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Detailing a Wall-to-Roof Truss Transition

An approachable Passive House–level assembly with raised-heel trusses, exterior wood-fiber insulation, and plywood air barrier

In the first post of this two-part series, I talked about a method for tying a foundation to the wall of a house built on a concrete-free slab. The result is an energy-efficient connection at the rim joist—an area notorious for air leakage. Here, I’ll explain detailing the transition from the wall to the roof system. As previously mentioned, both assemblies come from a certified Passive House build and they meet the zero-energy objective.

Plywood air barrier
CDX plywood air barrier and integrated MEP service cavity at parallel chord truss ceiling framing

Starting with the wall

The wall itself is a 2×6 stud wall with intermediate framing with studs at 24 in. o.c. We sheath the wall with 1/2-in. CDX plywood with the seams taped using Siga Wigluv to continue the air barrier up the wall. The stud bays are insulated with dense-packed cellulose, blown in and tested to a density of 3-1/2 lb. per sq. in. to prevent any future settling. This makes the CDX plywood the least permeable part of our assembly at approximately 10 perms, therefore making it the vapor control layer. This is the layer we are most concerned about seeing condensation on the coldest stretches of the year and therefore we want to keep it warm enough to keep it safe.

To keep the sheathing warm, on the exterior of the wall we install 1-3/8-in. Steico wood-fiber rigid insulation board. We like this product because of its low embodied carbon and ease of installation with the tongue-and-groove profile that can be installed “off layout,” meaning the seams don’t need to line up with the studs to install it. Because this is a wood product being installed in a climate that sees 42 inches of rain on average per year, we install a mechanically attached water-resistive barrier (WRB) to…

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6 Comments

  1. brendanalbano | | #1

    Having done it both ways (if I'm following your articles correctly), what determines whether you use plywood or use a membrane like Siga Majrex as the air barrier on the underside of the trusses?

    1. Expert Member
      Josh Salinger | | #4

      Good question and thanks for paying such close attention! Our team went back and forth on this detail for this particular home. Due to the need to hit a perfect trim reveal between the sloped vaulted areas and the window trim it made more sense to go with the thinner of the two options. Since the Majrex requires a 2x4 strapping it is actually thicker than the plywood. This is in the weeds, for sure. Plywood acts similar to a variable permeability membrane as it will change permeability based on the absolute moisture it is experiencing. Although it won't throttle quite as much as an Intello or Membrain product it will still control vapor entering into the assembly. We simply made this decision based on a design constraint.

      Interestingly, the carpentry team prefers to install the membranes as they go up quicker and are less hassle to hang on a tall sloped ceiling. This decision was made after the plans had been submitted for permit so we didn't have an easy path to make adjustments to the building height to switch to the plywood so we stuck with it.

      I would feel comfortable recommending either the membranes or the plywood for a vented roof assembly and would tell folks to go with what is either the most available, cost effective, easy to install or works best within any particular circumstances.

  2. creativedestruction | | #2

    14" of loose blown cellulose will not yield R60. It will be closer to R49. Not that this will result in a substantial difference to energy loss, but worth double checking calcs if targeting a PHIUS cert or current IECC.

    1. Expert Member
      Josh Salinger | | #3

      You caught a typo. We have 16" of cellulose in the attic. From the Greenfiber 'Santuary' cellulose spec: R-Value at 75 degrees Mean Temp at a settled thickness of 16.1" is R-60. This information along with the spec sheets have been uploaded to the Phius worksheet and approved.

  3. dillbill | | #5

    How are you sealing the area around the vented blocking/truss heels from water? I'm having trouble seeing how it's continuous above the taped cdx corner with the wrb outside on the insulation and what looks like tricky tight spaces between the blocking, strip of sheathing, and roof structure.

    1. Expert Member
      Josh Salinger | | #6

      I'm trying to follow your question. I think the problem may be that the pic you are referring to isn't yet complete. There is a front porch roof that is attached in that location that continues the slope of the roof down. Once this was installed, sheathing went over the tops of the trusses and over the vented porch framing. If you look deeper into the photo on the adjacent side wall you see how the trusses have tails that will create the overhangs that house the vented soffits. The vented blocking is protected by the installed overhangs, and in the case of the pic, the porch roof.

      I hope this clarifies things...

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