Smart Vapor Retarders: Not Just Your Grandmother’s Poly
New smart vapor retarders block most vapor diffusion when you want to eliminate risk of condensation, but allow vapor flow when you want drying potential
Nowhere in building design has there been more confusion or more dramatic change in recommended practice than with vapor retarders. Thirty years ago, we were told to always install a polyethylene (poly) vapor barrier on the warm side of the wall. Then we were told to forget the poly and go with an airtight layer of drywall (airtight drywall approach). Insulation contractors, meanwhile, often said to skip the vapor barrier; we need to let the wall or ceiling cavity dry out.
It made for a lot of confusion. And I’m not sure we’re totally out of the woods yet.
Some experts are now looking to vapor retarders whose vapor permeability changes based on the humidity conditions. We installed one of these new materials on our house.
Back when poly was the default choice as a vapor retarder (called vapor barriers back then), the recommended placement of that layer varied depending on where you lived. The rule was to install it on the “warm side.” In northern climates, that meant that the vapor retarder should be on the inside (installed on the inner face of wall studs and rafters) before installing drywall.
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The idea was that we wanted to prevent water vapor from migrating from inside the house (where it was warmer) outward through the building envelope. As vapor-laden air cools off, it is able to hold less moisture, and if it gets cold enough the moisture in the air will condense (i.e., it reaches the dew point)—causing problems by wetting the insulation or rotting wood framing. By installing the vapor retarder on the inside of the wall, we would keep that water vapor out of the wall cavity where it might condense.
In warmer climates, we were told to install the poly vapor retarder on the outside of the wall cavity, because the inside of the air-conditioned house was colder than the outside. In this case the risk was that condensation could occur with moisture laden air moving inward through the building enclosure and cooling off.
But what about places where some of the time it’s warmer inside than outside and at other times it’s just the opposite: colder inside than outside. It turns out that this is the case in most of the U.S. Even in chilly Vermont, where I’m based, most new houses are now being built with air-conditioning—and after the heat wave this past July, I'm sure we'll only see more of it.
Confused? So is most of the building industry.
Smart vapor retarders
One solution to the changing conditions of a house during the annual cycle is to install a vapor retarder whose permeability (a measure of how readily water vapor can pass through) varies based on the humidity. These are often referred to as smart vapor retarders. The goal is low permeability in the winter when humidity is low but it’s critically important to block moisture flow and prevent condensation, and high permeability in the summer when humidity is higher and you want drying potential to both the interior and exterior
It turns out that the plain old kraft paper facing on fiberglass batts has this variable permeability property—as leading building science expert Terry Brennan explained to me. As humidity increases (in the summer), it becomes more permeable to moisture, while in winter, when the humidity drops, it becomes less permeable and a better vapor retarder. Terry describes it as “poor man’s vapor retarder.”
About 15 years ago, researchers in Europe began working in a more focused way on variable-permeability vapor retarders. The first such product I heard about was MemBrain, made by CertainTeed’s parent company Saint-Gobain (headquartered in France) and available from CertainTeed in the U.S. MemBrain is a polyamide or nylon sheet with permeability that ranges from less than or equal to 1.0 perms in low humidity conditions to more than 10 perms under high-humidity conditions.
Two variable products are also made by Pro Clima in Germany and distributed by 475 High Performance Building Supply in Brooklyn, NY. Intello Plus is made from a polyethylene copolymer, and it varies in permeability from 0.17 in the winter to 13 in the summer. It comes in rolls 1.5 meters (59 – 1/16”) wide and 50 meters (164’) long.
DB+ is a less expensive, paper vapor retarder made by Pro Clima that varies in permeability from 0.8 perms with low humidity to 5.5 perms at high humidity. It is made mostly from recycled paper, and includes a fiberglass reinforcement grid. It comes in rolls 1.35 meters (53”) wide by 50 meters (164’) long. It is about 24% less expensive than Intello Plus.
Calculating moisture risk
There’s a software tool called WUFI that can be used to determine what the moisture dynamics are likely to be in a particular building assembly and climate. In our project, we were concerned about our roof assembly, because the sheathing was outside of the vented roof cavity. We worried that there might not be an adequate air barrier in the roof assembly.
Terry Brennan used WUFI to determine that as long as there is at least minimal roof ventilation we would be fine without a vapor retarder on the interior. But our roof dormers weren’t going to be vented and the main roof wouldn’t be vented above the roof valleys. So we decided to install a vapor retarder as an insurance policy.
To allow drying to either the interior or exterior, we decided to go with a variable-permeability product, and we opted for Pro Clima DB+. The performance isn’t quite as good as Pro Clima’s Intello Plus, but the cost was lower and DB+ had some environmental attributes—such as being made from 50% recycled paper and being recyclable.
Installation of the DB+ was pretty straightforward. It went up after the Spider insulation had been installed. It was held taught over the rafters and stapled in place. Following installation for several days there was a reasonably strong ammonia smell. Ken Levenson of 475 looked into this and found out for me that it is from the ammonium phosphate that is added as a flame retardant. By the time strapping and drywall went up, the smell was gone.
We didn’t bother with the vapor retarder on the walls, because there we have a well-sealed air barrier in the middle of the wall—made from Zip sheathing with edges taped and extra air sealing using the EcoSeal product from Knauf.
We’re happy. The drywall is now mostly installed, and we look forward to never having to worry about moisture accumulating in our insulation. At least until the next theory of moisture control comes along….
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
Published July 31, 2013