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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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Image Credits:

  1. Matthew H

Jan 18, 2013 8:58 AM ET

Interesting photo Martin.
by Chris Brown

Interesting photo Martin. Poly over kraft??? Secondly, (although not in keeping with your topic) is evidence of what initially seems to be a very good application of fiberglass batts that appears to have been compromised by another trade. In the bottom of each cavity the batts have obviously been disturbed to the point that each cavity is now compromized. One can't point the finger at the fiberglass material or the application technique in this photo! I see this far too often in the field. Other trades rarely hesitate to displace, damage or destroy the integrity of insulation applications, regardless of the insulating product used. If everyone working on these projects isn't in tune with the ultimate goal, we will find success difficult to attain. Either during inspection, or if and when an infrared image were to be taken of this wall after it is closed up, the insulation contractor will wrongly get the blame. By the way, fiberglass insulation materials are much more user friendly than ever before, especially those that have eliminated the phenolic binders. 20 years ago it was easier to understand why other trades didn't want to take the time to correctly replace batts that were removed. Today's batts aren't dusty or itchy for that matter. No excuse for not replacing them or repairing them.

Jan 18, 2013 9:08 AM ET

Edited Jan 18, 2013 9:17 AM ET.

Response to Chris Brown
by Martin Holladay

You guessed correctly: the photo does not show best practices.

Believe it or not, this type of installation used to be very common in Vermont. I used to see kraft-faced batts covered with interior poly all the time. To this day, there are plenty of builders who keep doing it the same way -- that is, the way they learned to do it back in the 1970s.

I'm not a big fan of fiberglass batts, and I think that blaming someone for not quite placing the batts perfectly is misguided. It's far more common to see batts with wrinkles or smushed-in sections than it is to see a perfect installation job -- and the fault lies more with the batts themselves than with the installer.

For more information on the topic, see Installing Fiberglass Right. In that article, I wrote: "It is the nature of a fiberglass batt to want to be installed sloppily. Unlike cellulose or spray polyurethane foam, a fiberglass batt doesn’t volunteer to fill a cavity completely; on the contrary, it tends to fight an installer’s attempt to make it fit snugly."

Jan 18, 2013 9:29 AM ET

Martin, I disagree. There is
by Chris Brown

Martin, I disagree. There is no doubt that that many find fault with fiberglass batts, and you have made it clear numerous times that you do not favor them. However, they are still the most widely used product for thermal applications. My point was simply this: The insulation installer did his job and did it correctly and yet it was compromized by someone else! We should not accept the fact that it could not have been repaired correctly, because it could have been! In the interest of full disclosure, I have been in the insulation business for over 45 years now, as a contractor and now as an employee of the manufacturer who's product is in the picture. My last point (and I will cease hijacking) is this: Do no harm! No trade should compromize the work of others. I know that is idealistic and I get that this is construction. What would it have looked like had this wall been sprayed with loosefill fiberglass, cellulose or foam? I don't care what insulation product was initially used in this wall. It was compromised and would have been regardless of the product used.

Jan 18, 2013 1:23 PM ET

Chris Brown: i see 1 major
by Jin Kazama

Chris Brown: i see 1 major fault in this picture
interior insulation ...

unfortunately, foam and or panel type exterior insulation will only rise up in popularity as
informed people such as many on this site ( Martin etc.. ) will push informative questionning and provide documentation to benefits all around.

People need to wake up and realize that 75% trade workers do very very bad jobs, destroys the previously well done jobs , and it will get worse. The new generation of young worker that are chatting on their cell phone while working really don't care at all about quality of work.
( let's assume that less than 25% of the young worker do care ..but i am beeing positivly unrealistic here )

I haven't seen a well done tile job in many many many years now, even in high end restaurants ,
hotels, luxury homes ..
Electricians drill holes wrong place in beams, destroys wall insulation and integrity...
Gypsum guys leave a mess in the walls cavity that "haunt" the future owners for 100 yeasr
( my parents house has had dust problems because of sanded dust at the bottom of inner walls partitions )

This will not change, however, using foam insulation at the exterior is much safer as far as integrity goes.

Then, polythene vapor barrier ... i am wondering how many holes per sq ft we would be able to find in most walls if we would be able to peak. Way too easily compromised.

Fiberglass bats insulation is still with us for only 1 reason = economic
Contractors= business= profit = fiberglass bats

Unfortunately, 95% of residential buildings here in Quebec are built using wood framed, fiberglass bats and interior poly sheets !

Good thing now R4 exterior sheathing thermal break for studs is mandatory !!

But again, everyone is trying to solve problems created by a wrong base system.

Jan 18, 2013 1:41 PM ET

Edited Jan 18, 2013 1:43 PM ET.

Vapor Movement
by Ron Keagle


When you refer to the governing factor that determines moisture content of siding being its temperature, I assume that you mean this: An insulated building will have colder siding and sheathing than an uninsulated building. When you decrease the temperature of the siding and sheathing, they can hold more moisture, so they will absorb more moisture.

When you say outward diffusion is not that significant, I can see two possible explanations, but I am not convinced by either one. The first and more obvious explanation is that the siding and sheathing, being chilled by the seasonal temperature reduction, will absorb more moisture; and so the vapor pressure in that vicinity will be higher than that of the living space. Therefore, there will simply be no impetus for interior vapor to diffuse outward. I am skeptical that the siding and sheathing can take on so much added moisture that they can cause a vapor migration toward the house interior

The other explanation is that outward diffusion through permeable materials does not move as much vapor as once thought. However, I know of no scientific proof of this.

You suggest that this wintertime moisture increase of the siding and sheathing will need an avenue to dry through during the summer, and for this reason, interior poly should be eliminated so as to permit inward drying to the living space.

Outward diffusion provides a continuous supply of condensing vapor to accumulate wetness day by day, all winter if it cannot escape by adequate ventilation. Whereas, the taking on of higher moisture content by the siding and sheathing due to the lower seasonal temperature is just a one-time adjustment corresponding with the lower average wintertime temperature. It does not continuously deliver moisture that accumulates as wetness, which later needs any extra means of drying. It can’t cause wetting that degrades the insulation, cause water damage, or grow mold. But outward diffusion definitely can cause these problems.

So, I do not see the need to eliminate the warm-side poly vapor barrier just to create the extra measure of inward drying to remove moisture that has built up over winter in the siding and sheathing. That extra moisture taken on by the siding and sheathing over winter can easily escape back to the outdoor air by the same route it came in. And while that moisture exists in the siding and sheathing, it can’t do any harm anyway.

Look at it this way: If the siding and sheathing could take on so much additional moisture over winter that they forced a need of inward drying to get rid of that moisture during the summer; then I would expect the siding and sheathing in winter to be covered with heavy frost that continues to wetting and freezing in the insulation. And even if this were observed, there would be no way to know whether it was being caused by the chilling of the siding and sheathing taking in moisture from the outdoors—OR—if it is being caused by outward diffusion.

So when you eliminate the vapor barrier to permit inward drying of moisture assumed to be coming from the outside, you might enable outward diffusion that would put moisture into the wall. So, the way I see it, eliminating the inward drying (by including a vapor barrier) might also eliminate the need for inward drying. Whereas, eliminating the vapor barrier to permit inward drying might create the very need for inward drying.

Jan 18, 2013 1:57 PM ET

Edited Jan 18, 2013 2:50 PM ET.

Response to Ron Keagle
by Martin Holladay

You have challenged me on this issue before, and I suspect that there isn't much I can write that will change your mind. If you want to install a polyethylene vapor barrier on the inside of your wall, you are of course free to do so. Depending on where your house is located, the details of your wall assembly, and whether or not you have an air conditioner, your house may be just fine -- even with interior poly.

You wrote, "When you say outward diffusion is not that significant, I can see two possible explanations, but I am not convinced by either one."

Diffusion has been both measured and modeled, so this question really isn't in dispute. See the Building Science Corp. graphic below -- one that has been widely reproduced -- to understand my point.

You wrote, "When you eliminate the vapor barrier to permit inward drying of moisture assumed to be coming from the outside, you might enable outward diffusion that would put moisture into the wall."

You're right, of course. However, in most climates, eliminating the poly puts you on the correct side of this pushme-pullyu equation -- in other words, your wall assembly will be dryer without the poly than with the poly.

But if you like interior vapor retarders, go ahead and install one. A "smart" retarder makes the most sense for someone with your concerns.

Vapor diffusion graphic.jpg

Jan 18, 2013 2:33 PM ET

Really Ron, do some online research! It's out there!
by Dana Dorsett

" I am skeptical that the siding and sheathing can take on so much added moisture that they can cause a vapor migration toward the house interior ."

Happens all the time, very intensely when dew or rain wetted brick or stucco gets heated by the sun. This has been measured and analyzed intensively by building scientists (and publish the data in peer reviewed journals.)

"The other explanation is that outward diffusion through permeable materials does not move as much vapor as once thought. However, I know of no scientific proof of this. "

There's no science to proving changes in opinions. There is plenty of (published) science about how vapor permeance of materials can be measured, and the rates at which moisture can diffuse through assemblies of measured, known ASTM C96 vapor permeance will move under different interior & exterior temperatures & humidities. This has been well studied, and provides the fundamental basis for modeling tools such as WUFI. The WUFI model has been well vetted in academic experimental assemblies as well as field monitoring/verification studies (yes, real "scientific proof", of just how much water vapor moves through walls in an air-tight assembly.)

I'll take the opinions and assertions of those who have spent their careers actually measuring stuff over the speculation of any arm-chair theoretician (even when that theoretician is ME. ;-) )

There's plenty of easy-to-digest compilations of this stuff readily searchable on the Building Science Corporation site, but the the more hard-core science on it is out there too. (Oak Ridge & Lawrence Berkeley National Lab websites are a good place to start too, but many universities with related engineering schools have web-accessible data too.) And that's just the English language stuff- there's plenty of European reasearch & modeling results out there too. (WUFI was originally developed and verified by the Fraunhofer-Institut für Bauphysik, but is now a freebie download from Oak Ridge National Labs. )

If you take the time learn how to use the WUFI tool beyond a "garbage-in = garbage out" level, it can be quite instructive about what a vapor barrier does for/to building assemblies.

Or you can keep making speculative arguments and complaining about how there is proof. But the science on this is mostly DONE, even if you've yet to discover it.

Jan 18, 2013 6:13 PM ET

a clarification
by floris keverling buisman

Good write up, Just to clarify, both INTELLO, DB+ from Proclima (that my company brings to the USA) and Membrain are smart vapor retarders.

Siga majpell is not a vapor variable membrane. It is 0.7 perm at any humidity.

This can make a big difference, especially if you are talking about solar driven diffusion and or inward drying potential (important under greenroofs or other assemblies that are vapor closed on the exterior) and yes, WUFI will show that these intelligent retarders can make a difference in well insulated assemblies.

Jan 18, 2013 6:31 PM ET

Response to Floris Keverling Buisman
by Martin Holladay

Thanks for the clarification. I have edited the article to reflect your correction.

Jan 19, 2013 2:48 AM ET

i am glad this was brought up
by John Klingel

i am glad this was brought up again. i seem to learn a little bit every time.

Jan 19, 2013 10:28 AM ET

Edited Jan 19, 2013 10:29 AM ET.

Permeance is the secondary concern
by albert rooks

Matin & Floris,

Thanks for the correction on the permeance of SIGA Miajpell.

Yes it has a fixed value. The overarching issue that it's trying do deal with is exfiltration (air leaks). In our thoughts and practices, the permeance is really secondary (as is the point of the article) and the task-at-hand is to produce (and use) a strong membrane that's tear and puncture resistant, easy to hang, fits class II permeance (rather than class I or III), as cost effectively as possible. -We're pretty happy with it. :)

Not to take away the features of "smart retarders", but I've got to point out that a fixed class II permeance has significant benefits: You know what you've got, - it's a constant value and near the common condition permeance of basic sheathing products of OSB and Plywood. When we look at proposed wall assemblies in WUFI, it is helpful to have a fixed value. That way we know that the calculations at that layer have the correct input data and there is no further need for interpretation.

It's not rocket science. As we move ahead in building envelop quality (or "enclosure" for those of the BSC ilk), we need simple straight up "exfiltration answers": A simple mind set of "stop the air leaks -but leave some permeance" will take the building industry long way.

Jan 19, 2013 11:55 AM ET

Edited Jan 19, 2013 12:02 PM ET.

by Ron Keagle

Jeez Dana, all I did was question a few things. I thought that was the point of discussions such as these. I am not sure what I said that you find yourself so at odds with.

Martin posted a diagram showing a great difference between vapor moving by diffusion compared to vapor hitchhiking on air. But that has nothing to do with my question which was asking for a quantification of diffusion over time.

I said this: "The other explanation is that outward diffusion through permeable materials does not move as much vapor as once thought. However, I know of no scientific proof of this. "

Clearly, this explanation has been given here numerous times, however, nobody has ever given the basis of knowing this. So my statement is 100% true. I did not say there is no scientific proof, although you seem to be reading that into my comment. The point of my comment was this: Why the discrepancy between what is said to be known now versus what was said to be known in the past?

In answer to the above question, from what Martin has said, I would have to conclude the following:

In the past, the claimed amount of vapor transmission due to diffusion was a deceitful exaggeration in order to protect the insulation industry. So diffusion was just a red herring put forth by the insulation industry as a defense against charges by the paint industry that insulation was making paint fall off. Furthermore, nobody bothered to verify it by scientific experiment.

It does raise this question: If the insulation industry falsely accused diffusion of causing dampness to the exterior that harmed paint adhesion, and thus called for a vapor barrier; why did the paint industry accept that remedy for diffusion if diffusion was not the cause? If the remedy was false, paint would have continued to fall off. So why did the paint industry accept the vapor barrier as solving the problem?

I also said this as you quoted in your response to me: “I am skeptical that the siding and sheathing can take on so much added moisture that they can cause a vapor migration toward the house interior.”

Sure I understand that reservoir cladding and bad flashing can lead to an influx of moisture. But if you read the rest of my comment, you will see that it is in the context of wintertime, and targets ONLY the claimed effect of seasonal average temperature drop causing a moisture increase in siding and sheathing.

And this premise that I am questioning goes on to say that the increased moisture accumulates over winter and migrates inward, spreading wetness to the insulation to the extent that inward drying as well as outward drying is needed once winter ends. THAT is the context of my saying, “I am skeptical that the siding and sheathing can take on so much added moisture that they can cause a vapor migration toward the house interior.” I remain skeptical.

Jan 19, 2013 12:19 PM ET

Edited Jan 19, 2013 12:21 PM ET.

Response to Ron Keagle
by Martin Holladay

The explanation provided by Teesdale, Rogers, and Rowley was convenient for those advocating the use of insulation, so it was widely believed. To anyone familiar with the history of science, it should come as no surprise that (as you put it) "nobody bothered to verify it by scientific experiment."

Building scientists had an incomplete understanding of air leakage though building envelopes until the 1970s. After all, the blower door wasn't even invented until the early 1970s. When researchers noticed damp sheathing, they mistakenly assumed that the moisture transport mechanism was vapor diffusion -- due in part to their ignorance of the fundamentals of air leakage. (In the 1930s and 1940s, air leakage through building envelopes had not yet been quantified.)

You write, "I remain skeptical." There is nothing wrong with skepticism; after all, skepticism has led to many scientific advances. But as far as I know, the researchers at Lawrence Berkeley National Laboratory, Oak Ridge National Laboratory, the Building Science Corporation, and Fraunhofer-Institut für Bauphysik have made repeated measurements in their labs to verify the principles that I have outlined in this article. You have not. If you want to put forward an alternate (skeptical) scientific theory, I suggest that you start making measurements.

Jan 19, 2013 1:39 PM ET

Vapor barriers to prevent offgassing of OSB and CCA treated wood
by marion marshall

Thanks Martin for explaining in detail why you recommend not using a Vapor in most instances. It was very helpful.

I am in a mild marine Climate in southern Chile ( similar to Portland Oregon) where likely using a vapor barrier would not cause problems from what I understand.

The house frame upper and lower floor was built with OSB which I understand contains formaldehyde and CCA pressure treated wood. These are common construction methods where I live.

In this case might you recommend a vapor barrier to prevent off-gassing from the CCA wood and formaldehyde in the OSB ?


Jan 19, 2013 1:55 PM ET

Response to Marion Marshall
by Martin Holladay

While water vapor can diffuse through many common building materials, I don't think that formaldehyde or CCA move through materials by diffusion -- at least not to a significant extent. If any chemist or physicist cares to correct me, I am prepared to be corrected.

If you want to prevent formaldehyde or CCA fumes from entering your house, all you really need is an air barrier.

I don't know what you are using as the interior finish material for your wall; perhaps you are using gypsum drywall or plaster. Either of these materials can be installed as an air barrier, as long as you pay attention to air sealing at the perimeter of the materials and penetrations.

If you are worried about offgassing of materials in your wall assembly, here's another suggestion: it might make more sense to install a supply-only ventilation system (which will slightly pressurize your house) rather than an exhaust-only ventilation system (which will slightly depressurize your house).

One of your statements is confusing to me. You wrote, "I am in a mild marine climate ... similar to Portland, Oregon, where likely using a vapor barrier would not cause problems." In fact, the installation of an interior vapor barrier in Portland, Oregon is not recommended, because it could cause problems.

Jan 19, 2013 2:55 PM ET

Reply to Martin Holiday
by marion marshall

Thanks Martin ..

Your reply clears matters up for me greatly , was a big help and now I have an action plan.

I hadn't thought about using supply only ventilation inside to slight pressurize your house to help with off gassing in a new home. Fortunately we had installed supply only ventilation so I will be using that often in the first year.

Also now I will not be installing a vapor barrier and instead I will be doing air sealing. That is sealing the edges and penetrations of the interior walls which (correct) are gypsum drywall and wood paneling I believe.

One final question per a vapor barrier - a contractor had recommended to me that we seal the fiberglass batts with closed cell polyurethane foam on top of the batts. (flash and batt system but inverted)

I could see how this could more perfectly seal the air cavities but perhaps this polyurethane spray foam would act as a vapor barrier also which is what I believe I want to avoid right now ? Perhaps a bad idea then ? Or Ok to do ?

If so would all closed cell polyurethane spray foam not be recommended for interior walls because it functions as a water vapor barrier and an air barrier (according to wikipedia) ?

Thanks in advance and sorry if I am confusing or misunderstanding the issue like I did with the Portland Oregon reference.

Should I just reply here or the other forum area ? Please let me know your preference

Jan 19, 2013 5:21 PM ET

Response to Marion Marshall
by Martin Holladay

For more information on air barriers and air sealing, see Questions and Answers About Air Barriers. You should also consider reading some of the articles linked to in the "Related Articles" box on that page.

If you want to follow the "flash and batt" system, the spray foam insulation should be installed on the exterior side of the stud cavities, not the interior side. For more information on the flash and batt system, you can search for "flash and batt" in the search box on the GBA site.

If you want a tight air barrier, you might prefer to use spray foam insulation without any batts. However, a good spray foam installer can create a tight air barrier with just 2 inches of closed-cell spray foam, so either approach can work.

Jan 20, 2013 12:41 PM ET

The purpose of the vapor barrier
by Bill Rose

Ron Keagle asked, "If the insulation industry falsely accused diffusion of causing dampness to the exterior that harmed paint adhesion, and thus called for a vapor barrier; why did the paint industry accept that remedy for diffusion if diffusion was not the cause?"

The point of the vapor barrier is not that it’s supposed to work — for insulation or for paint — technically, but that it’s supposed to be where fingers point when something goes wrong. And things go wrong in only a minority of cases.

When they went looking for buildings with moisture damage they presented a parade of 80 some cases, half of which were crawl space homes in Wisconsin. The vapor barrier “fix” worked for both industries, in the management sense not in the technical sense — it took them off the hook in the handful of cases where things went wrong.

Meanwhile, people pretty much stopped painting houses, and went to aluminum siding, Masonite siding, stained cedar, vinyl, Hardiboard.

The impression that these guys were nefariously contorting building science to their commercial ends is not really true. They were inventing building science, trying to get a handle on things, doing what they thought was best, but were all the while (Rogers, anyway) on the lookout to make sure insulation moved forward. It worked a little, technically. That was good enough, and they may have discovered later that it got them off the hook.


Jan 21, 2013 1:23 PM ET

by Ron Keagle

Regarding the graphic showing the difference between vapor transfer by diffusion compared to transfer by air leakage, I am curious about the effect of the diffusion alone. What were the temperature and RH conditions on the cold side of the drywall for the diffusion measure? What was the time period over which the 1/3 cup of water diffused?

Jan 21, 2013 1:57 PM ET

Edited Jan 21, 2013 2:01 PM ET.

Response to Ron Keagle
by Martin Holladay

Here is a link to one of the Building Science Corp. documents that includes the graphic:

The assumptions behind the graphic:

1. Interior conditions for both diffusion and air leakage are the same: 70 degrees F, 40% RH.
2. The climate is described as a "cold climate."
3. The time frame is described as "an entire heating season."
4. The diffusion is occurring through one 4x8 sheet of gypsum wallboard.
5. The air leakage is occurring through a hole measuring 1 square inch in a sheet of gypsum wallboard.

The amount of water that diffuses through the drywall under these conditions is 1/3 quart (about 11 fluid ounces), not 1/3 cup.

Jan 21, 2013 2:15 PM ET

Thanks Martin. That is what
by Ron Keagle

Thanks Martin. That is what I was seeking.

Jan 22, 2013 11:38 AM ET

The purpose of the vapor barrier
by Ron Keagle

Bill, Thanks for your clarification on that point.

Jan 22, 2013 5:16 PM ET

wow, incredible !! What a
by Jin Kazama

wow, incredible !! What a difference it makes !! no wonder why we see soo many failures around leaks in cold climates !!

Jan 23, 2013 5:59 PM ET

Edited Jan 23, 2013 6:00 PM ET.

air barrier questions
by Marc Bombois

I recently read the airtight drywall article at Fine Homebuilding and thought it a bit much and wondered if Martin agrees that's the way it must be done, or is there a simpler method to achieve airtight dywall? Unfortunately, it looks like poly is the easiest air barrier to install.

Also, what about using house wrap like Tyvek as an air barrier? Does nailed-on rain screen over the wrap wreck the air barrier? Seems like it would, or is the penetration of a nail through the wrap into solid wood airtight? Thanks for any thoughts.


Jan 23, 2013 6:06 PM ET

open cell vs closed cell spray foam in the roof
by Matt Cole

I am particularly interested in your statement regarding the use of open cell spray foam on the underside of the roof deck in warm attic spaces. as well as cathedral ceilings. My insulating contractor is advocating open cell spray foam to protect against water being trapped in the event of water intrusion from the exterior ( a leaky roof). You caution against this on account of the risk of vapor diffusion (from the interior) accumulating in the roof sheathing. I am currently working on a project with insulation soon to be installed and would be interested in any additional remarks you might have.

Jan 23, 2013 6:28 PM ET

Response to Marc Bombois
by Martin Holladay

I don't know which article you read in Fine Homebuilding, but if you read the article I wrote, that's the one you should pay attention to. Here is the link: Airtight Drywall.

You wrote, "It looks like poly is the easiest air barrier to install." Actually, very few builders try to use polyethylene as an air barrier any more. It's tricky, fussy work -- more work than the Airtight Drywall Approach. Every polyethylene seam has to fall on top of a piece of framing lumber. The poly seams all need to be sealed with Tremco acoustical sealant, a very messy product known as "black death." And you still need airtight electrical boxes. You also have to figure out a way to seal the poly to the electrical boxes -- which is harder than sealing the boxes to the drywall.

Q. "What about using housewrap like Tyvek as an air barrier?"

A. I don't recommend it. It's unlikely to be as tight as using the sheathing as your air barrier.

Q. "Does nailed-on rainscreen over the wrap wreck the air barrier?"

A. Any fastener that penetrates the housewrap has the potential to introduce an air leak -- including the fasteners used to attach the housewrap to the sheathing. More information here: Questions and Answers About Air Barriers.

Jan 23, 2013 6:31 PM ET

Response to Matt Cole
by Martin Holladay

I'm not sure what type of information you want. My information comes from Joe Lstiburek and Armin Rudd of the Building Science Corp., who found worrisome levels of moisture in roof sheathing behind open-cell foam in Massachusetts. Their conclusion: open-cell foam sprayed on the underside of roof sheathing is dangerous in Massachusetts or anywhere colder.

Use closed-cell foam, or cover the entire roof assembly with gypsum drywall painted with vapor-retarder paint.

Jan 23, 2013 8:54 PM ET

Thanks Martin.
by Marc Bombois

I had read this article by Myron Ferguson:

He seals gaps in framing as well as foaming or caulking any gaps in the drywall before taping. Egad. Why do that if tape and compound do the trick? No doubt he gets a good result but is it really necessary? Your article is simpler.

Jan 24, 2013 3:37 PM ET

In the name of physics...
by Derek Roff

If we invoke the name of physics in an explanation, it behooves us to present accurate observations of the essentials of a situation, and to clarify how all the important factors work together to produce the observed results. I don't think sufficient data was presented, to apply physics meaningfully to some of the questions raised in this discussion, especially the relationship of insulation to the moisture content of siding. Furthermore, some of the physics presented doesn't seem to be directly relevant to the questions.

We start with the poorly documented observation from the 20s and 30s, that paint peeled more in newly insulated houses, compared to uninsulated houses. The assertion was made by painters, that greater paint peeling was caused by increased wetness in the siding, brought about by the fact that "insulation draws moisture". But we don't really know if extra moisture was present, if so, how much, and whether increased moisture actually caused the peeling paint. We don't know if increased moisture was present in the insulation, nor where that moisture came from. There are enough unknowns up to this point, to make analysis unreliable, but let's move on.

It's not shown as a direct quote, but someone, perhaps William Rose, says, "When siding is cold, it draws moisture from the surrounding (exterior) air". Why would this be so? It doesn't matter if siding is cold, but rather whether it is colder than the air around it.

Certainly, if siding is colder than the surrounding air, then it could provide a surface on which moisture could condense out of that air. But siding tends to be warmer than the surrounding air, since it is part of the heat-flow path from the warm inside of the house to the cold, outside, wintry air. If the moisture levels in the siding and the air are roughly in equilibrium in the winter, then the slightly warmer siding would release moisture to the slightly colder air. On the other hand, if either the siding or the air has substantially greater moisture, then moisture will tend to flow from the wetter to the drier element. It violates a law of physics for drier air to transfer moisture to wetter siding, in the absence of factors not stated in the article.

Rose says, "At the same vapor pressure, lower temperatures means higher relative humidity and higher moisture content." He neglects to point out that water doesn't maintain the same vapor pressure at lower temperatures. As the link below shows, the vapor pressure of water at around 20 degrees F (-7 deg C) is less than 1/8th of its vapor pressure at 75 degrees F (24 deg C). The second link shows that relative humidity of the air in most American cities peaks in the late summer or fall, not in the winter when temperatures are the lowest.

Given that the houses under discussion aren't experiencing unvarying vapor pressures, nor higher relative humidity from the exterior air in contact with the siding, we would need more information to understand how Rose's comments could explain anything about houses.

There are other dubious assertions in the name of physics in this section of the article, but this post is already long. I'd love to see a more complete and credible explanation of the physics involved in siding, moisture, temperature, and the impact of insulation.

Jan 24, 2013 3:58 PM ET

Edited Jan 24, 2013 4:17 PM ET.

Response to Derek Roff
by Martin Holladay

First of all, you raise the question of whether the observations and opinions of house painters in the 1920s and 1930s were correct. While that topic may be worthy of discussion, it is irrelevant to the point in my article. I was giving an accurate account of history -- explaining how vapor barrier requirements became part of our building codes. The account is correct, and the events I summarized led to the code requirements -- regardless of whether the house painters were shrewd observers or sadly mistaken.

Your challenges to Bill Rose's explanations are similar to challenges made on this website by Ron Keagle. Bill Rose has graciously responded to Ron Keagle's challenges in the past. You may want to review those previous discussions. I'll include a couple of quotes here.

In the thread titled Fear of cold plywood sheathing in a cold climate, Bill Rose commented, “Wood moisture content depends strongly on RH of the surrounding air, somewhat regardless of the temperature or vapor pressure at which that RH is achieved. In one project I'm working on, the January outdoor RH is 84%, the January indoor RH is 10% and the June RH, indoors and out, is around 60%. That sort of pegs where the wood moisture content will be, or tend to be.”

In the thread titled Tongue and Groove Ceiling w/ 6mil VB - Enough?", Bill Rose commented, “You pose an experiment of chilling one block and keeping the other one at (room?) temperature. The colder one will get wetter. At freezing temperatures and below, the mechanism can involve forming frost on the surface (not bound water), which, when it melts, gets sucked into the wood. Or not, if the surface is fully saturated. All this is the flip side of lumber drying processes.”

I have sent an e-mail to Bill Rose. I hope that he can respond to some of your specific challenges.

Jan 24, 2013 4:46 PM ET

Good comment
by Bill Rose

Martin Holladay just drew attention to your comment.

I went to your reference, and the data do indeed support your claim that my contention of high RH during winters is questionable. Very interesting. This table gives the monthly average over several years of RH measurements taken at one time in the morning and one time in the afternoon. The notes explain which hours are selected. The hour selection is reasonable for the various zones.

I use a different NOAA dataset for my research. This is data from SURFRAD, and it has several benefits: 1) it includes radiation measurements, and 2) one of the sites (Bondville) is maintained by colleagues in my unit. The URL here allows a user to see the average monthly RH for a given year, where the average RH is the average of all RH measurements taken during the month, for a given year. These results for a site like Bondville paint a picture very much in support of my contention of high RH (80s) during winter and lower RH (50s) during summer. Southern states do not follow this pattern, but the northern-climate bias of building researchers is legendary.

So our differences on this matter come down to interpretation of available data. I am appreciative for the data you cited, and I’ll pipe down until I’m comfortable with a resolution of this.

On another matter you quote me correctly: "At the same vapor pressure, lower temperatures means higher relative humidity and higher moisture content." The moisture content I refer to here is the moisture content of materials that hold water, that are porous and hygroscopic, like wood. The RH/moisture content relation is captured in the sorption isotherm.

Looking back, it was not clear in my comment that I was referring to materials. Given these two clarifications — different datasets and a strict meaning of “moisture content” — I think I can stand behind my comments in light of your challenge. There are other matters you raise. I’ll respond if and when I can. Keeping it short… Thanks for the close reading.

Jan 24, 2013 7:10 PM ET

Edited Jan 24, 2013 7:12 PM ET.

Interesting Topic--
by Ron Keagle

From discussions here, this is my understanding regarding this topic of wall moisture originating with exterior hygroscopic materials being chilled during winter. I just wanted to lay this out to make sure we are all on the same page. It is an interesting topic.

As the average temperature drops from summer levels to winter levels, wood siding, sheathing, and the outer portions of studs will adsorb/absorb more moisture due to the temperature drop. What matters is that the colder wood can hold more moisture, so it will take it on from whatever source is available. And even if wintertime RH may be lower than that of summertime, it will be often high enough to provide a source of moisture adsorption of the colder siding, sheathing, and outer part of the studs.

Then, if the wall cavity is insulated with permeable insulation, the warmer areas toward the interior will cause drying of the extra moisture that has been acquired by the siding, sheathing, and outer parts of the studs. And as that moisture dries inward, it amounts to an inward vapor drive. As the siding, sheathing, and outer parts of the studs dry inward, they continue to take on more moisture due to their relatively lower temperature at the far exterior. So, you have a continuous pumping action that creates an inward vapor drive.

There is also an outward vapor drive due to diffusion. So these two opposing vapor drives meet somewhere within the wall cavity. This raises the question of which drive is dominant. If the inward drive were strong enough, it might totally cancel outward vapor drive. Indeed, if the inward driver were strong enough, it might just continue all the way through the wall and dry to the interior.

So, the inward vapor drive is originating from the siding, sheathing, and outer parts of the studs, all of which are acting as accumulator / reservoirs to hold an increasing moisture content due to adsorption brought on by falling temperature. And then, from that collecting reservoir, the moisture diffuses inward.

As that outer moisture begins its inward diffusion journey, it starts out already on the condensing side of the dew point in the wall.

However, according to Joe Lstiburek, the inward diffusion would not condense at the dew point. Instead, it will move on to a “condensing surface of interest” (a preferential nucleation site), which will be the inner wall surface.

But when this inward diffusion collides with the outward diffusion, it makes me wonder what nucleation site will be preferred. I guess it depends on which drive is stronger. Since a large number of insulated buildings were found to have wintertime frost on the interior side of the sheathing; and if this was not due to outward diffusion; then it seems as though this inward drive mechanism is quite strong compared to the outward diffusion drive.

Jan 24, 2013 10:22 PM ET

summer drying
by Gregory La Vardera

Martin said:
"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."

I think its worth framing that comment in the context of air-conditioning, or no air-conditioning, even though americans almost alway have/want AC.

If there is no AC then a wall cavity is not going to dry any more to inside than outside.

Jan 25, 2013 12:58 AM ET

Climate Zone?
by Marcus de la fleur

Martin, you referred to climate zones a number of times. Which climate zones are you referring to. Could you post a link to a map with those climate zones? Thx

Jan 25, 2013 8:25 AM ET

Response to Gregory La Vardera
by Martin Holladay

You're right that if a house is air conditioned, the indoor air will be dryer than if the house has no air conditioning.

But even in a house without air conditioning, there are many hours during the summer when the indoor air might be dry enough to encourage inward drying of a wall -- for example, during the morning hours, if the indoor air of the house has been cooled by the nighttime operation of a whole-house fan.

Weather conditions change, and the indoor air may be dry and cool under some conditions -- and omitting polyethylene allows the wall assembly to dry inward when conditions are right. If the polyethylene is there, the inward drying will never occur.

Jan 25, 2013 8:33 AM ET

Response to Marcus de la Fleur
by Martin Holladay

The DOE climate zone map can be found on our Q&A page:

Click the small image to enlarge the map.

I will also post the map here.

DOE climate zone map.jpg

Jan 25, 2013 10:15 AM ET

Summer Drying
by Gregory La Vardera

Martin, I appreciate your thoughts on this, although I find the prospect of limited morning or overnight drying in a house without AC to be a very slim benefit. I'm guessing this is more of a hunch than science?

If you live in a climate cold enough that you would not have AC - although I somehow doubt americans in any zone would be without AC - but if you were without AC you would do much better by keeping your poly and simply making your wall system more open to the exterior - building paper instead of housewrap, plywood instead of OSB.

Jan 25, 2013 10:29 AM ET

Add'l response to Derek Roff
by Bill Rose

You point out that observations from the 1930s were poorly documented. That’s correct. My effort has been to capture what documentation we do have and draw appropriate conclusions from that. See (not free).

We seem to agree on the fundamentals. At equilibrium, the temperatures and vapor pressures will determine the moisture content. Warming tends to dry materials, and conversely cooling tends to wet them. You note that siding is in the outward heat flow path, so the siding should receive some drying benefit from that. True, and with the introduction of insulation in the 30s that benefit got reduced.

I admitted to being a bit flummoxed by the monthly average RH data you cited at . I looked more closely at this data, particularly in the northern states. You note a peak in August-September. This is true for the average of all sites. For northern states, it is a sub-peak, lower than the peak in December-January. The trough for all sites is April-May. More to the point, the averages do not represent a wide span—mid 70s at the peaks, mid 60s in the troughs, which is narrower than I’d taken from my data source. It is commonplace to recognize wetter exterior materials during winter and drier materials during summer. The span in seasonal RH data does not explain the wider range in moisture content data. Adding radiation to the equation gets us there.

You seem to suggest that the quality of data, research and explanation is not really up to the level of what can be called physics. Perhaps you’re right. We do what we can.

Jan 25, 2013 10:31 AM ET

Response to Gregory La Vardera
by Martin Holladay

Your advice is risky, for the simple reason that no builder can assume that, for the life of the building, no one will ever install an air conditioner.

Especially with the prospect of global climate change, it's hard to imagine that many builders would make such a risky bet.

Jan 25, 2013 11:00 AM ET

Edited Jan 25, 2013 11:11 AM ET.

Thanks to Bill Rose, it's great to learn more.
by Derek Roff

I appreciate your response, Bill. I will try to read through the references that you provided, and understand these concepts more completely. In the meantime, one of the central points that we are discussing remains confusing to me. If I'm understanding what you just posted, you are saying that as you cool water-absorbing materials, such as wood siding, the moisture content goes up. Looking at Table 4-2, at the top of page 4 of the Forest Products Laboratory link that you provided, I see that the variation is tiny. For example, at 80% relative humidity, moisture content of wood at 50 degrees F is listed as 16.4%. When the temperature drops to 30 degrees F, the equilibrium moisture content is 16.5%. At 95% relative humidity, and at 5%, there is no difference at all in the listed figures over that temperature range. In fact, half the listed values show no change, while the other half show a difference of 0.1%. These differences seem very small to me. What am I misunderstanding?

Jan 25, 2013 12:03 PM ET

Response to Derek Roff
by Bill Rose

At the same RH, temperature differences mean practically nothing to the moisture content of wood. At the same outdoor vapor pressure (or absolute humidity), temperature differences lead to differences in RH thus to much larger changes in moisture content. Two houses side-by-side, during cold weather will be subject to the same vapor pressure. Warmer siding will be drier than cooler, surrounded by the same vapor pressure.

Jan 25, 2013 4:45 PM ET

More questions for Bill
by Derek Roff

Thanks for clarifying, Bill. What is the range of temperatures that you would expect for the siding in model/example side-by-side homes, with differing amounts of insulation?

Jan 25, 2013 5:29 PM ET

more response
by Bill Rose

Not much. A few degrees, maybe 3-5F. An IR scan would show differences in temperature house-to-house and at the same house.

Slight lowering of temperature leads to slight moisture effects. It's usually not a problem except where the cladding is not robust. There are many buildings and building parts that receive no vagrant heat, so they can be viewed as limiting cases for how bad it can get. Garages, porches, overhangs, etc. whatever works in these conditions works just fine in superinsulated buildings. We've seen a 19% loss in compressive strength in marble cladding between the surfaces that receive significant vagrant heat compared to those that do not, in a monumental NY building. The vagrant-heated marble had the same strength as marble warehoused at the source quarry, after 60 years. Can't identify the building, sorry. Just to say that in most situations the impact of added insulation can be ignored, but there are some cases where it cannot.

Jan 25, 2013 5:49 PM ET

Summer Drying Risk
by Gregory La Vardera

Martin, I think that is a strange picture of risk. If the person building the home does not have AC it would be more risky to use a wall assembly that would not perform as well for their use. What you are suggesting is accommodating an unknown future owner over the current owner.

You want to alleviate the risk of a future owner who may install AC without looking at the composition of the wall?

Jan 25, 2013 6:02 PM ET

Response to Gregory La Vardera
by Martin Holladay

If a wall assembly is so risky that the installation of an air conditioner would push the wall into failure, then I certainly wouldn't want to build it.

Jan 25, 2013 11:17 PM ET

Risky wall assembly
by Gregory La Vardera

But what you are saying now would preclude a vapor barrier in any situation. You are contradicting your statement here:

"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."

Jan 26, 2013 6:48 AM ET

Edited Jan 26, 2013 9:36 AM ET.

Response to Gregory La Vardera
by Martin Holladay

Just because a polyethylene vapor barrier can be beneficial in a certain type of house -- one that has no air conditioner -- doesn't mean that it's wise for a builder to install one. A wall has to be robust to keep a builder out of trouble. No builder wants a wet-wall problem, and no builder wants to be sued.

It is highly likely that any home in the US and southern Canada will have air conditioning at some point over the next 40 or 50 years.

So, if you live in Minnesota or Quebec, using MemBrain makes more sense than using polyethylene.

Jan 26, 2013 8:24 AM ET

Lots of interesting comments on this thread
by John Brooks

Thank you Ron Keagle and others for asking questions.

Jan 26, 2013 11:12 AM ET

Risky wall assembly
by Gregory La Vardera

Martin, enough. I understand what you saying. I am just trying to extrapolate that to its limits, or at least provoke you into identifying its limits.

So if you are north of southern Canada you are now suggesting that a wall assembly with Poly as I have described may be appropriate? But somebody can certainly decide to install air conditioning in those regions, invoking your "risk". But you are ok with that risk in those regions? Its no longer:
"If a wall assembly is so risky that the installation of an air conditioner would push the wall into failure, then I certainly wouldn't want to build it."

Really Martin, I don't expect you to continue to respond to this, because the point is the circle of reasoning you've outlined. You've said both "this is ok where there is likely no AC", AND you've said "NO wall that can be defeated by AC is OK". This can not be reconciled. I would not beat this to death if you did not characterize my advice as "risky", but you really left me no choice. I'll state my case, and then we can let it drop.

If you are building a house without air conditioning whether in northern canada, or in zones 5&6 right here in the US you would be better served by a wall assembly with Poly and a bias to being open to the exterior, than you would be by the interior drying, or both side drying assemblies loosely referred to in this post. It will have superior performance during the heating seasons, and no penalty for summer drying. In fact such walls are used widely throughout scandinavia, and I really have no idea how in the US we have such hubris to think we know better.

If somebody at some future date decides to retrofit a house, any house, with AC it would be very RISKY for them to do so without making a proper and appropriate investigation as to the composition of that wall, and to change it if need be. Let us please recognize where the risk really resides. No builder will take on risk and liability for building a proper wall for a house without a cooling system.

All that said it is no more effort to build such a wall with a smart vapor sheet than Poly, and so I can't imagine why anybody would not do so. Really the whole issue is moot, as is this strange sense of risk. Not because the Polyethylene boogie man that has been a popular green building science theme, but because its a better product and does a better job. Period.

Truly Martin, I think you create much more risk with the rather casual recommendation for the elimination of vapor retarders and you seem to have little worry over that. This advice is taken up by builders with no idea of making the wall air-tight, and no idea of the potential failure they may be inviting. If you are concerned with Risk, you should think about making your message more clear about the necessity for airtightness if you actually go down the road of no vapor retarder. I can assure that broad misunderstandings are already coming back to practitioners such as myself, many telling me that they read about it here. Many of them are bound to end up on the lower half of that clever BSC diagram with 30 qts of water in their wall. Whose risk is this?

Jan 26, 2013 11:50 AM ET

Response to Gregory La Vardera
by Martin Holladay

Houses differ. While most Canadian houses with poly are doing just fine, a few aren't. There are lots of variables: what's the siding? what's the sheathing? what's the rainfall? what's the indoor RH? how cold does it get? is there AC?

WUFI does a fairly good job of analyzing these factors, if the program is used by an experienced practitioner. Otherwise, most builders need to use rules of thumb.

In my writing, I do my best to provide useful rules of thumb. I don't want the typical Canadian homeowner with poly in their walls to worry. Most of these walls are fine. But just because these walls are fine, doesn't mean that a new home builder would choose to build a wall that way if he had a choice.

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