The Return of the Vapor Diffusion Bogeyman

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The Return of the Vapor Diffusion Bogeyman

For years, some builders assumed that they didn’t really need to worry about outward wintertime vapor diffusion — but it turns out that they might have to worry after all

Posted on May 15 2015 by Martin Holladay

Fully aware that I am engaging in gross oversimplification, I’m going to offer a cartoon version of the History of Vapor Barriers. (I’m not a cartoonist, though, so someone else will have to make the drawings.) Here goes:

Panel 1: In the late 1940s, residential building codes in the U.S. began requiring the installation of vapor barriers on the interior side of walls and ceilings. These requirements had complicated historical origins but were not based on credible building science.

Panel 2: During the 1960s, fiberglass batts were sold with kraft facing on one side. Builders were instructed that the kraft paper should face the interior of the building.

Panel 3: During the 1970s, when U.S. homeowners were hit with steep increases in energy prices, builders began paying more attention to insulation levels. Energy experts began advising builders that polyethylene sheeting was a better vapor barrier than kraft facing. By the late 1970s, most builders in the colder areas of the U.S. — and a significant number of builders in mixed climates and hot climates — were installing interior polyethylene vapor barriers.

Panel 4: During the 1980s and 1990s, building scientists in the U.S. and Canada realized that interior polyethylene prevents walls from drying inward during the summer, and that polyethylene was causing as many problems as it was preventing. As these scientists began studying the mechanisms by which moisture is transported from the interior of a house into walls and ceilings, they realized that the most important moisture transport mechanism was air leakage, not vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. . These scientists proclaimed, “Vapor diffusion is not the issue! We need to focus on air barriers, not vapor barriers. Code requirements for vapor barriers are misguided.” Led by Joseph Lstiburek, some of these scientists managed to implement code changes that allowed builders to omit interior polyethylene.

Panel 5: Journalists who listened to building scientists, including me, began advising builders that they don’t have to worry about vapor diffusion.

Panel 6: Oops! As builders began building airtight wall and roof assemblies, diffusion-related moisture problems started unexpectedly showing up. It was the Return of the Vapor Diffusion Bogeyman.

Damp sheathing behind open-cell spray foam

Before I address whether this cartoon summary is accurate, I’d like to fill in a few of the gaps in the narrative. Some readers may be wondering: What new diffusion-related problems are we talking about?

The first cluster of problems concerned unvented cathedral ceilings in cold climates insulated with open-cell spray foam. I reported on the problems ten years ago, in an article published in the April 2005 issue of Energy Design Update. The article quoted Clyde Potts, a builder in Big Fork, Montana. Potts installed IcyneneOpen-cell, low-density spray foam insulation that can be used in wall, floor, and roof assemblies. It has an R-value of about 3.6 per inch and a vapor permeability of about 10 perms at 5 inches thick. spray foam in the cathedral ceilings of his own home. Without installing an interior vapor retarder, he finished the ceiling with tongue-and-groove boards. Within a few months, moisture accumulation in the roof assembly was causing problems. “Water was getting through the foam,” Potts told me. “The water hit the roof sheathing and had nowhere to go. … So we pulled the boards off the ceiling. Then I cut out some sections of the Icynene, and I could see the roof sheathing was wet.”

I wrote a follow-up story about this type of failure a few months later, in an article that described wet wall and roof sheathing at a house in Warren, Vermont, owned by Elizabeth and Matt Moffitt. The wall and roof assemblies of this house, like Clyde Potts’ ceiling, were insulated with Icynene open-cell spray foam that was installed without an interior vapor retarder. Building consultant Henri deMarne told me, “The wall was opened up, and the whole thing was soaking wet. I took a handful of insulation, and squeezed it, and water dripped out of it like a sponge. When the roof was opened up from the outside, the roof insulation was wet as well.”

The disasters at these two houses both had the same important contributing factors: high indoor relative humidity and insufficient mechanical ventilation. That said, the failures at the Potts house and the Moffit house acted as canaries in the coal mine, focusing attention on cases of damp sheathing behind open-cell spray foam.

Because open-cell spray foam is an air barrier, it was clear that the moisture transport mechanism in these cases had nothing to do with exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration.. These failures occurred because of outward vapor diffusion.

Damp sheathing behind 12-inch-thick cellulose-insulated walls

The second category of cases — possibly but not necessarily worrisome — concern damp exterior sheathing on double-stud walls insulated with cellulose. In most of these cases, the moisture content of the OSB or plywood sheathing increases seasonally, generally peaking in February and returning to safe levels by April or May.

These cases have been discussed extensively on For those who haven’t seen the articles, here are the links:

So, with that groundwork laid, we can ask two questions: Is the vapor diffusion bogeyman back? And am I guilty of misleading builders with bad advice?

A review of an old story on vapor diffusion

I decided to look back at a few of the articles I have written on this topic over the years. In August 2002, I wrote an article for Energy Design Update titled, “Historic Summit Meeting Looks at Vapor Barriers.” In that article, I wrote, “While mold and construction-defect lawsuits are increasing, many building science researchers worry that some existing building codes mandate practices that contribute to building failures. For example, interior vapor barriers may have contributed to the EIFS crisis in North Carolina by slowing the rate of wall drying. … In hopes of forging a consensus and an action plan on issues related to vapor barriers, crawl space construction, and attic venting, [Joseph] Lstiburek and [Betsy] Pettit hosted a historic gathering at their offices on June 8-9. …

“Some experts have long suspected that in most of the U.S., interior poly vapor retarders do more harm than good. Concerns about keeping interior vapor out of walls are probably misplaced, since rain penetration and the migration of interior air into building assemblies are far more likely to cause problems than the diffusion of interior moisture into walls or roofs.”

In the article, I quoted building scientist Joseph Lstiburek, who said, “The problems we see now show that vapor barriers don’t keep our walls dry. Vapor barriers keep our walls wet.”

I also quoted Anton TenWolde, supervisory research physicist at the U.S. Forest Products Laboratory in Madison, Wisconsin, who said, “The calculations showed that even with very low air pressures across the assembly, and even with a very good air barrier, sufficient moisture can bypass the poly vapor retarder, degrading its performance. In practice it doesn’t matter what the permeance of the vapor retarder is, because the air leakage will go around it for moisture transfer. I came to the conclusion that the idea that we need a vapor barrier to keep our walls dry doesn’t hold a lot of water, so to speak.”

I also quoted building scientist John Straube, who said, “The question comes up, have we seen diffusion-related building failures? And the answer is, very few — maybe in rooms with a swimming pool. Assuming that the vapor came from the inside, you would have to have a very high load before you would see a problem.”

A review of vapor-barrier advice on GBA

In January 2013, I wrote an article for GBA called “Do I Need a Vapor Retarder?” In that article, I wrote: “Most buildings don’t need polyethylene anywhere, except directly under a concrete slab or on a crawl space floor.”

Responding to the question, “Can I just ignore vapor diffusion?”, I wrote, “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.
  • “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.”

I quote this article at length to refute the charge that that my advice lacked nuance. I’ll leave it to readers to determine whether my list of caveats was long enough, or whether it was incomplete.

If I were writing this list today, I would add one more bullet point: “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.”

High-R walls need a ventilated rainscreen gap

I recently telephoned Joe Lstiburek and asked him whether my advice to builders underestimated the risk of outward vapor diffusion during the winter. Lstiburek said, “I don’t think you were wrong. Fifteen years ago, remember the context. Back then, we weren’t talking about R-40 or R-50 walls. If we are building airtight walls with lots of insulation, we have to worry about diffusion. In the old days, we weren’t talking about R-50 airtight walls. We were talking about an R-20 wall. So you were not wrong. The context changed.”

Lstiburek brought up the topic of roof assemblies. “When it comes to attics and open-cell foam, I called my article on the topic ‘Mea Culpa Roofs.’ I did not appreciate that leaky ductwork in these attics was reducing the moisture loads in those attics. There was a lot of air change that occurred between the attic and the house as a result of the leaky ductwork, and that air exchange dramatically reduced the moisture load in the attic, and I didn’t appreciate the importance of that.

“I did realize the effect of vapor diffusion in cold climates — vapor diffusion through open-cell foam. If you use open-cell foam in an attic in a cold climate, you have to have a vapor retarder. So you were not wrong with your advice on the vapor diffusion stuff.”

I brought up the topic of cellulose-insulated double-stud walls, and asked a question about Lstiburek’s recommendation to include intermediary sheathing. “I have never recommended building a double-stud wall without the intermediary sheathing,” Lstiburek said. “Even back in the ’80s. But let me tell you what I didn’t appreciate back in the ’80s: We absolutely need a ventilated claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. with those high-R wall assemblies. The question is, What should the vapor permeability of the exterior sheathing be? In climate zones 1, 2, 3, 4, and 5, you can live with OSB. But in zones 6 and 7, you ought to go to gypsum sheathing or fiberboard. In those zones, I’d be worried that even plywood might not work. For a while, I didn’t appreciate that it’s best to be conservative on the outside — because of how effective we are on the air sealing. If you make the wall unbelievably airtight and add R-50 insulation, and you have a cold climate, you need to be ultraconservative on the outside. That’s a view that I have come to recently, in the last couple of years. The importance of the air space behind the cladding, coupled with a vapor-open sheathing — that is relatively recent understanding.”

“People are irrational”

Lstiburek continued, “Are there wall assemblies that are way more robust than a double-stud wall? Of course there are. The least risky is a 2x6 wall with advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope., Zip sheathing, 4 to 6 inches of exterior rock wool, and furring strips. But there are several different camps out there. There is the anti-foam camp. It’s irrational, but I understand irrationality. After all, I support the Toronto Maple Leafs.

“Then there are the people who hate any exterior insulation — the ones who say, ‘I can’t put cladding over several inches of foam or mineral wool.’ People are irrational. To me it’s a ‘Ginger or Mary Ann?’ question. There is no wrong answer. So I tell people, ‘If you want a double-stud wall, here are the things I want to see: the intermediary sheathing and the ventilated cladding — all the stuff we talked about earlier. So those are the double-stud wall people. Then there are the people who say, ‘You have to have a smart vapor retarder,’ the Loony Tunes out of New York. They hate plastic. They love their Intello stuff. They’re irrational, but that’s OK. All these walls can work.”

I need help with the math

I told Lstiburek that when some experts urged architects and builders to design walls by making vapor-flow calculations or “doing the math,” the advice confused me. I told him that I am a WUFI skeptic, and that I’m not sure which equations to trust any more.

Lstiburek answered, “The designer should never use math. That is ridiculous. For one thing, the math is so flawed! These calculations require such skill that you need to know the outcome before you run the math. So instead of using math, apply first principles and historic experience — period.

“I can’t tell you how much time and money we spend fixing flawed hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. simulations. I tell these clients, ‘You are getting an answer that is contrary to first principles and historic experience. If your modeling is saying you need an interior vapor barrier in Amarillo, Texas, that tells me that your analysis is wrong. Your simulation is wrong.

“I tell them what they have to do, and so they ask me, ‘How did you know you had to do that?’ I tell them, ‘I already knew what the right answer is.’ So why do the analysis?

Lstiburek continued, “Let’s say you ran a university department, and you wanted to do things correctly. Here’s what you would do: You build a bunch of assemblies, and then you stress them and try to push them to failure. You measure what happens, and you use the measurements to tune your model. Then you have to decide what the applicability of the model is. You can’t apply the model everywhere. The model has been tuned a certain way, for a certain climate. You can’t change too many of the factors more than a few percentage points or the model is no longer valid.”

Plywood sheathing is less risky than OSB

Next I telephoned William Rose, a research architect at the Building Research Council at the University of Illinois and author of Water in Buildings. Rose told me, “I feel that your take on vapor diffusion, it could be exactly mine — that is, let’s pay attention in very humid rooms like those with swimming pools.

“With regard to open-cell spray foam for cathedralized ceilings, I just did one in Kansas City. The client really wanted to use open-cell spray foam. This is in a museum building with a big old leaky ceiling. So I said, ‘How about if we first install 1 inch of cut-and-cobble extruded polystyrene, leaving 1/4 inch gaps all around? We’ll punch those pieces up against the underside of the 1x6 roof sheathing, to give us 1 inch of vapor-impermeable material toward the outside, and to give us a relief material, so that we are not applying spray foam up against the 1x6 tongue-and-groove.’

“The insulation installer asked, ‘Can I cut it up, or does it have to be continuous?’

“I said, ‘Go ahead and cut it up. We are handling air movement with the open-cell foam.’ The guy didn’t mind doing it. The insulation contractor loved having 1 inch of impermeable foam on the outside.”

Rose continued, “With this type of open-cell spray foam installation, I would apply caution. And I would apply caution on double-stud walls in cold climates if we don’t have assurance of low indoor humidity levels. Really, we are protecting the OSB with these measures. If it were plywood, I wouldn’t worry. Go right ahead, build a double-stud wall full of cellulose with plywood sheathing. I would give that a green light. But I would express caution with OSB.

“We can also say, ‘I want you to slather on a couple more layers of paint on the inside.’ We are usually looking at doing one coat. Why not slop on another coat or two? With the addition of more paint layers, over time, a building will always be more diffusion-resistant. I like doing that rather than relying on a spec from some industry representative. This is do-it-yourself building science, but I am in favor of that.”

I asked Rose whether multiple coats of interior paint reduce inward drying during the summer enough to raise worries. “In historic buildings, no. The historic preservation people will look at old walls and identify dozens of paint layers. So with regard to historic buildings, this is not really a concern. I don’t think multiple paint layers are comparable to the vinyl wall covering / Gulf Coast / hotel-motel effect.”

We have to make judgment calls

Rose continued, “I have never said, ‘Diffusion is gone; don’t worry about it.’ Historically, we have overblown the issue of diffusion. Now we are probably at about the right point. You still have people pushing both ways, and that’s OK. I don’t think we can make everything conflict-free, shy of establishing a building-science autocracy. We have to make judgment calls.

“One reason we were able to diminish the importance of vapor barriers is that our walls were never airtight. A lot of times, we’re not giving air flow through walls the credit it deserves for helping with drying. It’s a two-way process.”

Rose wants to develop a new software model

I asked Rose what he means when he says that we can answer questions about moisture in walls by “doing the math.”

Rose answered, “We talk about diffusion as the movement of water vapor through solid materials, while convection is movement around materials. I’m trying to build a model that fits halfway between the steady-state Glaser method — the steady-state ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. snapshot — and WUFI. The Glaser method has been put down over the years, more than it deserves to be. But the WUFI users don’t know what they are doing. They really really don’t. It is a real abuse of a really good tool. There are probably only a few dozen people who can use WUFI in a way that you can trust.”

Rose continued, “Anton [TenWolde] tells me it impossible — that it won’t work.”

I asked, “What won’t work?”

Rose answered, “Finding satisfactory ways to solve the transient equations for moisture and heat flows where you are dealing with time effects, not just spatial effects, in a satisfactory transparent way that a designer can use.”

Rose elaborated. “There is math, and people really ought to do it. That’s exactly what the math is for — dealing with the ‘yellow light’ areas. We want to be able to give a green light where we are comfortable, and a red light when we should. And we also need to come up with the tools to provide an answer for the caution areas. Right now, the ASHRAE steady-state method and WUFI are our two tools.

“I’ve used WUFI with clients on buildings several times, but I’ve never used it unless we have a season of monitored data, and I always calibrate the tool to the monitored data. That’s how I think WUFI should be used. And I would guess that the people who are good with WUFI, those several dozen people, are all people who have monitored buildings and have tried to calibrate WUFI to existing conditions.

“So if a client asked me to do it, I’d say ‘Sure.’ It means more billable hours.” (Needless to say, Rose is a responsible consultant; this remark was intended as humor.)

Rose concluded — only half joking, “The real reason that people use WUFI is to charge more billable hours to a gullible client.”

Pretty good building science

Lstiburek and Rose have somewhat different perspectives, but they both have a healthy respect for common sense. Lstiburek advises builders to base their designs on “first principles and historic experience,” while Rose advises builders to “slop on another coat or two of interior latex paint,” an approach he calls “do-it-yourself building science.”

I'm going to suggest another term for these math-free methods of building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. design: “pretty good building science.” Applying pretty good building science requires builders to listen to our elders and to pay attention to recent research, but it doesn't require us to perform any mathematical calculations.

Martin Holladay’s previous blog: “Books for Homeowners Interested in Saving Energy.”

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May 15, 2015 12:15 PM ET

The Takeaway
by Kevin Dickson, MSME

Well I'm ready to apply this to real life. I'm convinced that a 12" vapor open double stud wall assembly will be OK even with one layer of sheathing.

From inside to outside:

1. Painted sheetrock
2. 2x4 studs
3. Dense Packed cellulose insulation
4. 2x4 studs
5. 1/2 real plywood sheathing
6. Vapor-open air and weather barrier (
7. 1/4" min. furring strips
8. Exterior cladding


1. The house must be verified tight and ventilated to the 2015 IECC code .
2. The homeowner must be warned not to humidify above 40% RH

May 15, 2015 12:52 PM ET

Ice Box of the Nation
by Steve Vigoren

Zone 7 Northrn MN, 40 below F a few times every winter, 2-20 days every summer 90+F with high humidity. Sort of planning on going with sheet rocked 6" walls with fiberglass batts, osb, 4" rigid foam, airscreen, lap siding. Sounds like I should consider plywood instead of osb?

May 15, 2015 12:58 PM ET

Edited May 15, 2015 1:03 PM ET.

Reply to Kevin
by Steve Vigoren

I am also considering this type of wall. I like the intermediary sheathing idea, but am a little concerned about no sheathing on the outside, just vapor open air barrier and air screen strips?

May 15, 2015 1:13 PM ET

Response to Kevin Dickson
by Martin Holladay

If I remember correctly, you are in Climate Zone 5. Bill Rose certainly agrees with your stack-up. Joe Lstiburek would like to see intermediary sheathing as a vapor retarder, but not all experts agree with the need for that layer. Joe doesn't mind plywood sheathing in your climate zone, but in colder zones he wants to see fiberboard or gypsum sheathing.

As a former builder, I am always wary of any building envelope details that depend on homeowners to maintain certain interior RH levels to avoid rot and failure. I wouldn't build any wall in that category as a builder -- too risky. I want a wall that can survive homeowner mistakes -- a very robust wall.

I'm not saying your wall isn't robust -- just that I wouldn't want the integrity of my wall system to depend on whether a bath fan is operating correctly. When the bath fan conks out, I don't want the wall to fail.

May 15, 2015 1:19 PM ET

Edited May 15, 2015 1:22 PM ET.

Response to Steve Vigoren (Comment #2)
by Martin Holladay

Design tips for the type of wall you are planning to build (a 2x6 wall with 4 inches of exterior rigid foam) are found here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

The "cold OSB" worries apply to double-stud walls and other walls without exterior insulation. If you have 4 inches of rigid foam on the exterior side of your OSB, your OSB will be warm, cozy, and dry all winter long. So it won't be getting damp in February. There is no evidence whatsoever that you need to worry about the integrity of the OSB in that type of wall.

That said, plywood is a better quality sheathing than OSB. Some builders who take the long view have decided, "I just don't want any OSB in my building."

May 15, 2015 1:25 PM ET

Response to Steve Vigoren (Comment #3)
by Martin Holladay

In Comment #3, you address Kevin Dickson, writing that you are "also considering this type of wall" -- presumably Kevin's double-stud wall -- and that you are "a little concerned about no sheathing on the outside."

But Kevin's proposed wall has plywood sheathing on the exterior.

May 15, 2015 2:39 PM ET

Exterior Sheathing

Martin - Thank you for a nice review of this complex issue. All referenced people mention plywood, or fiberboard / gypsum in a cold weather situation for exterior sheathing. What about wood boards? In Maine, wood sheathing remains a viable choice. Yeah, it takes longer, but it diffuses well, can be relatively inexpensive, and it does have a long history of success. Why is this traditional material rarely mentioned?

May 15, 2015 2:43 PM ET

air leakage
by Peter Rogers

The one thing I'm still confused about is the impact of air leakage. Air leakage is evil because it can introduce moisture to sheathing, but as Mr. Rose pointed out, airflow can also help with drying. Clearly, this is nearly impossible to measure or model (if diffusion is difficult to model, I can ony imagine that moisture deposited by convection is unbelievably complicated). But unlike with diffusion I don't have a good grasp of the general principles that would inform when airflow would increase or decrease the moisture load inside a wall...

May 15, 2015 2:45 PM ET

Edited May 16, 2015 6:05 AM ET.

Response to Kevin Zorski
by Martin Holladay

I'm pretty sure that all of my articles on the topic mention that board sheathing is a good option. This article didn't, because Joe Lstiburek and Bill Rose didn't mention boards.

The main reason that boards aren't used more widely for sheathing is that sawmill boards don't have a grade stamp, and are therefore not approved by most building inspectors. Moreover, most construction workers aren't familiar with the use of boards for sheathing unless they live in a rural area and have relatives who work in a sawmill or who own a bandsaw mill. In most of the country, everything is about plywood or OSB.

I used rough boards from a local sawmill as wall sheathing and roof sheathing on my own house, and have built several other homes with board sheathing. Installed on the diagonal, boards make for a very strong (and of course, vapor-permeable) frame.

May 15, 2015 3:00 PM ET

Edited May 15, 2015 3:02 PM ET.

Response to Peter Rogers
by Martin Holladay

The reason we control air flow through building assemblies has very little to do with whether air flow through the assembly helps dry the assembly or adds moisture to the assembly. The reason that we control air flow through building assemblies is to save energy.

I think most building scientists would agree that uninsulated walls with a great deal of air leakage are the type of walls that are least likely to have moisture problems. Of course, you need a huge honking furnace to heat these old uninsulated houses, and hundreds of gallons of fuel oil.

We need to build our walls, ceilings, and floors to be as airtight as possible to lower our energy bills. Once we do that, we need to develop techniques to keep our materials dry and safe. In the old days, we could rely on the drying effect of air movement through those walls. We can't do that anymore.

May 15, 2015 3:20 PM ET

Kevin Zorski: I don't know
by Peter Rogers

Kevin Zorski: I don't know what the experts here will tell you, but as someone who lives in a 15 year old house with board sheathing, I'd argue against it. Primarily because of wind washing. There's housewrap on the exterior, but when the wind blows it goes straight through the gaps between those boards. I had to remove a section of drywall recently this winter to repair a leak, and it was shocking how much air blew through the sheathing. I'll also add that it allows ants and other crawlies more access points. to get into the wall.

May 15, 2015 3:51 PM ET

Edited May 15, 2015 4:02 PM ET.

Response to Martin
by Steve Vigoren

Thanks for the response, nice to know my osb will be ok. My comment to Kevin was confusing. I thought intermediary sheathing is on the outside of the inside of a double wall, and therefore would be warm. I guess I knew Kevin's sheathing was on the outside. I am interested in that type wall, but like the idea of sheathing on the inside where it will be easier to keep warm

May 15, 2015 3:58 PM ET

Response to Peter Rogers
by Martin Holladay

Of course every house needs at least one air barrier, and board sheathing isn't an air barrier. A house with board sheathing can use exterior rigid foam as an air barrier -- that's what my house has -- or the airtight drywall approach, or both.

May 15, 2015 4:00 PM ET

Edited May 15, 2015 4:04 PM ET.

Airflow through the assembly isn't to control moisture?
by Peter Rogers


Yes, I'm definitely planning on installing some exterior insulation over those boards, as soon as economically possible!

I think I had misinterpreted Bill Rose's comment to mean that airflow in insulated buildings could help dry them out in certain conditions. Based on your response, I'll assume he was only referring to uninsulated buildings.

So, of course we control airflow primarily to save energy. But airflow also causes all kinds of problems such as moisture accumulation, so it is really correct to say that avoiding moisture issues in assemblies is not a reason that we air seal buildings? I think it's both reasons, not primarily one or the other. (I'm finding this discussion confusing because most of these lessons I thought I had learned from articles on GBA including many of yours!)

I think your response pertains mainly to new construction? With older houses, if we look at the energy savings payback, air sealing is not always all that cheap or easy by comparison with insulating; air leakage can be extremely difficult to access, and therefore time consuming and costly to address. Either that, or it can be made up of thousands of tiny points of air leakage, and people don't want to pay for a tradesperson to spend hours upon hours doing a careful job with a caulking gun. From an energy standpoint, with an old leaky home, it makes way more sense in many cases to ignore the difficult job of reducing air changes (which is also a job with no guarantee of great success, overall) and simply blow some fluffy stuff into the walls and ceilings. But if you do that without paying attention to air sealing, then you can cause huge moisture issues down the road, correct? I come up against this problem every day, and therefore I like to point out to clients that simply insulating without air sealing can cause problems with moisture accumulation, and that it's not all just about the energy savings. Are you saying I'm wrong to give this advice??

May 15, 2015 4:15 PM ET

Edited May 16, 2015 6:07 AM ET.

Response to Peter Rogers
by Martin Holladay

Air movement through wall assemblies can add moisture to the lumber and sheathing, or it can reduce the moisture content of the lumber or sheathing. It usually does both, on alternating days, depending on the weather, the RH of the interior air, the RH of the exterior air, and the rate of air flow through the leak in question.

As building scientists say, "As long as the rate of drying exceeds the rate of wetting, your assembly should usually be OK."

There are some worst-case scenarios for air leakage: when outdoor temperatures are very cold (increasing the stack effect and making exterior sheathing very cold), and when indoor RH is high, air flow through a small hole can easily lead to the build-up of a layer of frost. This often happens in leaky cathedral ceilings. When the outdoor air temperature warms up, and the frost melts, it rains indoors.

So yes -- air leakage can cause problems. But air leakage can also dry out lumber and sheathing, especially when the moving air is dry.

May 15, 2015 4:28 PM ET

OK, thanks, so I'm not
by Peter Rogers

OK, thanks, so I'm not totally off base. That more or less confirms what I thought I knew on the subject.

May 15, 2015 5:10 PM ET

Climate still matters
by Dana Dorsett

Kevin Dickson's double studwall stackup might make it in zone 5, but it's marginal (ergo the 40% absolute max humidity prescriptive). In zone 4 and lower it should be fine.

But a layer of MemBrain between the studs and interior gypsum would make that stackup quite robust even in colder climate zones, and they wouldn't have to watch the interior relative humidity like a hawk. It's not particularly expensive stuff (on the order of 2x as expensive 6-mil polyethylene) and is more responsive than Intello, promoting faster drying. It has comparable vapor retardency to Intello at 30% interior RH and is still WAY more vapor retardent than standard latex paint at 40-50% RH, but becomes more vapor open than latex paint at 70%+ RH. Think of it as cheap insurance. I have no commercial interest in Certainteed,(the manufacturer of MemBrain), but trust third party test data from tested monitored assemblies- it works.

May 15, 2015 8:29 PM ET

Edited May 15, 2015 8:30 PM ET.

Wood sheathing
by Malcolm Taylor

Here on Vancouver Island we form our foundations out of 1"x8"s and then use the stripped boards to sheath as much of the house as we can. That's all well and good, but when you need to rely on the sheathing for shear, the boards have to be applied on the diagonal, and that makes the labour involved considerably higher.
In double wall construction, with an intermediate layer of sheathing, I wonder if it makes sense to apply skip sheathing - say 1"x4" at 16" o.c. - on the exterior. This might make flashings, openings and installing the WRB easier without significant additional labour and material costs.

May 16, 2015 2:03 AM ET

Response to Dana
by Kevin Dickson, MSME

Please direct me to the reports that have convinced you that MemBrain reduces vapor diffusion.

The data I have reviewed so far only convinces me that the reduced vapor transmission was due to the reduced air leakage. (A well installed "vapor retarder layer" makes a given wall assembly much tighter). In those tests, air leakage was neither measured nor controlled for. Note, a blower door test doesn't measure the air leakage during the long term test of the sheathing moisture content, only the relative tightness of the assembly.

Also, if the data convinces me that MemBrain is effective against diffusion, it will also convince me that the "condensation pumping" effect is negligible. (MemBrain can't reduce condensation pumping because it produces huge local pressure differences that would force air and vapor around the MemBrain and through the staple holes).

May 16, 2015 7:17 AM ET

my wall solution
by George LaLonde

New construction zone 6: 1/2" drywall on 2x6 studs w 5 1/2" rockwool insulation, 3/4" closed cell foam as a thermal break, 5/8" OSB, 30 lb felt, 1/4" Homeslicker rainscreen and finished w 1/2" painted cedar beveled siding. My thought is the stud cavity will dry to the inside and the OSB will dry to the outside.
Anyone see a problem w this?

May 16, 2015 7:50 AM ET

Response to George LaLonde
by Martin Holladay

Your approach is called "flash and batt." Two comments:

1. Although you refer to your 3/4 inch layer of closed-cell spray foam as a "thermal break," it won't stop thermal bridging through the studs if the spray foam is installed on the interior side of your OSB sheathing.

2. In Climate Zone 6, building codes require those who are following the "flash-and-batt" method between 2x6 wall framing to install a spray foam layer with a minimum R-value of R-11.25. Your suggested layer of 3/4 in. of foam is too thin to meet this requirement. For more information on flash-and-batt requirements, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

May 22, 2015 6:34 PM ET

To paint or not to paint
by Matthew Seabolt

I am in the process of building my personal home and read on How to Build an Insulated Cathedral Ceiling (GBA) that it is critical to install an air barrier under tongue and groove on a cathedral ceiling (i.e. drywall) when using "fluffy" insulation and I have done so. I have the code required R-15 rigid foam (for north Georgia) above my roof decking and insulated the rafter bays with roxul mineral wool insulation. I was wondering if I should paint it with vapor-retarder paint as an extra safety feature? My concern is, as Martin pointed out, diffusion works both ways and during the summer via my ac and during other seasons via my ultra air dehumidifier, my ceiling and walls will have the opportunity to dry if necessary. My primary concern arises because the house was framed in 2012 and I have been working on it as I have the money; I did not take out a loan as I don't want to risk ever losing my house to foreclosure and I presume there is a decent amount of moisture via air leaks inside of my ceiling already, which has the sheetrock installed but not yet been taped and mudded. I have the same question regarding my walls which have 1.5" of rigid foam (polyiso) on the exterior and 2x6 walls with roxul mineral wool batts. I could purchase a dehumidifier or portable ac in an attempt to get as much moisture out of the assembly as possible and then paint using a vapor retarder paint but I am not sure how effective that would be. Any thoughts?



May 23, 2015 7:24 AM ET

Response to Matthew Seabolt
by Martin Holladay

The reason that you installed R-15 of rigid foam above your roof sheathing was to keep your roof sheathing warm and dry during the winter. In your climate zone, R-15 is enough insulation to keep the roof sheathing above the dew point. If your roof sheathing is warm, you won't get any condensation or moisture accumulation in the sheathing. That problem is solved -- so stop worrying about moisture migration from the interior to the cold sheathing. You don't have any cold sheathing.

The type of roof assembly you have chosen is designed to dry to the interior, so you don't need vapor-retarder paint. It could even be argued that vapor-retarder paint is counterproductive, since it slows the rate of inward drying. For this type of assembly, inward drying is desirable. So don't install vapor-retarder paint.

That said, you always want a good air barrier on the interior side of your roof assembly. It looks like you have that. So relax.

By the way, the same advice I just gave you concerning your ceiling also applies to your walls.

May 29, 2015 5:36 PM ET

response to 19.# Kevin Dickson,
by Dana Dorsett

Kevin Writes:

"Please direct me to the reports that have convinced you that MemBrain reduces vapor diffusion.

The data I have reviewed so far only convinces me that the reduced vapor transmission was due to the reduced air leakage."


You might want to argue that the walls with the kraft facers or paint only were just leakier than those with poly or MemBrain, but these were EXTREMELY simple and small test (4' x 9') assemblies with ONE SHEET of wallboard to air seal, with no penetrations for electricity/plumbing etc. It's doubtful that a broad sheet of poly or MemBrain makes a single monolithic sheet of wallboard appreciably more air tight than just the wallboard itself.

Comparing the moisture content of the wood in similar assemblies with both vented & unvented siding to that of the MemBrain or poly vapor retarder units is useful, and is convincing enough for me.

Air leakage would not explain the difference between Wall 7 (latex only vapor retarder, unvented siding) and Wall 5 (kraft facers + low permeance oil paint vapor retarder), unless they magically made wall 5 a heluva lot more air tight than wall 7, and it's doubtul that the higher vapor permeance of plywood vs. OSB made the difference there.

The MemBrain walls (Wall 2 and Wall 8, both with unvented stucco siding) fared pretty well, despite the fact that the interior RH was maintained at an unusually high 50-55% RH @ 69F, which is a humidity level at which MemBrain isn't at it's most vapor-tight.

Look at the moisture content graphs for those walls. MCc 3 is the sheathing moisture sensor, which stayed well below 20% for all wood in both MemBrain walls. Wall 8 also had R5 EPS on the exterior, and nothing broke 15% moisture content. The peak moisture content of the sheathing on Wall 2 (unvented stucco siding) was about 17% for less than a week in February, and dropped below 15% by March. Wall 1 had a polyethylene vapor barrier, and it too peaked briefly in February, at about 15%.

Wall 5 had kraft + oil paint, unvented stucco, peaking at 20%, but over 15% for months.

Wall 7 had latex only, unfaced batt, unvented stucco, and the sheathing stayed over 30% from the beginning of December to half-past March.

But I'm sure it was just the air leaks... yeah thatzit. :-)

But seriously, ascribing those differences to air leakage in this instance would be a fairly severe insult to the building scientists running the test. MAYBE they're so incompetent that they don't understand the importance of air leakage, but it's a stretch to assume that. Their self-assessment (on page 6) is:

"There are limits to the conclusions that can be drawn from one year of testing. The test walls were
subjected to specific interior and exterior environmental loads. The test walls could have responded
differently to a different set of loads. Several examples to these limitations have already been noted.
Exterior moisture penetration is unlikely, and the transfer of moisture from the interior through air
leakage is unlikely. "

But sure, maybe it was air leaks, but they claim to have built it fairly tightly. The do not mention blower door testing to verify tightness prior to testing, but I'll accept their self-assessment in lieu of evidence to the contrary. YMMV.

Jun 1, 2015 10:45 PM ET

Another MemBrain (Smart Retarder) Study
by Kohta Ueno

For reference, here's another study on CertainTeed Membrain (polyamide-6 variable permeability vapor retarder), also from a Pacific Northwest climate, which is from monitoring we did 2003-2006.

Field Monitoring of Wall Vapor Control Strategies in the Pacific Northwest

Figure 6 summarizes it well--shows the sheathing MCs on the wetter wall. In wintertime, from wettest to driest: latex paint, Kraft paper/Smart Vapor Retarder facer (toss-up), SVR sheet, and polyethylene. Basically, the order in which the materials would resist vapor diffusion. Remember, this is the Pacific NW, so we often have interior RHs in the 40%+ range (risky, even with mild winters).

Later on, in disassembling the walls, we figured out that the reason why the SVR facer (i.e., facer attached to the batt) was wetter than SVR sheet: it was the installation method. We were asked to install the faced batt products inset stapled (yes, it filled me with pain). An inset stapled facer creates a great chimney on the interior, and voila--a perfect setup for convective looping to bypass your vapor retarder. Check out the MC map in Figure 8--the bit of damp sheathing right at the top of the stud bay, where the convective loop would hit. Sorry about the poor quality of that PDF--a better version is attached here.

Jun 1, 2015 10:46 PM ET

by Kohta Ueno

Okay, trying to attach that attachment again, which doesn't seem to be showing up.

2015-06-01 Luby Walls.jpg

Jun 2, 2015 7:57 AM ET

Response to Kohta Ueno
by Martin Holladay

Thanks very much for the link to your study.

Here are the key paragraphs from the "Conclusions" section:

"Two Class II vapor retarders (Kraft-faced batt and PA-6 [MemBrain]) provided acceptable performance, if the vapor control layer is not bypassed by convective airflow. Both of these materials allow drying of incidental moisture to the interior, if interior conditions are at a lower dewpoint than the assembly cavity.

"A Class I vapor retarder (polyethylene) provided acceptable performance as well in these walls. Although an increase in sill plate wood moisture content was seen in the summer due to an inward vapor gradient, it was well within the safe range. However, the use of a completely impermeable vapor control layer such as polyethylene eliminates drying to the interior; moisture sources could include inward vapor drives from saturated reservoir claddings, and the drying of incidental water leakage."

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