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Optimizing a fixed depth wall assembly?

codonell | Posted in Energy Efficiency and Durability on

Hello GBA community! I’m a nerdy engineer working with my builder in Zone 6 (Southwestern Ontario) on a wall assembly for an addition to a 100+ year old house.

The lot size doesn’t allow much space to work with and the current design is a compromise of aesthetics and efficiency. We don’t have much space to work with for the wall assembly, but I wanted to make it as efficient as possible. Constraints always limit possible solutions 🙂

Prospective wall:
* Drywall interior.
* Certainteed membrain smart vapor barrier to meet inspection in zone 6 and to avoid double-barrier problems.
* 2×6 16″ OC, using T-studs (thermally broken R-19 studs).
* ccSPF to fill the bays (means we don’t _need_ an interior vapor barrier, but I have a spec’d a smart vapor barrior to avoid failing inspection)
* 1/2″ plywood sheathing.
* WRB on the outside with taped seams.
* Vinyl siding.

The plywood can dry to the outside. The vinyl siding is well ventilated and doesn’t need a rainscreen.

I have about 1″ of extra space on the outside I could use in the assembly if needed.

I may fail to get T-studs due to availability.

– What can I do with the 1″ of extra space to optimize the wall assembly for energy efficiency?
– Would switching to ZIP R6 sheathing help? Is that the only optimization left?
– What if I have to use normal 2×6 studs?

– Adding more exterior insulation is not possible. For the 2×6 wall assembly I would need a lot of insulation on the outside to keep the sheathing below the dew point and I don’t have that space. Therefore I’m stuck putting all the insulation on the inside.

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  1. MattJF | | #1

    Read this if you haven't:

    You will avoid some of the thermal bridging, but will still not be able to fill the cavities. ccSPF generally has the highest $/R for the available options.

    Play with this tool:

    I don't have an answer for you as there are a lot of variables. Reclaimed external foam is generally cost effective and could be done with a 2x4 wall. If you go with T-studs, consider just dense packing them.

    This article is applicable to vinyl siding:

    1. codonell | | #5

      I don't follow your point about "still not be able to fill the cavities," could you please expand on that?

      I take your point about dense packing the T-studs. It need not be ccSPF between the naked T-studs, I could use dense packed cellulose.

      You reference an article about vinyl siding and furring strips. Could you expand on that a bit? Yes, I could use furring strips over the rigid foam, but one could equally nail through the foam right.

      Thanks for the reference to the r-value-calculator!

      1. MattJF | | #8

        Sorry I didn't fully explain things.

        With ccSPF you generally cannot completely fill the stud bays because it is very difficult to shave flush and overfilling consumes more of an already very expensive material. Best case is you can fit 5" in a 5.5" bay.

        As for cost, last time I ran the numbers. These vary base on location and are calculated as total wall R value with 20% framing fraction.

        Rockwool: $.072/R/SF
        ccSPF: $.278/R/SF

        As for vinyl siding, I thought the article would be interesting if you consider going with a lot of exterior foam. 1.5" or maybe 2" of exterior foam is would work to nail through. Beyond that it may get tricky and need furring and addressing any issues that brings up with vinyl.

  2. Expert Member
    NICK KEENAN | | #2

    ccSPF is the highest $/R, but it is also the highest R/space. If the thickness of the wall is your limiting factor you want it or polyiso. If you fill the bay the thermal bridging pointed out in the FHB article is not an issue.

    I had a similar situation and was very happy with cutting pieces of polyiso to fill the bays and then using cc spray foam to fill them and seal the assembly. I had to trim the spray foam but did not find it as onerous as some have suggested. It gives the same R-value as filling the bays with spray foam but is cheaper and better environmentally.

    1. codonell | | #6

      Thanks for the recommendations! Yes, perhaps cut-and-cobble here might work out well, though it's 5.5" of bay to fill with pieces of polyiso. I'll have to look at the labor costs.

  3. Expert Member
    NICK KEENAN | | #3

    For that given thickness, I think the best wall would be a 2x4 wall filled with insulation and then 2" of polyiso continuous on the inside. If you have an inch to spare run 1x3 strapping horizontally to hold the foam and give the drywall something to attach to, and space to run wiring.

    If the bay insulation is polyiso that gives nominal R33, with R15.5 over the framing. There is some evidence that polyiso loses insulation value at lower temperatures (see ) and that in cold climates polystyrene actually gives more insulation per inch.

  4. Expert Member
    Akos | | #4

    SB-12 limits you to what you can do there unless you provide simulation data for the assembly.

    The easiest one is just to go with the table for additions (R19 batts + R5 ci). This is much cheaper than the T stud SPF combo you are proposing and only 1" thicker.

    1. codonell | | #7

      Thanks for the note about SB-12.

      1. Yupster | | #9

        Not quite correct, SB-12 has a prescriptive table with a nominal R-value (R19+R5ci, and other combinations) and also an effective R-value, R15.96 (and other combinations with different effective values). No simulation required, just an effective R-value calculation, easy for those nerdy engineer types ;)

  5. Yupster | | #10

    I second the switch to 2x4 walls, assuming your building can be built with 2x4s. (Many can, there is an Ontario Building Code table that can tell you, or you can engineer it). Add 2" reclaimed roofing foam, fairly easy to find on Kijiji in my past experience, and R14 Roxul batts or something like the BIBS blown fiberglass in the stud bays. Both are better than fiberglass batts in my opinion and common in Ontario. I've found good dense pack cellulose installlers hard to find in my area of ON anyway.

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