UPDATED on May 15, 2013
Every couple of weeks, someone sends me an e-mail with a description of a proposed wall assembly and an urgent question: “Do I need a vapor retarder?” Energy experts have been answering the same question, repeatedly, for at least thirty years. Of course, even though I sometimes sigh when I read this recurring question, it’s still a perfectly good question.
The short answer is: if your wall doesn’t have a vapor retarder, there is no need to worry. Builders worry way too much about vapor diffusion and vapor retarders. It’s actually very rare for a building to have a problem caused by vapor diffusion.
A while back, I collected seven questions about vapor diffusion, and published them (along with my answers) in a blog called “Vapor Retarders and Vapor Barriers.” Since new questions keep showing up in my In box, I decided it was time for another Q&A roundup on vapor diffusion. Here are nine more questions on the topic.
Q. What is water vapor?
A. Water vapor is water in a gaseous state — that is, water that has evaporated. It is invisible. It is present in the air we inhale, and (in even greater concentrations) in the air we exhale.
When this invisible water vapor moves through building materials, the phenomenon is called vapor diffusion.
Q. Was the information I learned 30 years ago all wrong?
A. In the 1970s and early ’80s, builders were taught that it was important to install a vapor barrier (usually, polyethylene sheeting) on the warm-in-winter side of wall insulation and ceiling insulation. Most textbooks and magazines explained that a vapor barrier was needed to keep the walls dry during the winter, and that walls without vapor barriers would get wet.
This was bad advice, for several reasons. First of all, outward vapor diffusion through walls during the winter almost never leads to wet walls. When interior moisture causes moisture damage in walls or ceilings, the problem is almost always due to air leakage (exfiltration), not vapor diffusion.
Second, since an interior polyethylene vapor barrier prevents wall assemblies from drying inward during the summer, a layer of poly can actually make the wall wetter than it would be without the poly.
Q. What’s the difference between air leakage and vapor diffusion?
A. Water vapor can diffuse through vapor-permeable materials (for example, gypsum drywall) even when there are no air leakage pathways. If the air on one side of the drywall is hot and humid, and the air on the other side of the drywall is dry and cold, the drywall absorbs moisture from the humid side. Once the drywall is damp, some of the moisture in the damp drywall evaporates from the other side (the side facing dry air). This process of vapor transport through the drywall is called vapor diffusion. It happens even when the wall assembly is perfectly airtight.
Air leakage is a different phenomenon. If there is a hole in the drywall — at an electrical box, for example — then warm interior air can enter the wall cavity through the hole and escape through cracks in the wall sheathing — especially if there is a strong driving force, like the stack effect or a fan that is pressurizing the house. If the interior air is warm and humid, and the wall sheathing is cold, it’s possible for some of the moisture in the air to condense on the wall sheathing. (Although this phenomenon is often called condensation, it is more accurately referred to as adsorption or absorption. What happens is that the cold, dry sheathing becomes damp as the moisture from the indoor air is transferred to the sheathing.)
In the typical (somewhat leaky) wall, far more moisture is transported by air leaks than by vapor diffusion.
Q. Can you explain the difference between an air barrier and a vapor barrier?
A. An air barrier is a material that stops air leakage. A vapor barrier is a material that stops vapor diffusion.
Some building materials — for example, insect screening — allow the flow of air and water vapor. Insect screening is neither an air barrier nor a vapor barrier.
Other building materials — for example, gypsum drywall or plastic housewrap — are vapor-permeable but are still air barriers.
It’s also possible to have a building material — for example, a layer of vapor-barrier paint on a leaky plaster wall, or the kraft facing on fiberglass batts — that meets the legal definition for a vapor barrier (or vapor retarder) without being an air barrier.
Finally, it’s possible to have a building material — for example, polyethylene sheeting with taped seams — that acts as both a vapor barrier and an air barrier.
Q. How did requirements for vapor retarders get enshrined in our building codes?
A. William Rose, a research architect at the Building Research Council at the University of Illinois, has investigated this question. Rose reported his findings in his landmark book, Water in Buildings.
According to Rose, there were three main players in this drama:
- Larry V. Teesdale, a senior researcher at the U.S. Forest Products Laboratory;
- Tyler Stewart Rogers, a Harvard-trained architect; and
- Frank Rowley, a professor of mechanical engineering at the University of Minnesota.
During the 1930s, Teesdale, Rogers, and Rowley each contributed research or published articles that, directly or indirectly, responded to complaints of peeling paint on the exterior of recently insulated buildings. Rose wrote, “When insulation was introduced into wood-frame houses in the late 1920s and early 1930s, the paint began to peel. House painters often refused to paint insulated houses. The painters developed a pithy expression to describe what happens: ‘Insulation draws moisture.’”
Insulation manufacturers, insulation contractors, and many researchers (who, because of its obvious benefits, often promoted the increased use of insulation) took exception to the conclusion drawn by these complaining house painters. Tyler Rogers was particularly offended by the idea that insulation might make sheathing and siding wetter than they would otherwise be. In a seminal article, “Preventing Condensation in Insulated Structures,” published in the March 1938 issue of Architectural Record, Rogers wrote, “Architects, owners and research technicians have observed, in recent years, a small but growing number of buildings in which dampness or frost has developed in walls, roofs or attic spaces. Most of these were insulated houses. … The erroneous impression has spread that insulation ‘draws’ water into the walls and roofs … Obviously, insulation is not at fault — at least not alone.”
Rose’s analysis differs from Rogers’, however. Rose wrote, “Does insulation ‘draw’ moisture? Yes, insulation draws moisture to exterior materials. Insulation lowers the temperature of exterior materials. At the same vapor pressure, lower temperatures means higher relative humidity and higher moisture content. The painters were right. Paint holds more poorly on an insulated building, in general.”
Like it or not, physics provides an explanation for the observation that paint doesn’t last as long on an insulated building as it does on an uninsulated building. Adding insulation to a wall tends to make the sheathing and siding colder, and cold materials tend to be wetter than warm materials. When siding is cold, it draws moisture from the surrounding (exterior) air; the dampness is a function of its temperature. Rose wrote, “Deciding to insulate has the direct and immediate effect of causing those exterior materials (in cold weather) to be wetter. Historically, those advocating for insulation did not want to be seen as being responsible for additional wetness.”
Rose wrote that Teesdale, Rogers, and Rowley “created a version of hygrothermal building science for the United States that focused on moisture conditions in exterior materials during cold weather. The version they created was partial, and it was biased: It highlighted the importance of vapor transport, while it obscured the importance of temperature impact.” In other words, Teesdale, Rogers, and Rowley promoted the idea that the siding was getting damp because moisture was traveling through the wall assembly by diffusion from the interior. While this diffusion does occur, the amount of moisture transported via diffusion isn’t that significant; the governing factor determining the moisture content of the siding is its temperature, not the rate of diffusion through the wall.
Rose continued, “They produced prescriptive recommendations that later became code requirements, and these prescriptions embodied the incomplete and biased nature of their analysis. They supported their argument with a flawed and misleading analogy. They and their followers left a legacy of consumer fear of ill-defined moisture effects in buildings and of designers assigning excessive importance to prescriptive measures.”
The “misleading analogy” was a model of vapor transport through walls that was based on a flawed analogy with heat flow. The “prescriptive measures” that have caused so many headaches for builders were vapor barrier requirements in building codes.
According to Rose’s research, in January 1942, the Housing and Home Finance Agency established a requirement for an interior vapor barrier with a minimum permeance rating of 1.25 perm. This requirement was incorporated into the BOCA code — an early residential building code — in 1948.
The building code requirements for vapor barriers were the result of politics and technical errors, not scientific research. Rose wrote, “The authors of the condensation paradigm created a framework, a way of analyzing moisture conditions in buildings, that was distorted. It promoted vapor control, with a prescriptive requirement for vapor barriers in all buildings. At the same time, it masked an important physical principle — how materials at cold temperatures are wetted, and how, once wetted, the possibilities for vapor control mitigation are severely limited.”
For information on current building code requirements for vapor retarders, see Vapor Retarders and Vapor Barriers.
Q. Can I just ignore vapor diffusion?
A. Not quite, but almost. There are a few circumstances where builders need to pay attention to vapor diffusion:
- Vapor diffusion can be a significant moisture transport mechanism in certain rooms with high humidity — for example, greenhouses, rooms with indoor swimming pools, or rooms that are deliberately humidified — especially in a cold climate. If your building includes a greenhouse or indoor swimming pool, get expert advice on your wall and ceiling details before proceeding with the project.
- In a very cold climates (the colder sections of Climate Zone 7, as well as Climate Zone 8), the traditional use of interior polyethylene vapor barriers is often beneficial. That said, interior polyethylene can occasionally cause problems even in these climates, especially in buildings that are air-conditioned during the summer. When in doubt, a “smart” retarder with variable permeance is always safer than polyethylene.
- When open-cell spray foam is used on the underside of roof sheathing to create an unvented conditioned attic in a cold climate (climate zones 5 and colder), outward vapor diffusion during the winter can lead to damaging water accumulation in the roof sheathing. For this reason, it’s best to use closed-cell spray foam for this application in climate zones 5, 6, 7, and 8. If you insist on using open-cell spray foam, it must be protected on the interior with a layer of gypsum wallboard painted with vapor-retarder paint.
- Inward vapor diffusion during summer months can lead to problems in homes that include a “reservoir” siding (for example, brick veneer) and a vapor-permeable sheathing (for example, fiberboard). For more information on inward solar vapor drive, see When Sunshine Drives Moisture Into Walls.
- Wintertime moisture accumulation in exterior sheathing on cold-climate double-stud walls is associated with outward vapor diffusion. The following details may reduce this type of moisture accumulation: including a ventilated rainscreen gap between the siding and the sheathing; specifying vapor-permeable sheathing like fiberboard or DensGlass Gold; installing a layer of OSB or plywood sheathing in the center of the wall; and installing a smart vapor retarder on the interior side of the wall.
- It’s important to remember that diffusion can be a builder’s friend. During the summer, inward vapor diffusion through drywall can help to dry a damp wall assembly. That’s why the use of interior polyethylene or vinyl wallpaper often leads to problems.
Q. How do I figure out if a material is vapor-permeable?
A. There are published tables listing vapor permeance values for many common building materials. For example, you can refer to the posted on the Building Science Corporation web site.
Vapor permeance is measured in a lab; the relevant tests are governed by ASTM E96. There are two test procedures described by ASTM E96: procedure A (the “dry-cup” test) and procedure B (the “wet-cup” test). The 2007 Supplement to the IECC specifies that the permeance of a vapor retarder should be determined by procedure A, not procedure B.
It’s fair to say that procedure A measures the vapor permeance of a material when it is dry, while procedure B measures the vapor permeance of a material when it is damp. The permeance of many materials (including asphalt felt, plywood, and OSB) is variable: when these materials are dry, they have a relatively low permeance; when they are damp, their permeance rises. (Some people refer to materials with a variable permeance as “smart retarders.”)
Q. Is there any reason I have to know the exact perm rating for the materials I use?
A. No, but it’s sometimes useful to know whether a material falls into a broad category — in other words, whether the material is vapor-permeable, vapor-impermeable, or somewhere in between.
To simplify the situation, I’ll list a few materials that are considered “vapor-permeable” — that is, with a perm rating over 10 perms. These materials include gypsum drywall, plastic housewrap, fiberglass batts, cellulose insulation, asphalt-impregnated fiberboard sheathing, and 5 inches or less of open-cell spray foam.
Next, let’s list examples of materials that are considered “Class III vapor retarders” — that is, with a perm rating between 1.0 perm and 10 perms. These materials aren’t vapor barriers, but they slow down the flow of water vapor somewhat. Examples include stucco, one or two coats of latex paint, 1 inch of EPS foam insulation, and more than 5 inches of open-cell spray foam. (Note that the greater the thickness of a piece of foam insulation, the lower its permeance.)
The next category is a group of materials that are considered “Class II vapor retarders” — that is, with a perm rating between 0.1 perm and 1.0 perm. These materials slow down the flow of water vapor to a greater extent than materials that are considered Class III vapor retarders. Examples include plywood, OSB, the kraft facing on fiberglass batts, 1-inch-thick XPS foam insulation, and one coat of vapor-retarder paint applied to drywall.
Finally, the most impermeable materials are called “Class I vapor retarders” or “vapor barriers.” There materials include glass, sheet metal, aluminum foil, and polyethylene.
Q. Information overload! What’s the short version?
A. OK, we’ll break all this information down to a few rules:
- Most buildings don’t need polyethylene anywhere, except directly under a concrete slab or on a crawl space floor.
- The main reason to install an interior vapor retarder is to keep a building inspector happy.
- If a building inspector wants you to install a layer of interior polyethylene on a wall or ceiling, see if you can convince the inspector to accept a layer of vapor-retarder paint or a “smart” retarder (for example, or ) instead.
- Although most walls and ceilings don’t need an interior vapor barrier, it’s always a good idea to include an interior air barrier. Air leakage is far more likely to lead to problems than vapor diffusion.
Martin Holladay’s previous blog: “Nostalgia for the Hippie Building Heyday.”