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Choosing a Cold-Climate Wall Assembly

bwsct | Posted in General Questions on

Hello,
I’m hoping to have some renovations done to my house soon.  It was built in 1953 and has no insulation in the walls and balloon framed.  Its a combination of drywall and plaster and has cedar shake siding which needs some work or replacement with another siding product.  I’m in climate zone 5a.

Scenario 1 :  Blow in dense packed cellulose, add WRB, exterior insulation and siding.

Scenario 2 :  Gut interior drywall and create a wall assembly of drywall, smart membrane, mineral wool, sheathing, WRB,  exterior insulation and siding.

Will both those wall assemblies work?
Will wall assembly 1 be ok with no interior air barrier?
Is the dense packed cellulose going to work anywhere near as well as gutting the interior?

Thanks for your respoonses

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Replies

  1. Jon_R | | #1

    Make sure that the wall complies with the recommendations here (Table 2, A or B):

    https://www.appliedbuildingtech.com/system/files/200311_abtg_rr_1701-01_moisture_control_guidelines_figure_update_final.pdf

    Have at least one good, tested air barrier.

  2. Expert Member
    Rick Evans | | #2

    Brad,

    I like option #1. But if you are considering adding exterior insulation (which is better than option 2) then you can skip the cavity insulation altogether. Just make the exterior foam a bit thicker. Used re-claimed foam if possible.

    I would strip the siding and add a peel and stick like Henry blueskin over the existing sheathing. This will be your air barrier and WRB. I would then add 3-4" of foil-faced polyiso and then your 3/4" furring strips and siding.

    From a building science perspective, this is as good as it gets.

    1. bwsct | | #3

      Isn't option 2 going be a bigger improvement than option 1? Essentially doubling the R value compared to 1 since I'd add cavity insulation and exterior insulation? And also more likely to fill out all the cavities in the wall?

      1. Deleted | | #4

        Deleted

  3. Expert Member
    Rick Evans | | #5

    Hi Brad,

    You don't need an interior smart vapor retarder if you have sufficiently thick exterior rigid foam.
    If you are adding a new WRB then that can be your new air barrier. No need to put one inside. It will be damaged, poked, and likely discontinuous unless strapping is added.
    Also, if the walls are 2x4 walls, then rockwoool will add only an extra r-2 or so. Blowing in cellulose is always easier than cutting and installing batts.

    For these reasons, Option 1 wins in my opinion.

    Your option #2 (assuming 2x4 studs and let's say 2" of polyiso) would have a whole wall r value of around R-22. Option #1 + 2" of polyiso will get you around R-20.

    Since you are adding foam anyways, I would add 4" of exterior polyiso and skip the cavity insulation. This would give a whole wall r value of around R-2o without the hassle of cutting holes for cellulose and or trimming batts to fit a balloon framed stud wall. Call it option 3?

    When all of your insulation is on the exterior, then you've reached building science Nirvana. (Think about Lsteberek's rib analogy)

  4. creativedestruction | | #6

    +1 for one good, tested air barrier. Both approaches can work fine, but air barriers are best in the plane of the exterior sheathing. Best because there are limited interruptions there and it offers the most protection from temperature and movement extremes given both exterior and cavity insulation.

    I like option 1. Cellulose further reduces convection and infiltration. Use sufficient exterior insulation.

  5. bwsct | | #7

    Is a rain screen also ideal out side the WRB in scenario 1?

    1. creativedestruction | | #8

      Yes, rainscreens are always ideal outside of the WRB. Particularly so with higher rain exposure on higher insulating wall assemblies.

      1. Deleted | | #10

        Deleted

    2. WRanch | | #12

      Brad,

      The rain screen is typically between the foam and exterior cladding. It creates a gap to stop water intrusion and allow drying before your cladding is moisture damaged.

      This article from BSC offers additional explanation.

      https://www.buildingscience.com/documents/building-science-insights/bsi-111-double-play

      Additionally, it suggests a small gap between the foam and the WRB layer at the sheathing. Tyvek Drainwrap is one example of many available options to create this gap.

      "In Figure 1 we should have two “drainage gaps”. The first is obvious…immediately behind the cladding. No surprise there. The second? Ah, between the continuous exterior insulation and the “water control layer”. How much of a gap for the second gap? What works is somewhere between 1/32of an inch and 3/16 of an inch. Note that the larger the gap the more loss of thermal efficiency of the continuous exterior insulation. "

      These design decisions should consider the direction of drying for your wall assembly, inward or outward. Mind the Gap, Eh! by Joe Lstiburek is a good read.

  6. bwsct | | #9

    It sounds like option 1 is the best choice.

    The house has a concrete foundation. Is it still worth doing this insulation if the foundation is not insulated at all?

    1. WRanch | | #11

      Yes, the continuous exterior wall insulation is still worth it independent of foundation insulation. All the benefits of reduced thermal bridging and increased whole r-value are still at play. You can also retrofit the foundation with insulation.

      1. bwsct | | #13

        Thanks for reassurance on the foundation. I will try and address the foundation insulation at some point as well.

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