The post was simply labeled “Martin Holladay” — for the GreenBuildingAdvisor senior editor — but the question from architect Stephen Thompson went to the heart of one of the most contentious building questions in recent history: is a polyethylene vapor barrier a good idea?
Thompson tells Holladay he’s read much of what Holladay has had to say about vapor barriers, but he still is puzzled by several comments.
“You believe that polyethylene film is not to be used as a vapor barrier,” Thompson says in his Q&A post. “I could not disagree more; after 10 years of work in Minnesota, I’ve seen what can happen when VBs are left out. All of the expert seminars at the AIA Convention insist that VBs are essential in cold climates. After viewing their photos of buildings turned icicle, I must agree.”
Thompson makes two other points: that some GBA readers have implied an exterior vapor barrier is correct, when in fact that only makes sense in the deep South, and that some readers have referred to Tyvek, the house wrap made by DuPont, as a vapor or air barrer, which is not correct.
Although the practice is not required by code, some builders in cold climates continue to install a poly vapor barrier over the studs before the wall is covered with drywall. The poly is supposed to stop the migration of water vapor into wall cavities during the winter, where it would condense and cause problems such as mold or decay.
But as building scientists learned more about the movement of air and moisture through walls and roofs, the practice was discouraged in all but the most extreme climates. Thompson’s question, especially because it comes from a building professional, revives this old debate once again and is the focus of this week’s Q&A Spotlight.
Poly vapor barriers are a problem
It is GBA advisor Carl Seville who’s first to reply, with a comment that seems to summarize current thinking on the practice. Vapor barrier is really a misnomer, he says, when the poly should more correctly be called a vapor diffusion retarder. But leaving that aside for the moment, a poly layer can be “very problematic,” Seville says.
“They are a disaster in warm humid climates, and even in cold climates, if there is any air conditioning being used,” Seville writes. “There is a big opportunity for vapor drive to the interior where it will condense inside the wall cavity. Vapor barriers in extreme cold climates can serve a purpose if they are installed without any gaps or perforations — if there are, moisture laden air will flow right through any gaps, minimizing its effectiveness.
“In most warm climates, there is enough cold weather that walls dry to the exterior in the winter, so an exterior barrier is also problematic.”
Experts in the field believe that careful air sealing, not a vapor barrier, is the real key to keeping moisture vapor out of wall cavities.
“I’ll let Martin defend himself, but I must say a couple things,” adds Brett Moyer. “I think you really need to go back and review some Building Science 101. Who are these ‘experts’ at the AIA conference? ALL legitimate and respected building scientists do NOT recommend a vapor barrier/Class I vapor retarder anywhere but the most extreme cold climates. Homes in the lower 48 — Airtight Drywall Approach and a Class III vapor retarder (latex paint) is all you need.
“The key to a durable wall assembly is air sealing, not vapor barriers.”
The difference between air and moisture barriers
Thompson responds that despite this assertions, both the Canadian Building Council and the U.S. Department of Energy continue to recommend vapor barriers on the interior side of studs in cold climates.
But Lucas Durand isn’t convinced that the practice is endorsed so broadly. “In my opinion, the National Building Code of Canada does quite a good job in distinguishing between vapor retarders and air barriers,” he writes. “My experience has been, however, that despite having clear guidance on distinguishing between the two, every building professional I’ve had the pleasure of dealing with has somehow had it drilled into them that they are something like the same thing,” he adds.
“My best guess is that the building industry in Canada has built so many homes for so long that use a poly ‘VB’ as a vapor retarder and air barrier, they have somehow become synonymous with each other. The reality is you can get a lot more in terms of energy performance and durability if you understand the role of each as a separate entity within a wall system and make design choices that use sometimes different materials to optimum effect.”
Robert Hronek suggests Thompson spend some time reading up on the topic at the web site of the Building Science Corp., where Joseph Lstiburek discusses how much moisture an air leak can carry through a small tear in a polyethylene vapor barrier.
“Poly is rarely installed to the level of being an air barrier as well as a vapor barrier,” Hronek writes. “If its not an air barrier than it is not really a vapor barrier. Except in the most extreme climate a vapor retarder is all that is needed. Painted drywall is a vapor retarder. Add dense-pack or spray foam insulation and you don’t have the air flow carrying moisture into the wall. Take it farther with advanced air sealing and it gets better.”
Beware of moisture driven inward
Holladay says an interior polyethylene vapor barrier “can work” in cold climates such as Vermont, Minnesota, and Canada, providing the building is not air conditioned, and there is no exterior foam sheathing on the walls. Still, he says, it’s not a great idea.
Holladay directs Thompson to an article he wrote on vapor barriers and inward solar vapor drive. In it, Holladay discusses the origins of the vapor barrier recommendation, and some of the building disasters that resulted from using them in air-conditioned buildings.
A Cincinnati builder, Zaring Homes, was actually driven out of business by moisture problems in brick-clad houses it built.
But in the case of Zaring Homes, Thompson thinks the blame may have been unfairly leveled at the vapor barrier and not the fact that the brick veneer was installed without a “suitable cavity” behind it. A 2-in. air space behind the brick allows the masonry to dry out, and should be combined with brick vents and 2 in. of rigid foam insulation over the sheathing.
“… And so, I stubbornly question if this discussion isn’t more about solid moisture barriers preventing water and water vapor from entering assemblies from the outside as opposed to air-conditioned poly properly positioned in a poorly designed moisture barrier assembly?” Thompson says. “Your patience please. It’s difficult to teach old dogs new tricks.”
Holladay, however, thinks Thompson is confusing the effects of diffusion, water penetration, and air leakage. Inward solar vapor drive is a diffusion phenomenon that doesn’t have much to do with water penetration — and the problem can even occur in walls with an air gap.
“Your concern about interior poly is misplaced, because wintertime moisture migration into wall cavities is mostly an air leakage phenomenon; diffusion plays only a very minor role,” Holladay says. “You need an interior air barrier, not a vapor diffusion barrier, to address the problem of wintertime moisture migration into wall cavities. In your original post, you mentioned ‘icicles.’ Ice accumulation in wall cavities is not a result of diffusion.”
Why architects believe in vapor barriers
If architects are highly trained building professionals, why do some of them continue to promote the use of poly vapor barriers when experience suggests they can cause problems? Architect Bill Rose joins the discussion to offer this theory:
“Architects are pretty much the last remaining defenders of the vapor barrier. Why is that?” Rose writes. “I try to explain it using the ideas of performance and prescription. The client, and the public, want performance. Prescriptions (standards, codes, etc. …) are invented as shortcuts to performance, and they facilitate commerce. Two problems: 1) prescriptions never get better over time, and 2) institutions accrue around them drawing profit and protection.
“The vapor barrier is a prescription,” he adds. “It has outlived its usefulness. It detracts from performance at least as often as it provides benefit, all somewhat climate-dependent of course, and mostly it has no impact on performance. Buildings don’t need poly vapor barriers, architects need vapor barriers. My colleague Larry Elkin calls them ‘liability barriers.’ ”
Our expert’s opinion
Here’s what Peter Yost, GBA’s technical director, has to say:
Bear with me while I make a couple of key points before getting to the question at hand.
- 1. Building assemblies and materials get wet in one of four ways: bulk water, capillary water, air-transported vapor, and vapor diffusion.
- 2. In general, we need to worry about wetting in that order. There is the exception of solar-driven moisture being driven by vapor diffusion.
- 3. Building assemblies and materials dry in one of two ways: free drainage (bulk water) and by diffusion or evaporation.
- 4. We need to worry as much about how things dry as we do about how they get wet.
Whether we are talking about vapor barriers or retarders, we are talking about keeping building assemblies and materials from getting wet by the least important mechanism for wetting, which is vapor diffusion. And if we are focusing on the vapor permeability of just one component or material in the assembly, we are not dealing with how these assemblies and materials are going to dry.
If the choice is made to have an interior layer of polyethylene sheeting serve as the air barrier and the vapor retarder for outward moisture drive during the winter in a really cold climate — 9,000 heating degree days or more — be sure of three things:
- 1. That the polyethylene is actually a complete air barrier as proven by a blower-door test result.
- 2. That the assembly is designed to dry to the exterior.
- 3. And that the building is not air-conditioned during the summer.
Can you design and build assemblies that dry to the interior in really cold climates, have an interior air barrier not made of poly, and air condition that building in the summer? Yes, you can — any PERSIST OR REMOTE wall assembly can do just that, or any assembly designed using either or both of these approaches: vapor profile and dewpoint method analysis.
Finally, install mechanical systems that manage interior relative humidity in cold climates to reduce vapor drive — in other words, a ventilation system. That, a continuous air barrier, and designing/building assemblies to be able to dry are much more important than any prescription for an interior vapor retarder — much less a barrier — in cold climates.