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Redundancy of Vapor Retarder and Latex-Painted Plastered Blueboard for Vapor Retarder

dfvellone | Posted in General Questions on

Climate zone 6.
My wall stackup from the exterior is 1″ hemlock siding, 1″ airspace, housewrap, 2 1/2″ xps, 5″ studwall insulated with r23 roxul.

I understand that the 2 1/2″ xps is marginal for my climate zone and detailing the vapor retarder will be essential to performance.
I’ll be installing blueboard which will be plastered, primed, and painted with latex paint.
Would it be necessary or even just good insurance to install a vapor retarder prior to the blueboard, or will it be an unnecessary redundancy if the blueboard is detailed well?

For that matter, are there any issues with two layers of vapor retarder?

Thanks, Daniel

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  1. Expert Member
    Michael Maines | | #1

    Daniel, I'll start with a note that regular readers are probably tired of seeing, but I have to mention anyway--the blowing agents in XPS are potent greenhouse gasses, 1400 times worse than carbon dioxide, and worse than any other construction material. Conscientious builders find an alternative material with lower environmental impact, such as polyiso foam, EPS foam, Rockwool, or wood fiber insulation. If you can find a source for used XPS that's a different story. Additionally, XPS' advertised value of R-5/in degrades over time to about R-4.2/in, equal or close to the stable R-value of EPS.

    Walls with vapor-retarding exterior insulation in CZ6 need at least 33% of the R-value to be on the exterior to keep the condensing surface about the dewpoint and avoid moisture accumulation, allowing painted drywall, a class 3 vapor retarder, to be all that's required on the interior. With R-23 cavity insulation, that means at least R-11.5 on the exterior. 2.5" of XPS is a nominal R-12.5 but will age to about R-10.5. If the goal was just to get past the building official you would be fine, but I prefer to avoid risk and would either bump up your exterior insulation to a true R-12.5 or higher, and/or add a variable permeance membrane at the interior, such as Siga Majrex or Pro Clima Intello. I assume painted skim-coat plaster over blueboard performs approximately the same as drywall but I may be wrong--it's not used much where I am.

    The IRC building code simplifies the math to say that a Class 3 vapor retarder (i.e., painted drywall) is allowed if you have at least R-11.25 over a 2x6 wall, but some think their wording is a bit too simplified.

    1. charlie_sullivan | | #4

      Depending on Daniel's construction schedule, he might actually be able to get the new Foamular NGX version of XPS that drops the GWP from 1420 to 80, still higher than the other options you list, but vastly better than blueboard. It's supposed to be available throughout north america as of Jan 1st. There's not much reason to prefer it to EPS, but it might be more readily available than EPS at some point--we'll have to see how quickly it's actually made available. Scott Gibson is working on an article on this and I'm hoping he'll have more detail than I know about how soon it will be available.

      1. Expert Member
        Michael Maines | | #5

        Charlie, that's a good point. I would like to see the lifecycle analysis for the new XPS. I recently went through the spray foam industries' lifecycle analysis for HFC vs HFO blowing agents and although the GWP for those is 1030 vs 1, the total GWP of the HFO-blown foam is only about 24% of the HFC-blown. Still much better, but not a thousand times better as you might expect focusing only on the blowing agent. I imagine similar for XPS but want to see the numbers!

        1. charlie_sullivan | | #12

          Yes, I am not ready to start recommending it as a green choice--only as less of a disaster than it used to be. But it will be good to get more information.

    2. dfvellone | | #7

      Because of the valuable information I received on this forum, I accessed and used reclaimed xps for the entire project. Unfortunately at the time I went by the advertised r-value and also the reported cold weather performance of xps which claimed an increase in r-value as temperature decreased.
      Short of removing all the siding, I'm stuck with the 2.5" xps.

      1. charlie_sullivan | | #10

        Great that you got the reclaimed XPS!

        1. dfvellone | | #14

          Dana had pointed me in the direction of it, and prior to that I'd never heard of it's availability. Since then I've purchsed the entire sheating insulation for my new house, an additional 150 sheets of 3"×4'x3" reclaimed polyiso, and some thin sheets for here and there. All of it has been in beautiful condition, and a fraction of what it would cost new. There always seems to be large quantities available on my local craigslist as well. Better than the enormous savings is knowing that materials that may have one time been destined for the landfill get repurposed.

      2. Expert Member
        Michael Maines | | #13

        Daniel, the R-value does increase as the temperature decreases, I believe for all insulation other than polyiso. To what extent I'm not sure. Used is good!

  2. Jon_R | | #2

    In your marginal case, use a high-side of Class II interior side retarder (say .5 to 1 perms). For example, just vapor retarder paint. Support for this is here:

    More importantly, always air seal well and test it. The interior side is somewhat better.

    Just to be clear, typical exterior foam doesn't keep the sheathing above the dew point or prevent condensation/sorption/moisture accumulation. But it usually does limit it enough to avoid problems. Even more so if foam is the only sheathing. Understanding this is critical to understanding why perms (both interior and exterior) still matter.

    1. Expert Member
      Michael Maines | | #3

      I could have been more precise and written, "keeping the condensing surface above the dewpoint the vast majority of the time." For anyone who wants to understand the fine point Jon is making, you can read about it here:

    2. dfvellone | | #6

      The xps is the only sheathing.

      How is the assembly tested?

      1. Jon_R | | #8
  3. dfvellone | | #9

    Thanks for the information.
    My inclination is use vapor retarder paint. Is there any advantage or disadvantage If I also install Membrain prior to the blueboard?

    Thanks, Daniel
    I should add, this is a timberframe, so one of the reasons I'm inquiring about the use of a vapor retarder like Membrain is because its installation would likely give me some advantage to detailing the air sealing.

    1. charlie_sullivan | | #11

      I think it's a good idea to include the Membrain. It should be on the interior side of the wall cavity, not up against the XPS, in case that wasn't clear.

  4. Deleted | | #15


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