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Where to Put the Vapor Retarder on a Double-Stud Exterior Wall

JimmyCoro | Posted in Energy Efficiency and Durability on


I have a situation that I’m looking for some guidance on. There is a  home in zone 6 (WI) that has a portion of the basement with a 1/2 wall consisting of poured concrete, with another 1/2 wall on top of that consisting of wooden frame construction. I don’t think this is unusual, but what was built in front of the top part of the wall is a bit unusual. It’s essentially a double wall.

So, what we have for the upper part of the wall (approximately 46″ vertically) which runs around two sides of the house (the rest of the house has full basement):

Vinyl siding cladding – Tyvek wrap – 7/16 OSB – 2×6 wall with dense-pack cellulose (R21) – Certainteed Membrain VR – 1″ gap – 2×4 stud wall with fiberglass insulation – 5/8 drywall – 5/8 drywall. The final intent is to use plaster on the inner layer of drywall (plaster base).

When I say “gap” it isn’t a true gap – it’s filled with fiberglass insulation – I just mean that the stud walls are separated from each other.

Should this be an emergency with the VR in its current position removed or cut up / added to interior wall? I know the typical way to install VR in this climate is on the inner wall between insulation and the drywall, but part of me wonders if this is air sealed from the interior very well (all drywall seams caulked and outlets sealed – used putty pads on backside of all outlets), and it’s only a limited part of the house, maybe it will be ok? Not an expert on the subject especially in this scenario – any thoughts either way?

This is a newer house, about 5 yrs old.

Thank you.

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  1. Jon_R | | #1

    Normally a vapor retarder some distance into a wall is OK. But Membrain is special and stops being an effective retarder when it sees high relative humidity. So use another retarder on the interior side (to preserve an acceptable perm ratio).

  2. JimmyCoro | | #2

    Hello Jon R, thanks for the reply. Are you suggesting then to have TWO intact VRs, one in the location described (middle of wall) and another between drywall and studs? Or must the current VR be modified / removed / cut up?

    Either way, would a smart barrier / retarder like the Membrain be the suggested interior VR? That one would necessarily have holes in it due to outlets and things. The VR in the middle of the wall is completely solid (no holes) since all wiring was done in the interior wall space.

    Thank you for your help.

    1. Jon_R | | #5

      I'd say that leaving it there adds some beneficial air blocking and doesn't have a negative moisture effect.

      Exposed only to normal interior humidity (ie, inboard of all insulation), a smart retarder like Membrain is a good choice. Intello is better. Vapor retarding (< 1 perm) paint is OK.

  3. Expert Member
    PETER G ENGLE PE | | #3

    I disagree somewhat with Jon. The middle of the wall is a great place for both a VR and air barrier, especially when it can be essentially unbroken as yours is. Jon is concerned that, if the Membrain sees high humidity, then it might open up and allow more vapor to pass through it. But we have to consider what conditions might cause it to see high humidity. In the middle of the wall, it should never see anything close to condensing conditions, so that's not an issue. That leaves bulk water leaks. If it sees bulk water leaks, then you've got a problem regardless of the location of your VR and you would be happy that it opens up and at least provides a fighting chance of drying out during warmer weather. This sounds like a carefully designed and built wall and I'd leave it just as described. Latex paint on your interior drywall will still give you a bit of vapor management whether you need it or not.

    1. Jon_R | | #4

      > In the middle of the wall, it should never see anything close to condensing conditions

      Yes it will but condensing conditions at the MemBrain location isn't necessary. Membrain only needs to get to about 35% RH (not 100%) to stop being a Class II vapor retarder. And near the middle of the wall, this occurs any time it's even mildly cold outside. Very cold days and it will be >10 perms. Hopefully we agree that this risky, cold climate, cold sheathing wall shouldn't be left without a functional Class II vapor retarder (IRC R702.7 requires one).

      I suggest, a few calculations and this. Or the MemBrain installation instructions where we see "MemBrain is recommended for interior vapor retarder and air barrier applications, located at the “warm in winter” (moderate to high heating climates) side of the building envelope". Note: nothing about use in the "cool in Winter" middle of a wall.

      1. JimmyCoro | | #6

        So just to clarify - you are suggesting to leave the Membrain in the middle of the wall as-is, and install a second layer between interior studs and drywall?

      2. creativedestruction | | #8

        I'm with Peter on this. So long as the Membrane and interior drywall are installed airtight, I would keep them as is and sleep soundly. The wall has decent drying potential in both directions.

        Jon said "Membrain only needs to get to about 35% RH (not 100%) to stop being a Class II vapor retarder. And near the middle of the wall, this occurs any time it's even mildly cold outside."

        Class 2 vs class 3 vapor retarder definitions are arbitrary. The Joe that defined them said so himself-- he just needed to pick a number to establish a guiding principle. Nothing magic happens. Yes, the Membrain opens up a bit more than some would like to vapor flow, but I'm not convinced it's enough that it would cause an issue in your case. Airtightness matters more at reducing unwanted vapor movement; focus on that. Sealed at all perimeters, rough openings and penetrations. Avoid humidifying the basement spaces.

    2. JimmyCoro | | #7

      Do you feel that adding another Membrain as Jon describes would be a problem? Or just unnecessary?

      Thank you.

      1. Expert Member
        PETER G ENGLE PE | | #9

        Jon is not wrong with his comments above. In very cold weather, the Membrain will be more vapor-open than we would like to see, and the wall would not meet the IRC requirements for a Class II vapor retarder with that sort of construction. If you add another layer of Membrain behind the drywall, the wall would meet code and more importantly, it would have less chance of condensation on the sheathing.

        But all of this is a matter of degree, and there is a lot of guessing going on. The vapor permeance of the OSB sheathing tracks closely to the permeance of the Membrain until the RH goes past 70% or so. And, in cold weather, the RH of the air adjacent to the OSB will always be higher than the RH at the Membrain, so the OSB will generally be at least as permeable as the Membrain, and maybe a bit more. That means there will be no condensation on the sheathing. At the very coldest conditions, this may no longer be true and there could be some wetting of the sheathing. This risk would be reduced if you added another VR on the interior of the wall.

        But the OP has correctly identified the issue of penetrations in the interior VR as a risk for air and/or vapor leakage. Unless you detail the interior VR exceptionally well, it will probably be at least as leaky to vapor as the mid-wall unbroken VR and then you've done nothing but waste your money. AS Jason says above, limiting air leakage is far more important than any specific limit to vapor diffusion. IMVHO, the wall described is safe. An additional layer of interio VR would improve the wall, but may not be worth the expense.

        1. Jon_R | | #10

          > That means there will be no condensation on the sheathing

          Note that "exterior perms >= interior perms" does not imply "no condensation on the sheathing". In Z6 you need exterior wet perms at least 5X more than interior perms for sheathing moisture control (see Table 2A). Clearly this won't be met with just mid-wall MemBrain.

          Agreed, always air seal well. Multiple air barriers help.

        2. creativedestruction | | #12

          I noticed a similar thread by the same OP noting a bath in the basement. Use the bath fan religiously. I like timer switches or the humidity-sensing variety on them.

          1. JimmyCoro | | #13

            You are correct. Same house, different area. But yes I have grown to insist on having bath fan timers, especially in this area it will be especially important.

  4. Expert Member
    Dana Dorsett | | #11

    Jon has it absolutely right. In a zone 6 location the proximate entrained air in the fiberglass adjacent to the MemBrain is too high for it to work in that layer of the stackup:

    There is ~R20 to the exterior of the vapor retarder, ~R17 to the interior side, for about R37 total. In a Zone 6 location where the January outdoor mean temp is say +15F if the indoor temp is kept at 70F (a 55F difference) the average temp at the vapor retarder is roughly 15F + 55F* (R20/R37)= ~45F.

    With an indoor relative humidity 0f 35%, @ 70F the dew point of that air is 41F. If allowed to leak into the wall or diffuse through a high permeance finish, the dew point of 41F the RH of the entrained 45F air proximate to the MemBrain would be north of 86%. That is humid enough to make it substantially vapor open. The vapor retardency of ~ 5 perm interior latex paint helps (a lot if it's also air tight) and the air on the exterior side of the MemBrain has an average dew point of 15F (the average outdoor temp) since the moisture in the entrained air on that side is being sucked up by the sheathing, bringing the RH of the 45F air on the exterior side down to ~27%. But those factors combined isn't going to be low enough to completely overcome he effects of the high RH on the interior facing side of the MemBrain- it likely to be too vapor open to be completely protective during cold weather.

    It's a complex model- it's still doing something but probably not enough. Intello is tight enough that it might work there, but then again, maybe not. Installing a second layer of MemBrain on the interior side of the 2x4 studwall should fix it, since the proximate air on that layer would be 35% on the wallboard side and much much lower and about 11 % RH in the fiberglass closest to the interior, low enough to make it's vapor retardency <<1 perm.

    1. JimmyCoro | | #14

      Thank you for the response. There is a roll of Membrain around here somewhere so it's just a matter of removing a small number of chunks of drywall to install in those areas that already have been rocked. Small price to pay, in my opinion. Will be extra careful to get any electrical penetrations sealed as much as possible.

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