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Do I Need a Vapor Retarder?

Someday, builders will stop asking this recurring question — but unfortunately, that day has not yet come

Posted on Jan 18 2013 by Martin Holladay

UPDATED on May 15, 2015

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 diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. 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 (exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration.), 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. — especially if there is a strong driving force, like the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. 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 hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. 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 rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. 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 vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). 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 Building Materials Property Table posted on the Building Science Corporation web site.

Vapor permeance is measured in a lab; the relevant tests are governed by ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. 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 International Energy Conservation Code. 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 insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection., 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 EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. 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 XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. 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, MemBrain or Intello Plus) 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.”

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Jan 28, 2013 10:26 AM ET

rules of thumb
by Gregory La Vardera

Martin, I could not agree with you more, that the vast majority of houses get built using rules of thumb. We all know that 99.99% of houses will never see a WUFI analysis.

In that context it seems that a wall assembly that is open to dry to the inside, and relies on exterior foam insulation to keep the dew point outside the cavity is vastly more likely to have its performance altered by local weather conditions, material choices, out of design range temperatures, out of design range humidity levels - all things that could be examined and resolved with analysis, that we all acknowledge they will never get.

What I am saying is that it appears there is far more risk in these type of walls we see green building experts steer people towards. There does not seem to be much concern over that risk. I just think you are contradicting yourself, in the way you've discussed the wall with poly here. You are using risk as an excuse while clearly not concered about greater risks in other assemblies you recommend. Thats the point I want to make, so I'm done. You are welcome to the last word on this.

Jan 28, 2013 10:48 AM ET

Response to Gregory La Vardera
by Martin Holladay

I'm interested in dialogue -- not having the last word. Your input is valuable.

Like most builders, designers, and researchers, I had my doubts about the use of exterior rigid foam in the late 1970s, and wondered whether this method of building was risky. Nevertheless, when I built the house I'm living in now in 1981, I took a leap of faith and installed a layer of rigid foam on the exterior side of my sheathing.

Since then, I've had a few opportunities to open up my walls. Fortunately, they are dry as a bone.

I'm aware that my house is an example of an anecdote, not data. However, over the past few decades, I've also had many opportunities to review research papers on these issues. Every year, the evidence keeps accumulating that walls sheathed with exterior foam of adequate thickness perform extremely well, and stay remarkably dry.

In recent years, good data have been coming out of the Coquitlam test hut in British Columbia. (See, for example, "The Coquitlam Experience".)

It's fine to be skeptical, Gregory. I share your skepticism. However, eventually skepticism must yield to data.

Jan 28, 2013 12:31 PM ET

data vs skepticism
by Gregory La Vardera

There is not a drop of skepticism in my opinions Martin. Painting them as such makes them seem less valid, so lets invalidate that first. The cautions I expressed in my previous post are based on building science principals that are obvious. Clearly a wall with exterior foam can be designed well and can operate well. My concerns are that it can more easily fail, particularly when we go by rules of thumb.

For many years we have had good reason to look to exterior foam as a strategy. Continuous layers of exterior insulation is one of the best ways to improve performance of an assembly. But because of the properties of foam it necessitated this flip to the previous consensus on the vapor profile. I think we are beyond that now and better alternatives to foam are readily available. Rigid mineral wool, and affordable double stud assemblies make the necessity to flip the vapor profile moot.

What is disturbing is that it is never framed as an aside - "here is something you should do if you are going to use exterior foam". Instead this flipped vapor profile is now offered up as the preferred solution. It does not perform better, it does not cost less. Why would this be the predominant advice?

Jan 28, 2013 12:55 PM ET

Response to Gregory La Vardera
by Martin Holladay

You misunderstand: this isn't about rigid foam.

The homes built by Zaring Homes that failed due to inward solar vapor drive did not have exterior foam. (If they had, they would still be standing.) But they had interior poly.

Polyethylene contributed to the EIFS failures in North Carolina, as well as the "leaky condo" failures in British Columbia. Most of the British Columbia condos did not have exterior rigid foam.

I'm not suggesting that builders "flip" anything. I'm suggesting (a) a greater focus on air tightness, and (b) an understanding of the role of vapor diffusion in wall asemblies. (Hint: the role is minor.)

Even back in the 1980s, Canadian builders began to realize that poly wasn't really needed as a "vapor barrier." That's why R-2000 builders started to call it an "air/vapor barrier."

Now we can achieve good air tightness with gypsum wallboard, plywood, or OSB, so we don't have to use poly as an air barrier. As a result, our walls are more robust, and less likely to fail.

Jan 28, 2013 6:35 PM ET

by Gregory La Vardera

I'm not familiar with the Zaring Homes example so that point is lost on me.

Listen, I'm not pushing for poly, and I'm not pushing for foam. But if you want to posture that these walls are robust, at least the foam moves the dew point out of the cavity most of the time. Without that what has our rule of thumb boiled down to? Tape up the outside, caulk up the inside?

Taped sheathing vs house wrap, gyp board vs air tight gyp board. It seems very little has changed, certainly not enough to refer to such an assembly as "robust". You advise to forget about the vapor retarder is really not all that different than the way walls have been and are being made. Kraft retarders on fiberglass never made an air seal, and likely never made a vapor seal either. Then all that has changed is your light advice that "it’s always a good idea to include an interior air barrier". So we caulk the electrical boxes, maybe. Did we get every hole, every seam in that metal box? Were the electricians wires in the way? Ehh, that will do.

Do we have "robust" wall, or do we have 30qts of water heading into the cavity?

This is not a robust assembly, and we as a green building "movement" are kidding ourselves and an ignorant housing industry if we continue to present it as such.

The answer to the question your blog post is about is "No, you don't need a vapor retarder, if you tape, and caulk, and caulk some more. But if you want to build a robust wall system with the best performance you want to use a smart vapor control layer, and keep your wiring inboard of that sheet to minimize penetrations. Its no harder to build, it costs about the same, and its hard to imagine why you would not opt for the better solution.

Jan 29, 2013 9:13 AM ET

Edited Jan 30, 2013 9:03 AM ET.

Response to Gregory La Vardera
by Martin Holladay

For more on the Zaring Homes fiasco, see When Sunshine Drives Moisture Into Walls.

You wrote, "What has our rule of thumb boiled down to? Tape up the outside, caulk up the inside?"

Greg, your proposed rules for air sealing are insufficient, and they don't represent my advice. We probably agree on air sealing rules of thumb. It's more than just "tape up the outside, caulk up the inside." I imagine that you and I would give the same advice to any builder: pay attention to your air barrier everywhere, with particular attention to penetrations, material seams, and changes in building materials. Then verify the performance of your air barrier with a blower-door test, and seal up any leaks that the blower door reveals. (If you think that I have a laissez-faire attitude toward air sealing, you might want to read Questions and Answers About Air Barriers.)

I think that even a cursory reading of my articles here at GBA will verify that you are providing a thin parody of my recommendations when you write that I provide "light advice that 'it’s always a good idea to include an interior air barrier.'"

Here's the thing, though: you have to pay attention to air sealing details whether or not you install interior polyethylene. The polyethylene argument really has nothing to do with the question of air sealing details.

Do you know the old Buddhist saying: "Before enlightenment, chop wood and haul water. After enlightenment, chop wood and haul water"? Well, we can reformulate that saying: "With polyethylene, pay attention to air sealing. Without polyethylene, pay attention to air sealing."

We both want robust wall assemblies. There is no substitute for careful thought, study, and execution of details if we want a robust wall assembly. It's not just about where you put the rigid foam, or whether or not you have poly. We need whole-house thinking. Everything matters: your choice of sheathing, whether or not you have a rainscreen gap, your roof overhangs, your flashing details, your penetration details, and the permeance of your interior finish materials. It all matters.

Jan 29, 2013 11:23 AM ET

comments are based on your post
by Gregory La Vardera

Martin, my comments, and parody if you wish, was simply based on the content of your post above. The same thing a builder inexperienced with raising performance would read if they landed here after a google search for "vapor retarders".

If you are saying its not about one kind of material or another, then you are not being earnest. The only thing that drives me to even comment here is the repeated advice for what I consider to be low quality, failure vulnerable assemblies. I don't think there is any self awareness or perspective here on suggestions like "air tight' drywall, the prospect of suggesting to a builder that they seal the gaps around each receptacle and switch box, that they seal every hole and seam in an electrical box.

Perhaps the green building community has just become too secular, too immersed in a community that all agree with one another. Used to speaking with people who care, and no clue how to propose solutions that can be implemented by people who really don't care.

This gets back to your distorted sense of risk. Those people who don't care, sure they can be compelled to seal all that electrical work, but they will never do a good job at it. If your advice is to be a model widely adopted in the housing industry, you will inevitably have walls built with lots of leaks, and 30qts and more of water heading into your cavity. If you don't' believe me, just look at fiberglass batts. Same problem. Its not that you can't do it, its not that its so hard to do, its that its just so much more profitable to do a lousy job. Your airtight drywall is the same thing in a different suit. This is not a robust solution, and it will not lead to a robust assembly.

Its just lamentable that this is the advice handed out so often here.

Jan 29, 2013 11:35 AM ET

Edited Jan 30, 2013 9:05 AM ET.

Response to Gregory La Vardera
by Martin Holladay

You say that the average builder does a bad job of air sealing. I agree.

You say that journalists (like me) who write about building science issues should remind builders of the importance of air sealing. I agree.

Here's where I disagree: I disagree with your conclusion that "Its just lamentable that this is the advice handed out so often here."

In fact, I think that GBA is doing a pretty good job helping to educate builders about the importance of air sealing, and suggesting methods to reduce air leakage. I could create a list of articles in this post to prove my point, but you could find the articles easily by using the GBA search box and searching for "air sealing" or "air barrier."

You believe that the Airtight Drywall Approach is hard to do well. You may be right. It's also possible to create an air barrier at the wall sheathing and roof sheathing, as Marc Rosenbaum suggests. It's also possible to use both techniques. Every time a North American builder says, "These techniques are just too hard," the European builders who are building Passivhaus buildings scoff, "It's not hard! We do it routinely."

Look, I'm happy to listen to your feedback. You tell me that GBA is doing a lousy job explaining air sealing details to builders. I'll take it to heart and try to do better, because my goal is the same as yours: to help builders build better buildings.

Finally, I'd like to remind you that installing a layer of polyethylene on your walls doesn't solve the problem of air leaks. Builders who use interior poly still need to study air sealing.

Jan 29, 2013 12:33 PM ET

let me restate my point
by Gregory La Vardera

I'm not saying that GBA is doing a bad job advising people to do air sealing. I'm saying that if you are concerned about risk - which you claimed you were - then the advice (or lack of in this particular post) you are giving out about air sealing could be seen as risky. I'm using airtight drywall as an example of that.

This is all playing off your criticism of my advice to use poly in a wall with no AC - you called it risky. While in the same article you acknowledged that use of poly could be beneficial, and in the comments you were basically saying its always risky, you should never use it. I'm flagging that - contradiction.

And I've strayed from the primary question of poly into air sealing because I'm pointing out that generally your concern about poly as a risk compared to your lack of concern about airtight drywall as a risk also seems like a contradiction.

I'm not out to make some site-wide criticism of GBA here. If I am saying you were casual about reinforcing No Vapor Retarder with Must Air Seal, its because in this post it was treated very casually. Elsewhere on GBA it may not be so.

Using a sheet based vapor control layer does not solve anything by itself. Put it right behind the drywall, and you have a world of sealing problems. But its far easier to move the wiring in front of the sheet making all the penetrations go away, than it is go back and caulk every hole you can find in your drywall and electrical boxes.

Jan 29, 2013 1:02 PM ET

Edited Jan 29, 2013 1:04 PM ET.

Response to Gregory La Vardera
by Martin Holladay

It seems that you are raising a new topic, but that's fine. You're saying that we shouldn't discuss the Airtight Drywall Approach, because it is risky.

That raises an interesting question. I would reply that it's only risky if builders do a sloppy job. But I think you are saying that the chances are so high that builders will do a sloppy job that the entire approach shouldn't be recommended.

That's a valid point -- kind of analogous to my opinion about fiberglass batts. (It's my contention that installing fiberglass batts according to the manufacturers' instructions is so difficult that the entire method of insulation is risky.) I agree that we need to promote methods that work in the real world.

However, saying that the Airtight Drywall Approach is risky because many workers fail to do a good job is a little different from saying that Zaring Homes' use of polyethylene was risky. As installed in the Ohio homes built by Zaring, polyethylene contributed to failure, even though it was installed correctly. It pushed the wall assembly over the cliff.

It's certainly worth discussing which wall assembly types are easiest to build and most robust. This article -- the one on vapor retarders -- isn't really about the Airtight Drywall Approach. If a builder follows the guidelines in this article, the guidelines will work for a wall with the air barrier at the sheathing level. They will work for a double-stud wall. They will work for a foam-sheathed wall. They will even work for a 2x6 wall filled with fiberglass batts, as long as the builder pays attention to air sealing -- which in most cases requires verification with a blower door.

If you are verifying your air barrier with a blower door, and tracking down wayward air leaks when the blower door is operating, I don't really care where you put your air barrier.

Jan 29, 2013 1:18 PM ET

Edited Jan 29, 2013 1:18 PM ET.

by Ron Keagle


You said this:

"Finally, I'd like to remind you that installing a layer of polyethylene on your walls doesn't solve the problem of air leaks. Builders who use interior poly still need to study air sealing.”

I do not follow your point. If you are saying that a defective vapor barrier does not stop air leaks, then yes I agree. But a continuous poly vapor barrier with all seems properly sealed with no leaks, and no field damage will certainly stop all air leaks. It can’t be a vapor barrier if it has air leaks.

It can, however, be a layer of polyethylene, as you stipulate. And if that is thrown up with staple damage, unlapped seams, sloppy taping (if any), unsealed penetrations, and subsequent drywalling damage; then I agree that it does not solve the problem of air leaks.

Jan 29, 2013 1:46 PM ET

Response to Ron Keagle
by Martin Holladay

You wrote that a poly vapor barrier "can't be a vapor barrier if it has air leaks."

Actually, it can. Joe Lsitburek makes this point often. You are confusing diffusion with air leakage.

If a layer of polyethylene sheeting has holes in it so that it is 97% polyethylene and 3% holes, it will be worthless as an air barrier. But it will still be 97% effective at reducing the flow of water vapor by diffusion. So it's a vapor barrier, not an air barrier.

I've seen many, many homes with interior polyethylene. I have not yet seen any installations of polyethylene that qualify as an air barrier. They all leak.

But if you are conscientious, it is possible to install polytheylene as an air barrier. It's incredibly fussy and difficult work. (It probably falls into the category that Gregory La Vardera would refer to as "so difficult to do that it is risky to recommend.") I've read about a few builders who have done it -- some in Canada, including a few pioneers of the R-2000 movement in Saskatchewan, and at least one builder in Wisconsin, Steve Lentz.

To install polyethylene as an air barrier;
1. You need to install airtight electrical boxes.
2. You need to find a way to seal all of the electrical boxes to the poly.
3. You need to be sure every polyethylene seam falls over a framing member.
4. You need to seal every polyethylene seam with Tremco acoustical sealant.
5. You need to find a way to bridge the gap between the poly and the window frames. (Steve Lentz follows a complicated procedure that includes installing a polyethylene "bib" around each window before it is installed.)
6. You need to create details to bridge the air barrier gap between the concrete foundation and the wall poly.
7. You need to anticipate interruptions of the ceiling air barrier that occur when top-floor partitions are installed.

It's possible, but it's a lot of work. Probably more than A.D.A.

Jan 29, 2013 4:15 PM ET

Edited Jan 29, 2013 4:18 PM ET.

Diffusion vs. Air Leaks
by Ron Keagle


I am not confusing diffusion with air leakage. I fully understand both principles and know the difference.

You said:

“If a layer of polyethylene sheeting has holes in it so that it is 97% polyethylene and 3% holes, it will be worthless as an air barrier. But it will still be 97% effective at reducing the flow of water vapor by diffusion. So it's a vapor barrier, not an air barrier.”

I don’t understand your point. If the poly sheet has 3% holes, it is a defective vapor barrier. Furthermore, how could a poly barrier with 3% holes be a worthless air barrier, but be a 97% effective diffusion barrier? How do you conclude that 3% holes renders it "worthless" as an air barrier?

You said:

“I've seen many, many homes with interior polyethylene. I have not yet seen any installations of polyethylene that qualify as an air barrier. They all leak.”

I have seen a lot of those too. But again, I would never equate the mere existence of “interior polyethylene” with an adequately executed vapor barrier. Interior poly could be an adequately executed vapor barrier, but not if it leaks.

I understand all your points about the difficulty of executing a perfect vapor barrier. I believe that the vapor barrier and its sealing members must be carefully designed, particularly as it pertains to the window and door tunnels. I would put the electrical boxes and cabling in their own service cavity in order to eliminate the need so seal the vapor barrier around them.

I agree that it would be very difficult to execute a perfect vapor barrier in just any arbitrary architectural design. My consideration of this issue is only for new construction, superinsulated houses where the architecture is idealized for the superinsulation principles.

Jan 29, 2013 4:28 PM ET

Response to Ron Keagle
by Martin Holladay

You wrote, "I understand all your points about the difficulty of executing a perfect vapor barrier."

No, you don't -- because a vapor barrier doesn't have to be perfect. It can be quite sloppy, and still work fine.

We can measure diffusion in a lab. For example, if you paint 90% of a piece of gypsum drywall with vapor retarder paint, but leave 10% of the gypsum drywall unpainted, then the vapor retarder will be 90% as effective as a vapor retarder covering 100% of the gypsum drywall. The reduction in diffusion can be measured.

Perhaps it's easier to understand if you imagine a vapor barrier under a concrete slab. Let's say you put down a layer of 6-mil poly, but the poly is in terrible shape. It's full of holes. Maybe 5% of the poly is totally gone -- just plain missing. This sloppy vapor retarder will be 95% effective. The reduction in diffusion can be measured in a lab.

You can't be that sloppy with an air barrier. Huge amounts of air through small holes, especially if there is a driving force (for example, the stack effect).

Jan 29, 2013 6:15 PM ET

Edited Jan 29, 2013 6:18 PM ET.

Response to Martin
by Ron Keagle


Thanks for that explanation. Just to be clear, when I say "perfect" vapor barrier, I mean one that works just fine.

My question was mostly directed to your comment about how 3% holes in the vapor barrier would render it worthless as an air barrier. I am just wondering generally what you mean by these terms.

I understand your point about a lot of air escaping through small holes, but wonder how this correlates to the complete loss of air barrier function that you mentioned. I would think that holes in the air barrier would gradually degrade its function, just as would holes in the vapor barrier. The degradation would not perfectly match, but the general trend would be the same.

Jan 29, 2013 6:45 PM ET

Response to Ron Keagle
by Martin Holladay

You wrote, "I would think that holes in the air barrier would gradually degrade its function, just as would holes in the vapor barrier."

You're right, of course. But here's the thing: let's say that an earthquake or a tornado suddenly removes 10% of your walls and ceilings. Only 90% of the walls and ceiling remain. The wind is whipping through those holes.

As you can imagine, your house has become unheatable. There's no way your furnace can keep up. All of your heat is leaving the house through the huge holes created by the earthquake or the tornado.

That's very different from a situation where 10% of the vapor barrier is missing from underneath a concrete slab. The remaining 90% vapor barrier is adequate to reduce the vapor diffusion through the slab. No one notices the 10% problem.

So when does an air barrier become "useless"? With 0.1% holes? With 1% holes? With 3% holes?

I guess it depends on your airtightness goals. But if I install gypsum wallboard as my air barrier, I wouldn't want a 1 square foot hole in each sheet of drywall. (That's what a 3% defect in my air barrier means.)

Feb 19, 2013 5:37 PM ET

Great Article
by Matt Risinger

Great Article Martin. I read your posts but never seem to comment. Just wanted to thank you for your thoughtful blogging! I especially love the "Executive Summary" at the bottom. I do alot or whole house remodels in hot/humid Austin TX and I've seen firsthand the mold growing behind the poly sheeting of 90's houses I've remodeled. Attached is a photo of a 1995 built home I remodeled last year with mold behind the vapor barrier. Best, Matt Risinger


Feb 20, 2013 4:59 AM ET

Response to Matt Risinger
by Martin Holladay

Thanks for the feedback, and thanks for sharing the photo. I wasn't aware that polyethylene stupidity reached as far south as Austin, Texas.

Jun 22, 2013 4:24 PM ET

Tiny House Vapor issues
by Lara Mitchell

Thank you for the very informative articles, Martin. They are very useful.

I'm building a tiny house, that pretty much avoids most of the code issues. My house will be on a trailer. There will be galvanized steel flashing tack welded to the bottom of the entire frame, leaving little gaps between each tack spot. The idea is to keep out most road moisture during wet travel, and rodents/pests. I'm hoping that if there is some kind of accidental flood inside the house, most of the water can escape through the gaps in the flashing. I will be using loose sheep wool for the floor insulation, sheep wool rolls for the walls. The external house wrap will be Tyvek. I plan on adding rain screen and pest screen to the exterior walls and baffles to the roof to allow airflow.

Because the wool absorbs and releases moisture without compromising thermal efficiency, I want to make sure there is enough air for it to breathe. I will most likely put a marine-grade piece of plywood between the trailer bed and the sub floor framing, then the framing (which will be sunken into the trailer bed, since the cross members of the trailer will be welded to the bottom of the side rails to allow for more ceiling height in the loft), then the wool insulation, then the decking.

My concern is, of course, will water/air be able to get in and out of the sub floor effectively without causing any molding/rot issues? Also, from reading your articles, is it safe to say I don't need any further vapor/air retardants on the walls or floor?

Thank you for your consideration.


Jun 22, 2013 4:57 PM ET

Edited Jun 22, 2013 4:59 PM ET.

Response to Lara Mitchell
by Martin Holladay

Q. "Will water/air be able to get in and out of the subfloor effectively without causing any molding/rot issues?"

A. There is no reason that you want any air or water to get into your floor assembly. Ideally, your floor needs an air barrier -- usually, that is the subflooring, which is OSB or plywood adhered with construction adhesive -- and you want to do everything you can to keep water from entering. It sounds like you are using vapor-permeable materials, so I imagine it will dry out if it ever gets wet.

Q. "Also, from reading your articles, is it safe to say I don't need any further vapor/air retardants on the walls or floor?"

A. As I just wrote, you always need an air barrier. Pay attention to airtightness when installing your subfloor, and seal air leaks if there are any penetrations through your subfloor.

Jun 24, 2013 7:34 PM ET

Edited Jun 24, 2013 8:55 PM ET.

Tiny House Vapor issues
by Lara Mitchell

Thank you for your response, Martin.

Good advice, that.

My thoughts, then, are to use the Huber Zip System, then house wrap over that as a WRB, taping the heck out of everything. I've heard about this newer product called EcoSeal that is sprayed on inside the house at all joints to further seal leaks. It wouldn't be too expensive to do an entire tiny house, but then again, it might be harder to convince them to take the job since they want to send out a "professional" installer. Have you heard any feedback regarding this product?

Thank you again for sharing your knowledge.


Jun 25, 2013 3:53 AM ET

Response to Lara Mitchell
by Martin Holladay

GBA has published at least two articles on EcoSeal. Here are the links:

Air Sealing With Sprayable Caulk

EcoSeal: A New System for Air Sealing Homes

Feb 10, 2014 6:37 PM ET

Edited Feb 10, 2014 6:57 PM ET.

wine storage room in existing garage
by Jerry Walgamuth

I have an existing, one-story, 10x16, dry-walled and insulated garage, 1/3 of which will be converted into a wine storage room. Wine room will be conditioned to 58 degrees and 50 to 70% humidity. Southern California climate extremes = low 30's in winter and above 100 in the summer. Can I get by with airtight drywall specs or do I need vapor retarder?

Feb 11, 2014 7:27 AM ET

Response to Jerry Walgamuth
by Martin Holladay

In general, air leakage causes more problems than vapor diffusion. So the first principle for designing any insulated wall or ceiling is to pay attention to airtightness.

You didn't mention what type of insulation you are installing. If you choose to insulate with closed-cell spray foam or rigid foam, these insulation materials are already vapor retarders.

If you choose to insulate with a fluffy insulation like fiberglass batts, it would probably make sense to install a "smart" vapor retarder like MemBrain on the interior side of the wine cellar. The reason for this (conservative) approach is that the room is being maintained at a high humidity level.

Apr 4, 2018 10:43 AM ET

Vapor barrier paint for improper installation fix
by Tayson

Hi Martin,

If you are out there, thanks for all of the useful articles.

I have acted as general contractor for a few of my own projects. On the latest to home build, I used an improper installation method for my climate zone 6 which was one inch of closed-cell foam then wet spray cellulose. On the last project, my drywaller started too soon which resulted in wet wall cavities, which I assume will dry when the moisture drive reverses this summer, even with the one inch of foam. I read all of the perm ratings, and all of the data. In an attempt to remedy the situation in both homes, I have installed HRV systems, High CFM motion and humidity bathroom fans, and high CFM Auto on Hood ventilation systems. I am planning to repaint the latest project once the wall cavities are dry. I have been faced with the Dilemma of either caulking the electric boxes and sealing all areas that would allow air transfer then priming with a vapor barrier paint, or simply caulking all air leaks and using a typical primer and paint. My fear is the vapor barrier paint will cause more issues with respect to the walls drying in the event of future diffusion or moisture penetration. Any advice?

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