Don’t Talk to Me About Vapor Barriers!
I was at a meeting recently when the discussion turned to wall assemblies. One of the speakers then asked what we should do about vapor barriers. It’s what everyone wants to know, right? Should I use a vapor barrier? Should I install it on the inside or the outside? But I had a different question for the speaker.
I was at a meeting recently when the discussion turned to wall assemblies. One of the speakers then asked what we should do about vapor barriers. It’s what everyone wants to know, right? Should I use a vapor barrier? Should I install it on the inside or the outside? But I had a different question for the speaker.
In the world of building science, we spend an inordinate amount of time talking about vapor diffusion. In reality, though, it’s responsible for only a tiny, tiny fraction of all the cases of moisture damage. I’ve written about how the story of peeling paint put us on this misguided path of vapor barriers.
But wait! When I hear that term come up, I always have to ask a simple question:
What is a vapor barrier?
When you use that term, are you talking about 6 mil polyethylene? Are you talking about asphalt-impregnated kraft paper? Are you talking about house wrap?
Unfortunately, people use the term “vapor barrier” to mean all of those things and more. And that means I often don’t know what the heck you mean if you use that term.
If instead you mention a Class 1 vapor retarder, I know exactly what you mean. You’re talking about a material with a water vapor permeance of 0.1 or less. This is what a lot of people mean when they talk about vapor barriers, but there’s enough confusion that I still have to ask.
And if I have to ask, that means the term “vapor barrier” doesn’t help the conversation. Here’s what I’d rather hear when you talk about a material’s ability to deal with water vapor:
Impermeable – a Class 1 vapor retarder (≤ 0.1 perm)
Semi-impermeable – a Class 2 vapor retarder (between 0.1 and 1 perm)
Semi-permeable – a Class 3 vapor retarder (between 1 and 10 perms)
Permeable – not a vapor retarder (> 10 perms)
Let’s just drop the term “vapor barrier” completely and use words that everyone can understand.
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When and where should vapor
When and where should vapor retarders be installed? Is there a climate zone and/or a building function type where they should or should not be used? Where should a vapor retarded be installed in a composite wall assembly? There is thought that vapor retarders should not be installed in a wall assembly because it impeeds the ability of the wall to dry.
Sheesh! Once again you’re
Sheesh! Once again you’re trying to bring reality into the discussion. Your non building trades background is showing again!
It’s much easier to think of these things as on/off switches. Add plastic, the switch is off. If you consider that some moisture might penetrate a particular material then you need to think about what’s going on in the rest of the wall. Now you’ve added several seconds to the process and made many heads hurt.
Accept the magic. What you don’t know won’t hurt anyone ’til you’ve cashed the last check.
One of my favorite topics,
One of my favorite topics, Allison! If I had a nickle for every time I was asked "Where should I put the vapor barrier?" I would have… a lot of nickels. My answer was always some variation of the following: That’s a question that cannot be answered. Yes, there is water vapor in air and it has the potential to cause serious moisture problems. We can’t, however, address the question of where to put the “vapor barrier” until we understand the driving force that is likely to cause the water vapor to move. Will it move via diffusion, which is seldom a problem in my climate (5700 HAD)? Or will the vapor be carried along with air moving in response to an air pressure difference, which is always a concern? Until we stop using the term vapor barrier (VB) we’re not going to make much progress. I tried for many years to get folks to include the driving force in the name and refer to them as vapor diffusion barriers (or retarders) as opposed to air barriers. Not sure that my renaming campaign ever gained much traction, but I continue to make the case.
This is an important topic
This is an important topic especially here in northern Canada. Lately builders have been wrapping the OUTSIDE of the houses with foil faced isocyanurate and taping all the joints.
As well as 6 mil poly on the inside!
See a problem here?
You bet!
And the building inspectors allow this.
Even on leaky old retrofits.
I predict the largest moldy wall fiasco will start up very soon as all the moisture leaking through poor joints, staple holes, bad sub-trade installs starts collecting in the walls.
The most important thing is
The most important thing is to be sure that an assembly has good drying potential. Impermeable materials go under slabs and in crawl spaces. If you use them other places, you’ve got to make sure the assembly can still dry on both sides of the impermeable layer.
Actually, Bill, a big part of
Actually, Bill, a big part of the problem is the delay between when the last check is cashed and when the first problems show up. But that’s changing:
The Gap Between Stupid and Hurt Is Narrowing
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/77286/The-Gap-Between-Stupid-and-Hurt-Is-Narrowing
And it’s the water vapor that
And it’s the water vapor that moves into an assembly along with air that causes way more problems than water vapor diffusing through the materials.
Uh, yeah… Impermeable
Uh, yeah… Impermeable materials on both sides of a wall is a recipe for disaster.
One of my all-time heroes of
One of my all-time heroes of building science presented a paper in 1998 on air pressure in walls where his team researched the relationship between vapor retarders and air barriers. In it Anton TenWolde presented the obligatory pages of differential equations, graphs and tables and concluded with a simple statement. "…the effectiveness of the polyethylene vapor retarder almost certainly depended on its function as an air barrier, a function it was not intended to perform."
His research showed vapor retarders to work, mainly for the reason stated above and also stated "….without a carefully applied ABS (Air Barrier System), a 1-perm vapor retarder is rendered ineffective."
TenWolde’s discussion about moisture flow through wood frame walls states, "Obviously, when a vapor retarder is present or specified, the vapor transfer by air leakage needs to be limited to the amount that is transferred by diffusion through the vapor retarder, or less." I wonder how many of our homes and buildings are fully compliant with this?
Build it Tight and Ventilate Right.
This mantra hasn’t changed in decades, and I so wish everyone in building science and construction would think every problem through this filter. If we build tight buildings and control our indoor environment, we won’t have moisture problems. It’s not that difficult. We have great materials and all the brains and man-power needed to do a good job but somehow we "cheap-out" and think it’s OK.
Right now our world leaders are preparing to spend billions and billions of dollars to reduce greenhouse gas emissions, a large portion of which come from energy production and heating our buildings. Most of this problem would go away if we simply insulated and air-sealed our homes and buildings like we already know how; both our built environment and new construction. Think of the jobs we can create too……
Yep. Stop the water vapor
Yep. Stop the water vapor that gets carried along with air moving into an assembly and you eliminate the vast majority of moisture problems not caused by rain.
Allison,
Allison,
I know your in the south east as am I. I’m very interested in designing the right wall enclosure combination for our humid hot region. I live in Enterprise Alabama about 90 miles north of the Florida panhandle coast. Can you provide a few proven combinations of material that work well here and some examples that don’t?
Also curious on your thoughts on cellulose vs spray foam insulation in regards to indoor air quality ( off gassing ) and achieving efficient levels of R- values for the South.
Regards Mike
When and where should vapor
When and where should vapor retarders be installed? Is there a climate zone and/or a building function type where they should or should not be used? Where should a vapor retarded be installed in a composite wall assembly? There is thought that vapor retarders should not be installed in a wall assembly because it impeeds the ability of the wall to dry.
The most important thing is
The most important thing is to be sure that an assembly has good drying potential. Impermeable materials go under slabs and in crawl spaces. If you use them other places, you’ve got to make sure the assembly can still dry on both sides of the impermeable layer.
Sheesh! Once again you’re
Sheesh! Once again you’re trying to bring reality into the discussion. Your non building trades background is showing again!
It’s much easier to think of these things as on/off switches. Add plastic, the switch is off. If you consider that some moisture might penetrate a particular material then you need to think about what’s going on in the rest of the wall. Now you’ve added several seconds to the process and made many heads hurt.
Accept the magic. What you don’t know won’t hurt anyone ’til you’ve cashed the last check.
Actually, Bill, a big part of
Actually, Bill, a big part of the problem is the delay between when the last check is cashed and when the first problems show up. But that’s changing:
The Gap Between Stupid and Hurt Is Narrowing
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/77286/The-Gap-Between-Stupid-and-Hurt-Is-Narrowing
The narrowing gap is starting
The narrowing gap is starting to help. I’ve seen several builders over the past couple of years get hit with significant problems within a few months. The problem is admitting that thew problem is systematic and not just a one off.
One of my favorite topics,
One of my favorite topics, Allison! If I had a nickle for every time I was asked “Where should I put the vapor barrier?” I would have… a lot of nickels. My answer was always some variation of the following: That’s a question that cannot be answered. Yes, there is water vapor in air and it has the potential to cause serious moisture problems. We can’t, however, address the question of where to put the “vapor barrier” until we understand the driving force that is likely to cause the water vapor to move. Will it move via diffusion, which is seldom a problem in my climate (5700 HAD)? Or will the vapor be carried along with air moving in response to an air pressure difference, which is always a concern? Until we stop using the term vapor barrier (VB) we’re not going to make much progress. I tried for many years to get folks to include the driving force in the name and refer to them as vapor diffusion barriers (or retarders) as opposed to air barriers. Not sure that my renaming campaign ever gained much traction, but I continue to make the case.
And it’s the water vapor that
And it’s the water vapor that moves into an assembly along with air that causes way more problems than water vapor diffusing through the materials.
This is an important topic
This is an important topic especially here in northern Canada. Lately builders have been wrapping the OUTSIDE of the houses with foil faced isocyanurate and taping all the joints.
As well as 6 mil poly on the inside!
See a problem here?
You bet!
And the building inspectors allow this.
Even on leaky old retrofits.
I predict the largest moldy wall fiasco will start up very soon as all the moisture leaking through poor joints, staple holes, bad sub-trade installs starts collecting in the walls.
Uh, yeah… Impermeable
Uh, yeah… Impermeable materials on both sides of a wall is a recipe for disaster.
Andy, I could not agree more
Andy, I could not agree more with your concerns and I see that Allison also agrees.
One of my all-time heroes of
One of my all-time heroes of building science presented a paper in 1998 on air pressure in walls where his team researched the relationship between vapor retarders and air barriers. In it Anton TenWolde presented the obligatory pages of differential equations, graphs and tables and concluded with a simple statement. “…the effectiveness of the polyethylene vapor retarder almost certainly depended on its function as an air barrier, a function it was not intended to perform.”
His research showed vapor retarders to work, mainly for the reason stated above and also stated “….without a carefully applied ABS (Air Barrier System), a 1-perm vapor retarder is rendered ineffective.”
TenWolde’s discussion about moisture flow through wood frame walls states, “Obviously, when a vapor retarder is present or specified, the vapor transfer by air leakage needs to be limited to the amount that is transferred by diffusion through the vapor retarder, or less.” I wonder how many of our homes and buildings are fully compliant with this?
Build it Tight and Ventilate Right.
This mantra hasn’t changed in decades, and I so wish everyone in building science and construction would think every problem through this filter. If we build tight buildings and control our indoor environment, we won’t have moisture problems. It’s not that difficult. We have great materials and all the brains and man-power needed to do a good job but somehow we “cheap-out” and think it’s OK.
Right now our world leaders are preparing to spend billions and billions of dollars to reduce greenhouse gas emissions, a large portion of which come from energy production and heating our buildings. Most of this problem would go away if we simply insulated and air-sealed our homes and buildings like we already know how; both our built environment and new construction. Think of the jobs we can create too……
Yep. Stop the water vapor
Yep. Stop the water vapor that gets carried along with air moving into an assembly and you eliminate the vast majority of moisture problems not caused by rain.
Allison,
Allison,
I know your in the south east as am I. I’m very interested in designing the right wall enclosure combination for our humid hot region. I live in Enterprise Alabama about 90 miles north of the Florida panhandle coast. Can you provide a few proven combinations of material that work well here and some examples that don’t?
Also curious on your thoughts on cellulose vs spray foam insulation in regards to indoor air quality ( off gassing ) and achieving efficient levels of R- values for the South.
Regards Mike
I first learned about "
I first learned about "vapor barriers" well over 30 years ago, when their importance was stressed for Super-Insulating houses in colder climate zones (CZ-6), which I had begun to do. Air barriers were somewhat of a less concern back then, although that is the reason that I developed my modified double-wall system.
In wanting to do a superior job, I developed a way to install the "vapor retarder" ~ in the "middle" of a modified double-wall system. This eliminated the need for detailed work to be done around electrical and other penetrations, where air sealing was often difficult and expensive to do. I developed simple ways to install a very effective vapor barrier (poly) in the middle of my walls, which can be adjusted for different climate zones. I soon learned that this system was also a very effective air barrier, which is today recognized as much more important. This system is also very economical to install and to check on its quality of installation.
Andy, I could not agree more
Andy, I could not agree more with your concerns and I see that Allison also agrees.
The narrowing gap is starting
The narrowing gap is starting to help. I’ve seen several builders over the past couple of years get hit with significant problems within a few months. The problem is admitting that thew problem is systematic and not just a one off.
I first learned about "
I first learned about “vapor barriers” well over 30 years ago, when their importance was stressed for Super-Insulating houses in colder climate zones (CZ-6), which I had begun to do. Air barriers were somewhat of a less concern back then, although that is the reason that I developed my modified double-wall system.
In wanting to do a superior job, I developed a way to install the “vapor retarder” ~ in the “middle” of a modified double-wall system. This eliminated the need for detailed work to be done around electrical and other penetrations, where air sealing was often difficult and expensive to do. I developed simple ways to install a very effective vapor barrier (poly) in the middle of my walls, which can be adjusted for different climate zones. I soon learned that this system was also a very effective air barrier, which is today recognized as much more important. This system is also very economical to install and to check on its quality of installation.
DR. I have a question about a
DR. I have a question about a Crawlspace I inspected this week. Info about the Home. 1940’s craftsman. Grade under home is 1-1.5 feet below outside grade. Homes adjacent, both sides, are less than 10 ft away on both sides. NO gutters on the three homes. The Subject home in the center here has mortared brick peirs with a brick "underpinning" on the outside, This brick wall may have some structural significance. The question…The outside is painted, I guess several times over the years. If I seal the crawlspace with VB on the ground and Closed Cell on the Outside walls, will that create a double vapor barrier on the bricks and not allow the bricks to dry when wicking water climbs the wall?
David,
David,
You are right to be concerned. Dr. Joe did a presentation over 25 years ago in which he stated that a concrete pier painted on all sides would wick moisture vertically around 6 miles. Concrete (brick) walls need to breath to open air. Dr. Joe further cited that sealing concrete on both sides had led to rotting sill plates in Canada. Removing a band of paint on the exterior might help, but site unseen (by me) my recommendation is worth what I charged you for it. I highly recommend you do more research before sealing the interior wall. The exterior aesthetics may cost extra, but not as much as replacing sill boards with PT, which I highly doubt was placed there in 1940. Good luck!
DR. I have a question about a
DR. I have a question about a Crawlspace I inspected this week. Info about the Home. 1940’s craftsman. Grade under home is 1-1.5 feet below outside grade. Homes adjacent, both sides, are less than 10 ft away on both sides. NO gutters on the three homes. The Subject home in the center here has mortared brick peirs with a brick “underpinning” on the outside, This brick wall may have some structural significance. The question…The outside is painted, I guess several times over the years. If I seal the crawlspace with VB on the ground and Closed Cell on the Outside walls, will that create a double vapor barrier on the bricks and not allow the bricks to dry when wicking water climbs the wall?
David,
David,
You are right to be concerned. Dr. Joe did a presentation over 25 years ago in which he stated that a concrete pier painted on all sides would wick moisture vertically around 6 miles. Concrete (brick) walls need to breath to open air. Dr. Joe further cited that sealing concrete on both sides had led to rotting sill plates in Canada. Removing a band of paint on the exterior might help, but site unseen (by me) my recommendation is worth what I charged you for it. I highly recommend you do more research before sealing the interior wall. The exterior aesthetics may cost extra, but not as much as replacing sill boards with PT, which I highly doubt was placed there in 1940. Good luck!