Tuesday July 30, 2013

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. 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 increasingly calling for vapor retarders whose vapor permeability changes based on the humidity conditions. We installed one of these new materials on our house.

Changing recommendations

Back when poly was the default choice as a vapor retarder, 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.

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 can hold less moisture, and if it gets cold enough the moisture in the air will condense -- 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 moisture out of the wall cavity where that water vapor 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, the risk was that condensation could occur with moisture laden air moving inward through the building enclosure and cooling off.

But it wasn’t really clear what you were supposed to do in places -- really most of the U.S. -- where some of the time it’s warmer inside than out and at other times it’s colder inside.

Confused? So was (and 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 -- as my friend (and leading building science expert) Terry Brennan explained. 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."

Maybe 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 greater than or equal to 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, N.Y. 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 percent 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 some 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 a sort-of 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 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 got back to 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.

Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (, both based in Brattleboro. Send comments or suggestions for future columns to


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