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Wall & roof assembly questions for Zone 7/8 — new build

Chris Armstrong | Posted in Green Building Techniques on

New build in zone 7/8, Colorado mountains at 8,600 feet elevation. Construction will start in a few months so I still have time to work through some details.

I have a few questions on the wall and roof assemblies that I was hoping to get opinions on.

The above grade walls will be 2×6 with dense pack cellulose cavity insulation, sheathed in plywood with a WRB and then 4″ of rigid mineral wool overinsulation.

The below grade walls will be 8-12″ CIP concrete, interior 2″ EPS foam with taped seams, then 2×4 stud walls

There is a cathedral ceiling in part of the house that will be 16-18″ TJI’s with cavity insulation. The interior finish is a structural tongue and groove.

The remaining ceilings are flat with traditional roof trusses and will be insulated with loose fill cellulose.

My questions:

– For the above grade walls I was planning on using a smart interior membrane as both vapor and air barrier. I was not planning on using a service cavity, instead I was planning to use air tight electrical boxes and chase walls for plumbing. I am wondering if the interior membrane is necessary or if it makes sense to tape the seams in the sheathing instead and use that as the air barrier. I am not sure if a vapor barrier is required by code, I believe they are still on 2009 IBC.

– Detail for continuation of air barrier at rim and band joists – I was planning on using this detail from four seven five and inboarding the band joists by 2″ to allow the WRB to be wrapped around the joists and tied in to the interior membrane.

https://foursevenfive.files.wordpress.com/2012/02/wall_fig-17_010000small.jpg

The 2″ cavity exterior of the band joist would be insulated with roxul and then sheathed over. I wanted to avoid using spray foam in the build but I wonder if it would be more cost effective and/ or fool proof.

– In the basement cavity walls, since they are separated from the concrete by 2″ of EPS can I use dense pack cellulose, or would it be safer to use roxul? No interior membrane would be used on these walls, the taped EPS would serve as air/ vapor barrier.

– For the cathedral ceiling – this part of the house is timber framed and the tongue and groove ceiling is structural. It would be built from the inside out by applying interior membrane to the top side of the tongue and groove, then over framing the ceiling with I-joists on top of that. The cavity will be insulated then WRB will be installed over the top of the I-Joists, furring will then be installed over this to create a vent space. Should I insulate this cavity with dense pack cellulose or would Roxul be a better choice?

– I was planning on using either Siga or ProClima products for the membranes and associated tapes and adhesives. Is there a clear better choice between the two? It appears to me that Proclima Intello plus may have an advantage over Siga Majpell for the interior membrane simply because it is a smart membrane. Both brands have very good reputations for the quality of their tapes. Siga is distributed by a local vendor so I would not have to mail order it, this has me leaning toward using Siga unless there is a good argument for Proclima. According to my take offs the cumulative cost for either brand will be very comparable.

– From anybody that has experience with mineral wool overinsulation, I am installing wood siding on furring strips as the wall cladding. Any input on the type of screws to use for attaching the furring strips? Heco topix therm are recommended, they are a double threaded screw and seem like a great option but they are expensive. I have also looked at pancake head SIPS and timber screws, are these a valid option and what other options are there that I haven’t considered?

Thanks for any input.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Chris,
    Lots of questions. It looks like I will tackle a few questions at a time.

    Q. "For the above grade walls I was planning on using a smart interior membrane as both vapor and air barrier."

    A. That will work, although it would also be possible to use drywall as your interior air barrier (especially since you plan to use airtight electrical boxes). Vapor retarder paint would satisfy the code requirement for a vapor retarder.

    Q. "I am wondering if the interior membrane is necessary."

    A. No, it's not.

    Q. "I am wondering if it makes sense to tape the seams in the sheathing instead and use that as the air barrier."

    A. I would certainly tape the seams of the plywood wall sheathing if I were you. An exterior air barrier is a good idea. Just because you have one air barrier doesn't mean that you shouldn't consider having two air barriers. Here is a link to an article on this topic: One Air Barrier or Two?

    Q. "I am not sure if a vapor barrier is required by code."

    A. It is not. Most codes require an interior vapor retarder, not an interior vapor barrier. However, there is no requirement for an interior vapor retarder is you install continuous insulation on the exterior side of the sheathing. Here are links to two articles with more information:

    Do I Need a Vapor Retarder?

    Calculating the Minimum Thickness of Rigid Foam Sheathing

  2. GBA Editor
    Martin Holladay | | #2

    Chris,
    Q. "In the basement cavity walls, since they are separated from the concrete by 2 inches of EPS can I use dense-packed cellulose, or would it be safer to use Roxul?"

    A. I would never install cellulose in a basement wall. Basements are occasionally subject to flooding events, and you don't want to install cellulose anywhere that might get wet. Mineral wool is a better choice.

  3. Bill Dietze | | #3

    Chris, what zip code are you building in? Curious if we're near each other.

  4. Chris Armstrong | | #4

    We are in Grand Lake

  5. Bill Dietze | | #5

    Chris, then I'm quite far - Placerville, CO: south and west. 350 mi by car. Warmer where I am - climate Zone 5 like January, but still ~9000 HDD at 9000' elevattion.

  6. Chris Armstrong | | #6

    Received some comments from an insulation sub looking at our job.

    He recommended polyiso on the interior face of the foundation instead of EPS. My reason for choosing EPS is to eliminate risk of wicking which I have heard can happen with polyiso. Curious to hear thoughts on:
    Polyiso vs eps in this location
    is 2" sufficient thickness assuming taped seams and 2x4 cavity wall with roxul
    does having it on the interior of the foundation wall reduce the derating in cold temps
    If we do use polyiso in this location should it be faced and with what

    He also said he has had a lot of success with fully adhered membranes in this type of assembly (exterior rigid mineral wool/ rain screen) and recommended grace vycor env-s which is vapor permeable at 15 perms. I chose plywood instead of osb for sheathing due to it being more vapor permeable than osb (10 perms vs 2) and its properties of increasing permeance as it gets wetter to around 35 perms at 100% mean RH. I wouldn't want to restrict this property by applying a membrane with only 15 perms - am I overthinking this? There are other fully adhered vapor permeable products on the market that have higher permeability: Henry blue skin 100 @ 33 perms, Henry blue skin 160 @ 29 perms, cosella dorken delta vent SA @ 31 perms dry cup, 50 perms wet cup, vaproshield wrap shield SA @ 50 perms, All per astm E-96 except vaproshield which is E398-13?

    Thoughts on self adhered vs taped sheathing with mechanically fastened wrb? Self adhered is more expensive per SF, I don't see a lot of discussion of self adhered membranes in QA.

  7. GBA Editor
    Martin Holladay | | #7

    Chris,
    Q. "He recommended polyiso on the interior face of the foundation instead of EPS. My reason for choosing EPS is to eliminate risk of wicking which I have heard can happen with polyiso. Curious to hear thoughts on polyiso vs EPS in this location."

    A. Either product works fine. If you have a damp slab -- which you shouldn't, if this is new construction -- you might consider leaving a 1/2-inch gap between the wall foam and the slab. But really, this shouldn't be necessary for a new, properly detailed basement.

    Q. "Is 2 inches of rigid foam sufficient thickness assuming taped seams and 2x4 cavity wall with Roxul?"

    A. Yes.

    Q. "Does having it [polyiso] on the interior of the foundation wall reduce the derating in cold temps?"

    A. The performance degradation is temperature-related. But your basement wall is (mostly) in contact with soil, not the exterior air, so it won't get as cold as polyiso installed on the exterior side of your above-grade walls. If you are worried, specify EPS.

    Q. "If we do use polyiso in this location, should it be faced? And with what?"

    A. I prefer foil-faced polyiso, because it is the easiest type of rigid foam to tape.

    Q. "He also said he has had a lot of success with fully adhered membranes in this type of [above-grade wall] -- (exterior rigid mineral wool/ rainscreen) -- and recommended Grace Vycor Env-S which is vapor- permeable at 15 perms. I chose plywood instead of OSB for sheathing due to it being more vapor-permeable than OSB (10 perms vs 2) and its properties of increasing permeance as it gets wetter to around 35 perms at 100% mean RH. I wouldn't want to restrict this property by applying a membrane with only 15 perms. Am I overthinking this?"

    A. Yes. If your wall is properly designed -- with enough continuous insulation on the exterior side of your plywood wall sheathing to keep the sheathing above the dew point during the winter -- then your plywood will be warm and dry, and won't need to dry to the exterior.

    Q. "Thoughts on self-adhered membrane vs. taped sheathing with mechanically fastened WRB?"

    A. You should specify the WRB you prefer. If you want a robust assembly, the inclusion of a ventilated rainscreen gap is more important than your choice of WRB.

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