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Community and Q&A

Conflicting articles on GBA about vapor retarders/vapor barriers

BuildingFun | Posted in General Questions on

Many articles on GBA suggest vapor barriers are no longer needed if the drywall layer is fully air sealed.   However, many other articles tout the importance of “smart vapor retarders or vapor barriers” including this one…

Mineral Wool Insulation Isn’t Like Fiberglass – GreenBuildingAdvisor

“Smart vapor retarders

And third, air sealing. Mineral wool only comes in unfaced batts. No foil or kraft paper vapor retarders are offered. This means an independent vapor retarder must be installed.Simply said, this is the best way to create an airtight envelope for the house.

My favorite product in this regard is a variable-permeability membrane, a so-called “smart” membrane, because the permeability self-adjusts to suit conditions. An example is the MemBrain vapor retarder from CertainTeed. This is their branding of the smart membrane product made by their European owner, Saint-Gobain.

Another high-quality variable-permeability membrane is Intello Plus and DB+ by ProClima. This vapor retarder membrane has a low permeability level in dry conditions, but if the humidity level within the wall gets high, the material will open up to allow the moisture to dry to the other side. The ProClima Intello membrane is notable because it is well reinforced. It will not tear or split from stapling, and this tolerance of handling makes it easier to work with.”

Is this outdated advice in this article?

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  1. Expert Member
    BILL WICHERS | | #1

    My own general rule is that if you use exterior cotinuous insulation with at least the minimum R value for your climate zone and wall assembly, then an interior vapor retarder is not needed. If you have no exterior insulation, than the interior vapor retarder IS needed, but can be simple stuff like the kraft facer on kraft faced batts.

    I always install the smart vapor retarder for additional insurance though, since it goes up quick and the cheaper ones like MemBrain aren't very expensive. I put this in even when using MORE than code minimum levels of exterior continuous insulation.


    1. Expert Member
      DCcontrarian | | #2

      In a cold climate you want to control humidity for two reasons. Most important, you want to keep condensation from happening inside your walls to avoid the associated problems. But you also want to keep the humidity inside the house, otherwise the house gets uncomfortably dry. Or more specifically, you don't want to lose more humidity than the occupants create.

      The exterior continuous will prevent condensation, but depending on what it's made of it may not be enough of a retarder.

      1. maine_tyler | | #3

        Im not sure anyone should ever choose a wall vapor retarder/barrier simply for the reason of keeping moisture inside a house. It's not that significant of a loss compared to exfiltration. Really tight houses that under ventilate typically have the opposite problem anyways (too much winter humidity) depending on indoor usage, and thus venting solves the issue (or makes it too dry if venting is too much).
        But there are benefits to keeping the moisture out of the wall for condensation / sorption reasons.

  2. maine_tyler | | #4


    It depends on your climate.

    I too have found certain statements about vapor retarders here to be a little confusing. I think the notion that vapor diffusion is seldom a cause of moisture issues is somewhat contradicted by the requirement to use a class II retarder in standard (no exterior insulation) assemblies in colder climates. It CAN be an issue still. It may seldom be a cause because people are using them. If no-one ever used class II or smart retarders in these colder regions, could we still say vapor diffusion is a non-issue? It's not a complete non-issue.

    I think the reason you see the emphasis on not needing vapor *barriers* in particular is because there has historically been widespread misapplication of vapor barriers (especially in warmer climates) and many were led to erroneously believe that vapor barriers or retarders are the most significant moisture throttle, when in reality it is air barrier that will most successfully reduce moisture transport into/out of walls.

    So a class 1 vapor *barrier* in a wall is seldom if ever needed (and in many cases is a bad idea!), but a class II retarder (and a smart retarder is even better) in colder climates is still generally recommended (in walls without exterior insulation of adequate thickness). Warmer climates is a different story.

    Martin recently clued me into his excellent article that lays out what the current IRC says about this:

  3. Expert Member
    PETER Engle | | #5

    Martin's article is a good one, well worth reading. I think that part of the confusion is because this is a somewhat complicated subject and most of us try to explain these things in simple terms.

    The type of vapor retarder, and its location in the wall vary with the construction type and climate zone. In a heating climate, you want it on the inside. In a cooling climate you want it on the outside. Since most of us live in mixed climates, we'd really like to have it in the middle and some wall systems actually do this intentionally. When we apply the ratio rules for cavity/exterior insulation, this is part of what we're doing. In effect, we make the sheathing the least permeable part of the wall, and therefore, the potential condensing surface. But when it's in the middle of the thermal envelope, the sheathing doesn't get cold enough in either heating or cooling modes to condense any moisture. So we don't need much of a vapor retarder (or any really) on the inside, as long as we make sure to get that insulation ratio right.

    In really cold climates (Zone 7 and up), the inside of the walls gets really cold and most "safe" wall designs will include a class 1 vapor retarder (barrier) on the inside. The cooling times in these areas are very short, if any cooling is needed at all, so having a poly film on the interior is safe in that climate. But, if you put poly on the interior in Miami, disaster happens. You could probably put it on the outside in Miami, but there are better alternatives.

    So the real answer, as in most things, is "it depends."

    1. maine_tyler | | #6

      >"In really cold climates (Zone 7 and up), the inside of the walls gets really cold and most "safe" wall designs will include a class 1 vapor retarder (barrier) on the inside."

      Peter, it appears the IRC is fine with use of class II even in these climate zones. Maybe there are parts of the code I am missing that further qualify that? Are you saying class I in these climate zones is simply preferable (safer) even if class II is 'allowed'?

      I also assume if adequately thick exterior insulation is used in these climates, you would no longer recommend a class 1 interior? (I know that there are examples of class 1 interiors coupled with low perm exterior foam working, but that's not my question).

      1. Expert Member
        PETER Engle | | #8

        I'm fortunate to live and work in Zone 6, not 7+, and so the difference between class 1 and 2 is merely academic for me. In general, even in zone 7+, I am more comfortable with Class 2 than 1. However, there are far too many examples of Zone 7 housing built successfully with Class 1 vapor barriers that they should be included in the discussion.

        I may have been a bit unclear in my statement above. The IRC does permit Class 2 in these zones, and I have no problem with that. At the same time, many builders in Zone 7 insist on Class 1, and that's OK, too. Even in zone 6, class 1 VBs can work just fine though class 2 is probably "better" or "safer" for most walls. We're seeing more installation of A/C equipment in zone 6 and we should consider vapor drive in both heating and cooling conditions. South of zone 6, I would be very hesitant to use a class 1. I have also become a fan of variable vapor retarders ("smart membranes"), though they may be unnecessary with otherwise properly designed and built walls.

        We must also remember that there is a giant caveat to all of these discussions: We assume that whatever wall system specified is properly constructed with attention to details, especially air-sealing details. We must also assume that interior humidity is controlled to remain within whatever design parameters were applied, ie. greenhouses, indoor pools and other high-humidity spaces require different designs than standard residential spaces, which can also be different than certain workspaces that have little or no internal moisture generation. Even within "standard" residential spaces, I've seen reasonably well built houses rotted out by owners who, for whatever reason, insist on maintaining humidity at 50% or higher in cold weather.

        1. maine_tyler | | #9

          Makes perfect sense Pete.

    2. Expert Member
      BILL WICHERS | | #7

      It's worth adding that smart vapor retarders are less likely to introduce new problems compared to using vapor barriers like polyethylene sheet. Vapor barriers can sometimes "trap" moisture, limiting drying, and ultimately causing problems in the wall assembly. Since smart vapor retarders are able to open up when exposed to high local moisture levels, they allow for some drying and are much less likely to be a risk when added to any kind of wall assembly.


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