Variable-perm (smart) vapour-retarders – GreenBuildingAdvisor

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