Replies

1. Expert Member
   BILL WICHERS | Jul 31, 2026 02:37pm | #1

   Interesting question, and I looked into this a bit. I'm going to use MemBrain as an example, since their data sheets came up first in my search (yes, very scientific of me I know :D). They specifically state that their product reacts to *relative* humidity, so %RH. That's important. What that can be extrapolated out to mean is that the material will behave similarly at different tempertures, depending on the relative humidity levels. That would imply that if you put the MemBrain layer closer to the exterior, it will be in a colder part of the wall, where a higher %RH occurs at a lower absolute moisture level in terms of total water per unit volume. I think that would mean the MemBrain would be more vapor open on average over time, and this would concern me.

   Based on the above, I would say that NO, a smart vapor retarder doesn't let you put it in colder parts of the wall when trying to avoid condensation risk. You still need to put it towards the warmer side for best performance. I've always considered the big benefit to a "smart" vapor retarder is that it can open up to allow moisture to escape, rather than trapping it the way a sheet of polyethylene does. This is why we generally consider smart vapor retarders to be safer than absolute vapor barriers like polyethylene, but the relative risk does vary a bit with conditions.

   Smart vapor retarders, being in that they "see" relative humidity levels, also have what I like to think of as a sort of "proximity effect". What I mean by that is that if the interior of the wall is at a high humidity level relative to the exterior of the wall, on the "dry side" of the vapor retarder, the smart vapor retarder will still "open up" due to the high relative moisture content INSIDE the wall, which is in close proximity to the vapor retarder. Think of this as a sandwich in a zip lock bag -- even if the sandwich is cold, you will tend to see more condensation INSIDE the bag, due to the high moisture content in there due to the sandwich, compared with condensation on the OUTSIDE of the bag. If that ziplock bag was made of smart vapor retarder material, it would "open up" to allow that higher inside moisture level to escape until it's closer to equilibrium with the outside of the bag, at which point the smart vapor retarder will start to "see" a sort of average humidity level between the inside and outside, and it will "close" when that level gets below whatever the transition threshold is for the spacific "smart" material in question (CertainTeed lists that as around 60% for MemBrain).

   I like to think of the smart vapor barrier as helping to keep the inside of the wall at an average moisture level closer to the dry side average, by restricting moisture entry to the wall but allowing for moisture to escape. It's obviously not perfect, but it's better than poly that has no variability at all.

   Plywood and OSB are not as selective. The graph I see in a GBA article shows plywood starting to be more vapor open at much higher levels, with a "knee" (start of where the rate of change starts to increase more rapidly) of around 84% or so, so quite a bit higher than MemBrain. This means that plywood would not "open up" until moisture levels inside the wall became significantly higher in comparison to what MemBrain would allow. OSB didn't really have a knee, and stayed less permeable overall. This means plywood has more drying ability compared with OSB.

   A lot more about those other materials is in the GBA article here:
   https://www.greenbuildingadvisor.com/article/permeability-of-oriented-strand-board

   The MemBrain document I referenced is available here:
   https://certainteed.widen.net/content/hbh1fgzmub/pdf/30-28-159_MemBrainTechnicalBrochure_July_2019.pdf?u=nwk4fd

   I would expect other smart vapor retarders to perform in the same general way, but with the specific data points to be different (i.e. specific permeabilities may be different at different specific humidity levels, etc, between different products).

   Bill

1. Expert Member
   MALCOLM TAYLOR | Jul 31, 2026 03:41pm | #2

   Bill,

   Thanks, (as usual) that's very useful.

   Edit: Here is what Mike Maines wrote in another thread:
   "I design a lot of double-stud walls in CZ6 and always include a variable permeance membrane at the interior side, as highly insulated walls have little heat energy available to push moisture through sheathing so I want to keep as much interior water vapor out as possible. The farthest into the wall assembly I've put the vapor control layer is 1/4 of the R-value and monitoring shows that it works just fine there."

1. Expert Member
   Michael Maines | Aug 01, 2026 12:30pm | #3

   Malcolm, while I stand by that statement, I also trust Ben Bogie's monitoring that indicates responsive membranes (the IRC's current name for them) may not be necessary: https://www.greenbuildingadvisor.com/article/are-smart-vapor-retarders-cheap-insurance. (One of his monitored projects is one I designed and originally had access to his monitoring but lost the password at some point.) I am still recommending interior membranes because they are a code requirement and logic says they are a net positive in the climate zones where I work, 6 and sometimes 5.

   As for my 1/4 distance note, that's based on a simple dewpoint analysis at ASHRAE design temps and is not as scientifically derived as it could be. In a cold climate, the closer the membrane is to the interior, the lower the RH and the better it will be at blocking moisture diffusion moving outward.

2. Expert Member
   MALCOLM TAYLOR | Aug 01, 2026 12:47pm | #4

   Mike,

   Thanks - and sorry for beating this to death. For some reason I'm having a hard time understanding the mechanisms involved.

   From what I took from Bill and your explanations, it isn't that the variable-perm membranes (or other permeable vapour-retarders like OSB or plywood) will accumulate condensation on their inner-face the way poly would if placed too far into the wall, it's that they become increasingly ineffective in keeping moisture out when they are moved to a colder location. Hopefully I've got that right.

1. Expert Member
   Michael Maines | Aug 01, 2026 02:34pm | #5

   Malcolm, correct, because for the same amount of moisture available, the colder the surfaces, the higher the relative humidity and the more the membrane pores open up. Basically, the farther into the wall assembly, the less effective the membrane is at slowing moisture diffusion.

   There are a lot of variable at play so it's not as simple as only saying "keep a responsive vapor retarder at the interior face;" climate zone, wall assembly and interior humidity levels all play large roles. If the membrane is doubling as the primary air control layer, setting it into the wall may be worth a slightly elevated risk of moisture movement. If it's a heating-dominated climate and the assembly is completely vapor-open to the exterior, built with durable materials, it's less important than with a riskier assembly.

   As often discussed, air movement is a far higher risk than moisture diffusion, but moisture still moves via diffusion so it's worth paying attention to in my opinion.

   Plywood and OSB are also responsive vapor retarders, they just don't have as wide a swing as the polyamide films. My favorite wall assembly is still the one we used when I worked at Ecocor: a 2x4 wall with taped Zip sheathing for air and moisture control, then 12" of exterior cellulose packed inside I-joist outriggers, contained with a vapor-open WRB (and no exterior sheathing). The 2x4 wall was a service cavity and also insulated.

1. Expert Member
   MALCOLM TAYLOR | Aug 01, 2026 04:27pm | #6

   Mike,

   Thanks again, that's very helpful. I think I've got in now.