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Community and Q&A

Wall and cathedral ceiling assemblies

Jon_Lawrence | Posted in Plans Review on

I am trying to finalize my assemblies for my Passive House and would like to solicit some feedback at this point. The project is located in NJ in zone 4a, although I am only a few miles from zone 5. I would like to avoid high GWP products. Attached is my proposed wall/cathedral ceiling assembly.

The wall assembly is as follows:
1) Fiber cement siding
2) 1×4 vertical furring strips to provide a nailing surface for the siding and a rain screen.
3) 3” of Roxul ComfortBoard (thermal barrier)
4) 7/16″ Zip sheating (air barrier and WRB)
5) 2×6 studs, 16” OC filled with DP cellulose
6) IntelloPlus air barrier/smart vapor retarder
7) 2×3 horizontal furring strips installed on edge for service cavity
8) 5/8” gypsum

The cathedral ceiling assembly is as follows:
1) Asphalt shingles
2) GAF ThermaCal1 nailbase comprised of 7/16” OSB, 1” vent cavity, 4” Polyiso (thermal barrier – air/vapor barrier?)
3) 5/8” Zip sheathing (air barrier and WRB, may be able to use plywood if nailbase does provide the air/vapor barrier)
4) 2×10 rafters with 9.25” DP cellulose
5) IntelloPlus air barrier/smart vapor retarder
6) 2×4 service cavity on flat section for light fixtures, no service cavity on sloped section.
7) 5/8” gypsum

The gable roof assembly above the attic would be similar to the cathedral ceiling assembly except that the Intello would wrap up 2nd floor wall, then between the top plate and the floor joist, then up the outside edge of the floor joist, then between the rafter and the attic floor and then up the inside of the rafters.

When I ran my energy model, I was able to meet PH standards using triple pane windows and lower levels of exterior insulation. The reason I have used 4” of Polyiso is to keep my exterior/interior R-value ratios above the minimum needed to avoid condensation (I de-rated the Polyiso to 5 as condensation would become a concern at close to 0 degrees outside temps). I did the same for the walls, but I am not sure if the same rules apply to mineral wool since it has a high perm rating allowing the wall to dry to the exterior. Just for kicks, I also ran the model using these insulation levels and code minimum U-.3 windows and was not able to meet PH standards. The house has a lot of glazing, but a favorable surface area to volume ration of 17%.

I like the wall assembly because it allows drying to both the interior and exterior. I am unsure about the Polyiso on the roof. I saw Kohta’s presentation at NESEA and it left me with an uneasy feeling about my planned conditioned attic and cathedral ceiling. Ironically I could avoid exterior roof insulation and still meet PH standards, but the code requires me to add more insulation which could potentially place my roof at risk for failure. I know Roxul has commercial roofing products, but I have not seen any residential exterior roofing products from them. Maybe I am safe with the IntelloPlus air barrier and keeping the wintertime RH under 40%, especially when temps drop into the teens?



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  1. GBA Editor
    Martin Holladay | | #1

    Your plan looks good to me.

    Presumably, you are aware of the squishy nature of Roxul ComfortBoard -- and you are willing to put up with the aggravation of making the furring strips co-planar, and you will proceed happily in spite of your knowledge that rigid foam would be easier to install.

    The IntelloPlus is unnecessary for the ceiling assembly, but it's harmless if you use it.

    You don't really ask any questions.

    You noted, "I am unsure about the polyiso on the roof." That's a vague statement. What makes you uneasy? I think that the polyiso will perform just fine.

  2. Jon_Lawrence | | #2


    Thanks for the response. I was afraid that I was going to wake-up to find only Chinese text.

    I have used Roxul batts before and, while I like the product, I am amazed that people were able to use them for continuous exterior insulation prior to the introduction of ComfortBoard. And yes, the ComfortBoard is still a bit squishy, albeit much less so, and we will have to keep the clutches on our drills at the same setting. My goal is to create a very durable wall by using a vapor-open exterior insulation, so the trade-off is needing to pay more attention to get the furring strips all in the same plane. I may go with the 110 over the 80 for its higher compressive resistance.

    I live in 475 country, so I have been brain washed into believing that the more air barriers, the better. I have to put netting up for the DP anyways, so a little belt and suspenders in the form of Intello is ok by me. We have not done any value engineering yet, so that may not make it to the final plan, at least as the walls are concerned.

    I apologize for the vagueness regarding the Polyiso, so let my clarify. Those pictures of moldy OSB from Kohta's presentation are engrained in my brain. My proposed roof assembly will only dry to the inside so I have less room for error. The nice thing about using a naibase product in this scenario is that any bulk water that may penetrate the shingles will dry via the vent cavity. So my concern should only be condensation. I think with the correct ratio of exterior to interior insulation, managing wintertime humidity, and at least in the roof sections, using Intello for additional vapor management, I should be ok. I feel much better knowing that you think so too.

    I am working on the foundation details now and will post those as soon as I am done.


  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "Those pictures of moldy OSB from Kohta's presentation are engrained in my brain."

    Then it's a good idea for you to install one or more layers of rigid foam on the exterior side of your roof sheathing. The rigid foam keeps the OSB warm, dry, and mold-free.

    Without the rigid foam, the OSB would be cold during the winter -- and therefore much likelier to develop mold.

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