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Vapor and air barrier for continuous insulation retrofit

Abeth | Posted in Energy Efficiency and Durability on

Hello,

I am planning an energy retrofit as prescribed by an energy evaluation. My 1993 two-story home needs continuous insulation added and I am struggling to find recommendations on an appropriate WRB that can also serve as my air barrier.

The house is 2×6 framed with fiberglass batts (R-17.3 effective) and poly vapor barrier which is not taped (and even found to be missing on one basement wall!) and the sheathing is 1/2″ ply. I am in Western Canada (Zone 5-6) and my energy evaluation has spec’d adding R-11.1 continuous insulation. I will be using 3″ of comfortboard for R-12.6. I’ve read up on dew point and also used a calculator to find indeed my dew point will be outside the sheathing.

Researching on GBA has led me to understand that I need a WRB that can allow this old construction to breathe, however, I also need this layer to become the definitive air barrier. One recommendation I found on GBA was
SOLITEX MENTO 1000 (38 perms). Speaking with a local builder building new homes with the same design as my retrofit (2×6 framed, combined WRB/air barrier, continuous insulation) I found they are using  Siga Majvest (54 perms). Both products meet CAN/ULC-S741-08/ ASTM E2178 specs for air leakage.

My question is: what PERMS spec do I need for my retrofit? Is higher better, and is there a number I should not go below (low perms solutions such as BlueSkin and Vycor come to mind) at risk of trapping moisture in my walls?

Thanks for the help,
Phil

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Replies

  1. boxfactory | | #1

    I personally don’t have any helpful information, but I noticed that two of the products you are considering are in this video. It occurs that this bit of backyard experimentation might be useful in demonstrating how the products work in the real world, or it might not be useful. I’d be curious what others think.

    https://m.youtube.com/watch?v=NyigYpjWVg8

    Ben

  2. Expert Member
    Michael Maines | | #2

    Phil, the ideal WRB perm rating depends on several factors, but the main ones are whether you're using a "reservoir cladding," whether you have a rain screen gap and how dry your climate is. This article explains why: https://www.greenbuildingadvisor.com/article/when-sunshine-drives-moisture-into-walls. (Be sure to read the comments as well.)

    That said, with continuous exterior insulation and what I assume is a dry climate, there is little risk of inward vapor drive, so in your case the perm rating isn't particularly important, and higher is probably better.

    My go-to WRBs are Siga Majvest or Pro Clima Mento, and we just used Rothoblaas Transpir self-adhered WRB on a project with good results.

  3. Abeth | | #3

    Boxfactory, thanks for the vid. I'll do some research on this type of testing.

    Micheal, I've read through the vapor drive article and see that my interior poly vapor barrier could be an issue for condensation in the summer as we will be airconditioning via a heat pump at the end of this retrofit. I should note my whole wall detail and climate:

    Old construction: drywall , un-taped poly , 2x6 24oc studs with fiberglass batts, 1/2" ply
    New retrofit: Combined air and vapor barrier, 3" Comfortboard, 3/4" furring (rain screen), metal siding.

    Climate: Castlegar has a humid continental climate (Köppen Dfb) or an inland oceanic climate (Cfb), bordering an inland warm-summer mediterranean climate (Köppen Csb). https://en.wikipedia.org/wiki/Castlegar,_British_Columbia#Climate:~:text=Climate%5Bedit,3%5D%5B16%5D We are basically on the wetter side of a high semi-desert and wet interior forest boundary.

    From your explanation, and given my wall system should have an R-value of 30, highly breathable continuous insulation, and a non-reservoir cladding, I understand that any moisture accumlulation within my old wall will travel outwards through a high perm WRB. If I used a lower perm WRB such as Vycor or Blueskin (I mention these because my materials supplier has offered them), is there a risk of moisture being unable to exit the wall?

    I will ask my supplier about your suggested products and I may be able to get the Mento.

    Thank you again for your professional suggestions,
    Phil

  4. Expert Member
    AKOS TOTH | | #4

    With your assembly, any reasonable WRB will work. You don't need anything fancy here. With plywood sheathing the simplest way to air seal is to tape the seams of it with a quality tape (ie Zip, Tescon Vanna or 3m8067). This is less work than dealing with a self adhered membrane and works just as well. Make sure to seal the sheathing to the foundation, read the specs as not all tapes work for this and some need a primer. The WRB can be standard house wrap.

    Working with 3" of rigid mineral wool is not easy, I would talk to your installer first to make sure they are comfortable with it. Specing 2x strapping will make getting it level and flat easier but it is still a lot of fussy work. If you must have mineral wool, I would look at keeping it down to about 1" to make install easier. If you want more R value, permeable rigid insulation (unfaced EPS/GPS or permeable polyiso) is a much simpler install.

    1. timot1 | | #5

      Is EPS/GPS considered "permeable insulation"? It's perm rating is only 2.5 to 5 perm. Comfortboard has a perm rating of 30. Seems a lot more restrictive.

      1. Expert Member
        AKOS TOTH | | #6

        In most cases, walls don't need permeable insulation. This is especially the case when you have sufficient rigid for condensation control as the OP proposed.

        Having a permeable rigid does make the assembly more robust if you are dealing with older windows without proper flashing or in an area with a lot of wind driven rain.

        The rigid is typically going over OSB which is 1 perm, so going up to 30 perm rigid won't add all that much.

        1. timot1 | | #7

          So would say using that using comfortboard might only be worth it when using plywood as opposed to osb sheathing?

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