The Return of the Vapor Diffusion Bogeyman
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
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 GBAGreenBuildingAdvisor.com. For those who haven’t seen the articles, here are the links:
- How Risky Is Cold OSB Wall Sheathing?
- Monitoring Moisture Levels in Double-Stud Walls
- Is Cold Sheathing in Double-Wall Construction at Risk?
- Lstiburek’s Ideal Double-Stud Wall Design
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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