevant.

          The easiest way to get a redundant air barrier, and what I prefer to do, is to detail the interior drywall airtight, which means a bead of sealant around the perimeter of the wall (you don't need to seal intersections between panels in the middle of the wall, the mud/tape at joints takes care of that already), and then to do the same with the exterior sheathing as the secondary air barrier. By keeping the air barriers as planes -- flat things like drywall and plywood -- you help to avoid having as many complex intersections that need fancy things to keep the air barrier intact.

          Regarding the original question and the "perfect wall" though, I agree with the other posters that you're better off building a wall that works for your climate zone. You're not going to move your house somewhere else, so there is no need to make it "work" in other climate zones than the one you build it in initially. There are still issues sourcing some materials too, so using commonly available materials helps to prevent you from running into schedule busting shortages and leadtime issues.


          1. Expert Member
            DCcontrarian | | #7

            That's true if you make the interior drywall your air barrier. It's when you make the exterior sheathing your air barrier that the transition at the ceiling becomes complicated.

            There are pros and cons either way. The drywall tends to have a lot more penetrations than the sheathing.

          2. andy_ | | #10

            Granted, I don't have a ton of experience with this approach, but from standard builds I see a lot of other air leak paths in the drywall than just the ceiling to framing. The electrical outlets, can lights, smoke detectors, multi story electrical runs (though those should be fire blocked), the drywall being above the floor by 1/2", countless framing penetrations, corners, etc all seem like they could add up to significant leakage. Top plates are notorious air leak sources.
            The other thing I worry about is the drywall crew doing any sealing when they're being paid by the sheet or square foot and have immense pressure to work fast in order to be profitable. Introducing a new step for them might not go so well or be 100% executed unless you have a really good crew.
            I don't want to say that airtight drywall is impossible, just that it's hard for me to see it as the silver bullet some think and even a decent outcome will still take very careful attention to detail and extra effort.
            I can see why builders like Jake Bruton have been trying other approaches like service cavities below an air seal layered attic.

    2. TateL | | #4

      I am in zone 5.

  2. matthew25 | | #2

    Scott’s method doesn’t use exterior insulation if I recall correctly. If you want to use exterior insulation, do the method Matt did on his house with the cut down outriggers. That’s how I plan to do it when we finally get a chance to build new.

    1. Tim_O | | #8

      Scott was building his own house with exterior insulation, but I don't know how he did the roof to wall transition.

      To the OP, Scott has an Instagram and youtube channel of his own, might be worth an ask if you are really interested in pursuing that method.

  3. Expert Member

    I think applied eaves, especially when there is exterior insulation, are a disproportionately complex solution to maintaining the continuity of the air-barrier. There are a lot easier ways to bridge between the walls and roof, wherever the two air-barriers are located.

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