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Building Science

What Happens When You Put a Plastic Vapor Barrier in Your Wall?

A look at the physics of humid air and cool surfaces

Image 1 of 2
A polyethylene vapor barrier in an above-grade wall does indeed stop water vapor from moving through. Most walls don't need a vapor barrier, though, because drying is more important than wetting in most cases.
A polyethylene vapor barrier in an above-grade wall does indeed stop water vapor from moving through. Most walls don't need a vapor barrier, though, because drying is more important than wetting in most cases. Polyethylene vapor barrier in a basement wall in a $4 million dollar home in Atlanta
Image Credit: Energy Vanguard

A lot of people have heard advice about vapor barriers and vapor retarders. Many of them have walked away confused. A big part of the problem, I think, is that they’ve been told what to do — “Put it on the warm-in-winter side,” or “Never use one” — but they haven’t had the physics of what happens explained to them.

In this article, I’m not going to get into the details of vapor barriers or all the possible scenarios of different wall assemblies and moisture loads. I’m simply going to explain what happens in a wall cavity with and without a plastic vapor barrier installed.

Scenario 1: plastic on the inside, hot humid weather

I’m writing this article because one of our HERS raters came across a house in Charleston, South Carolina that had polyethylene under the drywall, on the interior side of the wall assembly. Polyethylene, often shortened to poly, is a Class I vapor retarder, which is usually what’s meant by the term vapor barrier. If you’re at all familiar with the climate in Charleston and understand moisture, you know that can’t be a good thing.

I was there one day in June a few years ago and saw condensation on the outside of a window… at one o’clock in the afternoon of a sunny day. The dew point of the outdoor air was 78° F. The window had a single pane of glass. They were running the air conditioner, so the indoor temperature was probably 75 or below. Humid air hits cool surface. Condensation results.

Now, imagine that pane of glass is actually a sheet of polyethylene. Next, imagine that a layer of drywall separates the poly from the indoor air. Then build a wood-frame wall outside the poly, complete with cladding and air permeable insulation in the cavities. Will that poly be protected from the outdoor humidity? Or will it, like the window I saw, be dripping with condensation?

If it’s a typical wall, chances are good that water vapor in the outdoor air will find its way into the wall cavity, eventually finding the sheet of poly, pressed up against the drywall. If that wall allows outdoor air to infiltrate and the poly is below the dew point, condensation is the likely result. If those conditions last long enough, the condensed water will run down the poly, get the wood framing wet, and begin to rot out the wall.

The truth, however, is that the water vapor in the outdoor air is rarely the source of moisture that rots out a wall. More likely is that moisture from a wet foundation makes its way up into the wall by capillary action, or bulk water from leaks around openings gets into the wall cavity. The presence of an interior vapor barrier makes drying out the cavity harder to do, though.

Without poly beneath the drywall, water vapor hits the drywall and diffuses through to the drier (in summer) indoor air. By installing a sheet of poly there, you cut off that drying mechanism and water that finds its way into walls can stay there longer and do more damage.

Scenario 2: plastic on the inside, cold weather

In cold weather, a sheet of poly on the interior side of a wall probably won’t cause any problems. The humid air is indoors, and the dry air is outdoors. The sheet of poly still cuts off drying to the indoors, but it keeps the water vapor in the humid indoor air away from the cold surfaces inside the wall. This is what building scientists proposed as the solution for walls that wouldn’t hold paint back in the early days of insulation. It didn’t solve the paint problem, though, because water vapor from the indoor air wasn’t the main source of moisture.

Scenario 3: plastic on the outside, cold weather

Plastic on the outer surface of a wall in cold weather could cause problems. The humid air is indoors. The cool surface is the sheathing, assuming no exterior insulation. If water vapor diffuses or infiltrates into the wall cavity and finds the cool surface, moisture problems can occur.

Of course, you can have moisture problems here even without the exterior vapor barrier because of what Bill Rose calls the rule of material wetting. That is, warm materials dry more quickly than cold materials. If humid air from indoors finds cold sheathing and starts accumulating there, the moisture will stay there longer because it’s difficult to dry at low temperatures, even though the outdoor air is dry in terms of absolute humidity.

Scenario 4: plastic on the outside, hot humid weather

A vapor barrier can cause problems if it prevents drying to a drier space. In a building with air conditioning during hot humid weather, the drier space is indoors. The humid air is outdoors. Putting a vapor barrier on the inside, as we saw in Scenario 1 above, can lead to problems because any humid air that gets into the wall cavity is blocked from drying to the inside.

If the vapor barrier is on the outside, it prevents the humid air from diffusing into the wall cavity and finding the cold surface on the other side of the cavity, the back side of the drywall. So, like a vapor barrier on the inner surface in cold weather, putting one on the outer surface in hot weather isn’t likely to have a moisture problem due to vapor diffusion. It doesn’t mean you won’t have a moisture problem, though, because moisture can get into the walls by means other than diffusion.

It’s not just a climate issue

We can summarize the vapor barrier issue like this:

  • A Class I vapor retarder’s job is to keep water vapor in humid air from diffusing through one side of a wall and finding a cool surface inside the wall.
  • When a Class I vapor retarder is on the side of a wall where the dry air is (i.e., outside in winter or inside in summer), moisture problems can occur.
  • A Class I vapor retarder reduces the movement of water vapor by diffusion. Holes in the vapor barrier that allow humid air through may allow a lot more water vapor into an assembly than the vapor barrier is stopping. Because of this, air sealing is more important than vapor retarders.

If you’re in a place like Miami, where it’ll almost never be colder outdoors than indoors, a Class I vapor retarder on the outer surface of a wall assembly may be OK. If you’re in Maine and never use an air conditioner, a Class I vapor retarder on the inner surface may be OK. If you’re in a cold climate, however, and do use air conditioning, you need to be careful with interior vapor barriers like polyethylene. You could be creating the kind of problems I described in Scenario 1 above.

Enhancing drying vs. preventing wetness

If designers and builders want buildings to do their jobs properly and not fail prematurely, it’s important to understanding moisture. We know now that mid-twentieth century building science incorrectly ascribed magical properties to vapor barriers. Water vapor from indoor air wasn’t the source of most moisture problems. Bulk water from deficiencies in drainage planes, flashing, and other moisture management details caused most of the problems.

Building science has progressed since then. We know that vapor barriers can cause problems, but we still have homes like the one in Charleston with poly in the walls. And we have $4 million dollar homes with poly on the walls, too. I saw the one shown in the photo below when Martin Holladay came to Atlanta last year. The photo was taken in the basement, and the kneewalls in the attic also were covered with poly.

Our understanding now is that it’s generally more important for wall assemblies to be able to dry than it is to block water vapor with materials like polyethylene. In most cases, you’re fine with the vapor retarding ability of the materials already in your walls, and you don’t need to use Class I vapor retarders.

Here’s what Bill Rose wrote in his book, Water in Buildings: “Given the fact that a very small percentage of building problems (1 to 5% at most in the author’s experience) are associated with wetting by water vapor diffusion, the argument for enhanced drying potential becomes much stronger.”

In other words, the potential for creating problems is greater when you use Class I vapor barriers that prevent drying, so be absolutely certain you need one before going that route.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. user-1140531 | | #1


    The original justification for a warm-side vapor barrier was to solve the wall moisture problem that arose with the advent of filling the walls with insulation, which at that time was fiberglass. The insulation lowered the temperature of the inside surface of the sheathing below the dew point of the outward vapor flow. So the vapor barrier was introduced to stop the outward vapor flow.

    More recently, however, that original explanation of the moisture problem has been replaced by a new explanation involving the fact that cold sheathing holds more water than warm sheathing. So we have explanation #1 being replaced by explanation #2.

    Above, you say that explanation #2 is ultimately caused by bulk water intrusion. So, If one were to eliminate all bulk water intrusion, would that eliminate the wetting of cold sheathing which can hold more moisture than warm sheathing?

  2. user882465 | | #2

    Ron Keagle's Question
    Bulk water is a problem. But so is moisture carried by warm air, which comes from the interior in winter. That is why Alison speaks of the importance of air sealing. Air carries moisture in its gaseous state, which condenses when it reaches a cold surface. See

  3. fitchplate | | #3

    All in all ...
    All in all, Class 1 vapor barriers are a bad idea. I would even venture to say that Class II (at least on the low end < 1.0 perm) VDR’s have limited if any useful application nowadays. Seen too many places using, heard too many builders proclaiming: vapor barriers for infiltration/exfiltration control … that is, as a cheap and easy air-seal. Bring out the dehumidifiers, defrost the HRV's.

  4. user-1140531 | | #4

    Reply to Bill Burke

    I understand your point, however, Referring to the erroneous conclusion of earlier building scientists, Allison said this, “Water vapor from indoor air wasn't the source of most moisture problems. Bulk water from deficiencies in drainage planes, flashing, and other moisture management details caused most of the problems.”

    And he said this: “In cold weather, a sheet of poly on the interior side of a wall probably won't cause any problems. The humid air is indoors, and the dry air is outdoors. The sheet of poly still cuts off drying to the indoors, but it keeps the water vapor in the humid indoor air away from the cold surfaces inside the wall. This is what building scientists proposed as the solution for walls that wouldn't hold paint back in the early days of insulation. It didn't solve the paint problem, though, because water vapor from the indoor air wasn't the main source of moisture.”

    I don’t understand how poly can have the ability to “keep the water vapor in the humid indoor air away from the cold surfaces inside of the wall,” and yet be incapable of solving the paint adhesion problem because for that problem, “water vapor from the indoor air is not the main source of moisture.”

  5. Expert Member

    Poly has its place.
    In some climates poly still has a place as an air barrier. I agree in the long term its use should be phased out but for now it is a viable option in some places.
    Here in British Columbia both the code and inspectors became very serious about air sealing over twenty years ago. The poly had to be sealed at all sills and penetrations. The advantage to this approach were two fold: It was done as a separate task and subject to visual inspection. This had a knock on effect of making insulators take their jobs seriously and we never had the poor batt installations I see are common elsewhere.
    So BC and much of the rest of Canada has a system of insuring tight houses in which a small group might because of their summer cooling have some vulnerability in the poly being present. The alternatives, an airtight drywall approach, or in more sophisticated building envelopes burying the air sealing in another part of the enclosure, while fine in high end houses, presents real problems for most housing production. It can't be inspected until the problems have occurred (by a blower door test) and disrupts what is for now a system in which the building trades have been educated and understand.
    As poly is abandoned in many parts of the States, builders are interpreting the problems caused by it's vapour impermeability as a reason to just abandon air sealing altogether, rather than adopting another strategy. In northern climates I don't think it is a good idea to get rid of a system that in the main works, until an alternative method becomes widely understood and disseminated.

  6. user-831308 | | #6

    on the contrary
    "As poly is abandoned in many parts of the States, builders are interpreting the problems caused by it's vapour impermeability as a reason to just abandon air sealing altogether"

    I don't know. I believe that the builders that are abandoning the use of poly are now becoming more educated on the fundamental difference between air sealing and vapor sealing. Besides, prescriptive air sealing has been in the code books (and has been enforced) now for many years down here. Of course I live in Maryland, where we immediately adopt the latest version of the IRC and IECC. Although, perhaps I live in a bubble (and one likely made of poly.)

  7. Perry525 | | #7

    Warm wet room. cold frame.
    Let's try and add some detail. Wood is hygroscopic. Molecules of water vapor are small enough to enter the pores of wood when it is below dew point and raise its water content, to the point where mold can form and where wood rot can start.

    Water vapor doesn't settle inside warm wood, that is to say wood that is above dew point. Therefore, having the insulation on the room side of a wall would seem to be wrong, as the temperature of the frame will be somewhere approximate to the temperature outside and at times condensation will result inside the wood, however, experience shows that where the wood is warmed and dried by the summer sun each year this isn't usually a problem.

    The principal problem we need to overcome is, that wood is poor insulation, and heat moving to cold as the first law of thermodynamics, causes heat loss not only through the frame, but in all directions through any solid. Meaning heat is also lost down into the ground and up into the roof and on into the sky.

    In winter heating areas, it is desirable to have the insulation on the room side of the frame, thereby stopping our heat reaching the frame and slipping away. The way to do this is to fix sheets of closed cell insulation on the room side of the frame, to cover the insulation with plastic sheet, followed by drywall which is then kept at room temperature, thereby being above dew point at all times and avoiding condensation.

    This solution neatly gets round the usual problem, of poorly fitted water vapor barriers, where because water vapor molecules are so tiny they are able to pass through gaps that are too small to see, and they then do untold damage inside the frame.

  8. lutro | | #8

    Water vapor and wood
    I'm not in agreement with Roger Anthony, regarding his statements about water vapor and wood. Water vapor will enter (and leave) wood slowly, but relentlessly, regardless of the dew point. If humidity conditions are fairly stable, the wood will eventually reach equilibrium moisture content (EMC), where water vapor molecules are entering and leaving the wood at equal rates. If wood is drier than the EMC for the average temperature and relative humidity of the air surrounding it, then it will gain moisture, even if the wood and the air are constantly above the dew point.

    In the wall of a house, the relative humidity and the temperature will vary with time of day, weather, and season. However, it remains true that staying above the dew point will not prevent moisture uptake by wooden framing members.

  9. Myrtleboone | | #9

    Poly in a double wall assembly-cold climate
    I am building a double-walled home in northern Maine. I have come up with a wall that includes a dedicated poly air barrier that joins to the poly below the slab. It rests interior to 2" of r-board polyiso which is is found between my two Roxul-filled stud walls. The poly layer lies interior to ~R-35 and exterior to ~R-15, seemingly below the dew point region. Being on the warm side of the wall (more than 2/3 R-value inward), and having an HRV to control humidity levels, am I safe to assume that condensation on the poly won't be an issue? Thought of excluding the poly and just taping the interior polyiso seams to create my air barrier, but thought it would involves more labor, more expense (tape) and the chance for the seal to become compromised with settling of the house.

  10. GBA Editor
    Martin Holladay | | #10

    Response to Matthew Michaud
    Polyethylene can be used safely the way you intend to use it -- because you live in such a cold climate.

  11. ErgoDesk | | #11

    Better Building Materials
    Building with Wood is a real problem we all seem to just tolerate, rather that suggest better materials that don't give us all the grief. For example, EPS Composites, that, Insulate and cladding without all those confusing layers.

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Wood has been successfully used in human habitations for almost as long humans have been on earth. It is abundant, environmentally friendly and biodegrades when its lifespan is over. We are just having some teething pains over very recent changes in the way we insulate. No need to throw the baby out with the bathwater.

  13. user-1140531 | | #13

    The Case For Poly
    Building science recognizes the following methods for moisture accumulating in wall cavities:

    1) Air leaks that carry vapor in the airflow.
    2) Diffusion equalizing vapor pressure across a boundary.
    3) Bulk water leaks from the exterior.
    4) Cold sheathing adsorption.
    5) Summertime reverse vapor drive.

    In much of the discussion on this topic, items #1 and #3 are treated as though they are inevitable. Particularly item #1 is often framed as though it is nearly impossible to prevent. Therefore, due to the inevitability of wetting the wall cavity from these two moisture sources, the suggested remedy is to allow the wall cavity with all of the drying potential that is possible.

    I prefer to build without air leaks or water leaks from bulk water intrusion. Building a wall cavity that can dry out bulk water intrusion seems like furnishing the interior with outdoor carpet and furniture in case the roof leaks. Superinsulated houses demand attention to details and high quality workmanship. Therefore it does not seem unreasonable to expect the flashing details to keep the water out.

    Assuming that more vapor is likely to enter a wall cavity when carried airflow through holes than will enter by diffusion, we are told that preventing holes is more important than providing a vapor barrier that stops diffusion. But why assume that the two have to be mutually exclusive?

    The often cited claim that diffusion is a minor vapor transfer mechanism is based on diffusion through a material. Whereas a hole through material allows diffusion through air or free equalization of differing vapor pressures. So a hole in a vapor barrier partially defeats the vapor barrier just as a hole in an air barrier partially defeats an air barrier. A hole in either barrier lets vapor pass while carried by airflow, and by movement through diffusion. Therefore, it is essential that either type of barrier be free of holes.

    An air barrier that is free of holes stops vapor that is carried by airflow, but does not stop vapor transferred by diffusion. But a vapor barrier that is free of holes stops both airflow and diffusion.

    I like the certainty of a poly vapor barrier. If installed properly, it stops air and diffusion. Couple it with proper flashing and exterior details, and the wall cavity stays dry. And just for backup, the cavity can dry to the exterior.

    I agree that there is even more backup if the wall can dry to the interior, but the interior is a big source of moisture that wants to move outward much of the time. Rather than letting this moisture into the wall and countering that problem with the remedy that the moisture can dry back out the way it came in, I would prefer to keep it out of the wall in the first place. Otherwise, it strikes me as putting holes in the Titanic so any water that happens to get in can get back out.

    So my approach is to use a poly vapor barrier to stop airflow, and diffusion; and to build without any opportunity for bulk water intrusion. I won’t have any holes for air, diffusion, or bulk water to enter the insulation cavity. That leaves only items #4 and #5 to worry about. If items #1-3 are eliminated, I don’t see why item #4 would be a problem. To be a problem, it needs a significant source of wintertime moisture, and without items #1-3, there is no such significant source.

    That leaves only item #5 to be concerned about. Certainly, this has the potential to deposit moisture inside of the wall cavity if the outdoor dew point is high enough and the interior air conditioning is cold enough. Clearly, the hottest, humid summertime climates can provide the conditions for item #5 to become a problem. The only question is where the climate dividing line is that separates the areas where the problem is likely from the areas where it is not. Generally, it is said to be a problem everywhere except for the far northern regions of Canada.

    However, there are other factors that influence whether item #5 will be a problem. I have no problem with it in central Minnesota. Part of the reason is that I do not cool the interior lower than around 75 degrees. If I liked air conditioning at 68 degrees, there might be some condensation inside the insulation cavities. But the dew point rarely gets as high as 75 degrees, and if it does, it is only for part of a day on a few days during the summer. Another point to consider is that there is some R-value between the warm-side vapor barrier and the conditioned living space. So a 75 degree air temperature in the living space will not cool the vapor barrier as low as 75 degrees. A service cavity between the living space and the vapor barrier will also add some R-value, further separating the vapor barrier temperature from the dew point. So I don’t have a problem with item #5.

  14. 1900DisasterHome | | #14

    HELP!!! I bought a remodeled home which is an energy disaster. The master bedroom is a cathedral ceiling and doesn't stay warm enough. There is a very thin space between the master and the roof, I don't think there is much insulation in between, perhaps the minimum that they could get by with. My furnace runs all day (it has its own problems but that's for another day) but the master doesn't get warm at all, it stays below 60 degrees on a cold day 40 degrees or lower. The other rooms can hit upper 60's. I live in central Maryland, and last winter my heating bill was over $400 to keep the house at 55 degrees. I am working on air sealing everything because of terrible stack effects, ducts that are pumping heat into crawl space etc. But I am wondering if it is OK to put foam boards (R-20 4") on the cathedral ceiling, either directly, or by spacing it from the drywall with some 1/2 inch strips and then and then caulking/sealing any gaps between boards with foil tape . Based on this article, it sounds like the main risk would be in the summer if I run AC too cold, condensation could happen in the drywall? But also I see condensation on the windows inside at the bottom (windows are at the ceiling) when I put foam board against them, in the morning when I wake up if I take off the foam board. Of course the glass is probably much colder than the drywall, so there is less chance that this could happen. But I am wondering if there are any other unforeseen issues that I might create by doing this.

  15. user-7484530 | | #15

    I am convinced that VB’s are not a good idea in southern Canada for residential construction. Moisture issues are generally not due to water vapor moving from the inside of the wall to the outside of the wall, but rather a consequence of other factors, such as exfiltration, which move 100 times more water. That being said, making sure that interior vapor does not become a potential source of moisture is the role of the interior of wall VB, and as such, I have questions.

    What builders assume happens with an interior of wall VB :

    - A) When it is cold and dry outside the VB will prevent moisture from diffusing into the wall assembly and then condensing once it hits the dew point.

    - B) When it is warm and humid outside the VB will prevent moisture that may have accumulated in the wall from drying to the inside.

    In Canada, even in Ottawa, we assume A to be true, and to be more important than B when it comes to reducing the moisture content in a wall due to our long winters. (To be clear, as I understand it, A is not more important than B. That is to say A is not more beneficial to the wall assembly than B is harmful to the wall assembly. )

    My questions are below, though certainly feel free to correct the above if it is wrong.
    1) What drives vapor diffusion?
    a. In your All About Vapor article you assign difference in Vapor Pressure (VP) and Temperature, independent of each other, as the most important factors in determining the direction of Vapor Diffusion. High Vapor Pressure to low vapor pressure and hot to cold. Would the difference in Relative Humidity (VP/Saturated VP at original temp x100) or ((SVP at dew point/ SVP at the original air temperature)x100), not be the driving factor here? I’ve never understood why the absolute amount of water drives vapor, and not the amount of water that the air can hold.
    2) In Ottawa, and the northern US, is there a large difference in relative humidity in the winter and summer? Though the climate is varied I would assume that there is a significantly greater difference in vapor pressure between summer and winter than there is RH? This has implications for vapor drive.
    3) In the summer, if the outside air is warm and has higher vapor pressure than the inside air, I assume this will drive vapor inwards. Yet if RH is what drives vapor, then there would be much less of a vapor drive as the different temperatures inside and outside would create potentially similar RH.
    4) The same question applies to the wintertime. In the winter, if the outside air is cool and dry like it is in Canada, this would assume a strong vapor drive to the outside, yet not if RH is the actual drive.
    5) Assuming that the vapor drive is the product of vapor pressure and temp. Where do we land on the pushme pullyu equation as you put it? In other words, how much water is driven inside relative to outside in a given year due to vapor diffusion? I find this is part of the crux of the issue, though never really answered. Talk to an old school builder and he’ll say, yeah the vapor drive is 99% driven from the inside out.
    6) I understand that moisture will need to dry to the inside at some point, and that the vapor drive will go both ways. Nonetheless on what empirical basis are we saying that it is better to have vapor move through the assembly, than to completely stop the vapor moving from the inside out? I could suggest that in the VB’s particular case, the amount of reduced moisture is relatively unimportant, but the need for drying, especially due to water infiltration, may be relatively important as it carries much more moisture. How can I prove this in an unimpeachable way? The counter is always there: for most of the year, the vapor drive supposedly goes the other way.
    7) Could the water being driven outside condense more than the water being driven inside due to potentially greater sudden drops in exterior air temperature as it exits the buildings insulation in a cold winter’s day than when it enters the building during a warm summer’s day? Would it cause more damage due to this? I always wonder about putting an AB/VB right on top of the insulation. I understand its purpose as a drainage plane, but I wonder if it serves as a drainage plane for moisture going the other way: vapor that has just condensed as it exited the thermal cladding of the home. ( BTW I checked and Tyvek is apparently directional when it comes to water vapor, but not when it comes to moisture…am I wrong, if not then is this not an issue?)
    8) As such the follow-up is, considering both the amount of vapor that moves either in or out of the home, and what that vapor does when it travels, which causes the most harm?
    9) What percentage of vapor can be accounted for by the cladding choice? If I choose brick, and it holds water and heat very well, it may not provide a path for vapor (it is warmer than and wetter than the air) but rather provide a source of vapor for the assembly by itself.

  16. Expert Member
    MALCOLM TAYLOR | | #16


    I'll add a few th0ughts but forgive me if it a bit of a scatter-gun reply.

    - In B it isn't just that the moisture can't dry to the inside, it's that the vapour-barrier becomes a condensing surface. A VB should never be a drainage plane, or have moisture accumulate on them. They are a barrier to stop diffusion. If they are located in the correct position, the moisture shouldn't condense because it is at the same temperature as the interior (or in summer the exterior) air which is not at its dew point.

    - The importance of A depends on the drying capacity of the wall to the outside. In very vapour-open wall-assemblies the amount of moisture making its way into the wall through diffusion isn't significant. But in walls with a low-perm exterior, like those with an inch or so of foam (as is now common in Ontario), even with a good air-barrier the amount of moisture that can make it's way into the all though diffusion can be enough to cause problems. In these assemblies the exterior rate of drying needs to exceed the interior rate, and that means the presence of a VB is important.

    - I think old time builders got it wrong, and confused bulk water infiltration (due to their poor detailing) for diffusion from the outside.

    - I understand the concern with wrong-side VB's in air conditioned buildings, but do we have any evidence this has caused problems in our Canadian climate? (Maybe there is and I just haven't heard about it).

    - Cladding choice definitely matters. In the absence of a reservoir cladding, like brick or stone, which retains significant amounts of water, there isn't enough moisture in humid summertime outside air for sun to drive it into the wall assembly. But those reservoirs claddings can cause significant harm to sheathing if the WRB is a high-perm 0ne. The harm isn't due to the presence or location of a VB, it is that the exterior sheathing can take on too much water due to inward solar vapour-drive.

    - If I could suggest a method to design wall assemblies it would be to sort with Steve Baczek's method of considering the effects of water, air, vapour and thermal control in that order. The decision as to what level of interior vapour control depends on what is necessary to ensure it is exceeded by the permeability of the exterior. If drying exceeds wetting, diffusion isn't a problem.

    Sorry if this hasn't really answered any of your questions directly, and hope it hasn't just caused more confusion.

    1. user-7484530 | | #19

      Hello Malcolm,
      You don't answer the questions per say, but you do bring up extra issues! Thank you, all comments and thoughts welcome.

      - About B, agreed, it's implicit that the issue is condensation. Could you explain what you mean by "the moisture shouldn't condense because it is at the same temperature as the interior". If enough moisture accumulates due to vapor drive wouldn't the Vapor Pressure reach saturation VP even if the interior is not that cool?

      - The issue seems to get a little complicated with Isobrace, foam or independant polystyrene on the outside of the studs. On one hand, as you point out, it stops vapor diffusion to the outside, presenting a good case for a VB on the inside, but on the other, having a VB on the inside and a low perm exterior on the outside leaves nowhere for vapor to diffuse. That being said, this is what happens in theory and we don't know what the drying rate vs the interior rate is. Or do we?

      - Old time builders def got it wrong. Complete agreement there.

      - I think evidence would be hard to come by and substantiate - no one might recognize it for what it might be, and at this point, everyone would look for a hole in the assembly and would be right to do so. Are you suggesting that despite the hundreds of articles on this site, and from the Building Science Corp and so on, you would still recommend a VB behind the gypse?

      - Claddding matters, but your saying interior vapor drive is mostly dependant on the perm of the WB/AB? Makes sense.

      1. Expert Member
        MALCOLM TAYLOR | | #22

        - Canadian codes require that the vapour-barrier be as close to the interior as necessary to prevent condensation at design conditions. The implicit assumption is that what causes interior moisture to condense is reaching its dew p0int. A VB located behind the drywall will never become a condensing surface, while one located 3/4 of the way through the wall has a good chance of doing so - although both are subject to the same vapour-drive.

        - We can get a good idea what the interior wetting rate is vs the exterior drying rate simply by knowing the permeability of the materials. And the apparent success of walls with what should be inadequate foam, seems to show that as long as the ratio of the two allows greater drying than wetting, the walls work.

        - I'm not saying that poly is preferable to other vapour retarders, but the evidence in a Canadian climate seems to be that it isn't the source of moisture problems when they occur. The present advantage of using poly is that it is A) Understood and accepted as an easily inspected combined air and vapour barrier. B) Integrates with the normal sequence of construction. So in the absence of an obvious problem, and until some other approach displaces it and gains widespread acceptance in the construction industry here, I have no problem using poly.

        As a general comment: I'm not at all sure having to constantly think about these things on a theoretic level makes sense. It's useful for us here on GBA and among those driving innovation, but as a general approach I think we should find a few building assemblies for each climate that work over time, are efficient, and easy to implement. Each house shouldn't need a reassessment of the building science behind its design. That's essentially where we are here in BC. The houses simply follow the code. In the absence of construction error, the buildings don't have moisture problems, are quite efficient, and perform well over time, without the designer or builder needing to re-visit or invent new assemblies.

        1. user-7484530 | | #23

          - Yes but with enough hot days, you could very well find yourself with enough vapor pressure to hit saturation irrespective of the temperature.

          - Poly costs a little bit of labor and it might actually cause issues. The entire point of about 50 GBA articles is to rectify its inappropriate use. I would consider it important to come to a conclusion concerning its use instead of ignoring it and turning a blind eye to the damage it could cause or the wasted labor, however minor, it demands.

          - I'm trying to figure out if poly is relevant or not, not necessarily reinvent the wheel. The fact that we can't seem to come up with good answers as to something so relatively simple, speaks volumes about the quality of construction in North America, the average quality of thought in building science, and the likelihood of major wall assembly issues as yet unknown. In other words, I strongly disagree with your last comment. Many builders scoffed at the idea of putting up pressure equalized rainscreen walls but half a decade ago. To them it was too complicated, too strange, unproven, and not worth their time. Many of their houses rotted as a consequence. Theory and thought, is actually required. Doubly so when you consider that empirical evidence is hard to come by : issues don't exist in isolation, it's difficult to replicate site conditions, and often impossible to provide the required timescales. You probably can't prove that poly is a bad idea in Florida by looking at rotting walls alone, you need building science to fill in the blanks.

          The notion that people should leave well enough alone when it comes to building construction, is what provides the industry the inertia to continue on as it has for the last 100 years. It makes for incredibly poor construction that future generations of homeowners will have to pay for. The quality of residential construction in both BC and Ontario is cringeworthy to anyone who has lived outside of North America. People have barely begun to think of wall assemblies.

          1. Expert Member
            MALCOLM TAYLOR | | #24

            "with enough hot days, you could very well find yourself with enough vapour pressure to hit saturation irrespective of the temperature."
            - But do we? There are a number of building scientists monitoring walls in various Canadian cities, and we also have the chance to look at what we find when we renovate. I haven't seen any evidence that wrong-sided VBs are causing a problem. In the absence of that, what's the worry?

            " Poly costs a little bit of labor and it might actually cause issues. The entire point of about 50 GBA articles is to rectify its inappropriate use"

            - Compared to other methods of air-sealing and providing the code required VB, poly is cheap. I can't find any admonition in GBA against it's use in cold climates, only that there may be better methods of achieving the same result. The more recent conclusions of Building Science Corp seem to indicate that vapour retarders are more important in cold weather homes than previously thought - especially those with exterior foam.

            I sure didn't want to advocate for building as usual. What I don't think is useful is needing a full analysis of every house as though it was a case study. GBA is full of poster's building sections - each proposing a unique assembly, each with their own potential drawbacks. In the PNW where I build we now have a good idea what works here and what is problematic.

            And I do think we are in a very good place right now. If you look at the BC code and the building science behind it catalogued in the appendices, you can see it it being driven by very good building science. To me the way forward is to continue to have science-based innovation incorporated into standards that create widespread improvement, not put the onus on individual builders to come up with assemblies that may or may not work. That the discussion we are having - between two architects - is so fraught with complex unresolved questions, either speaks to us both being a bit dim (I hope not!), or it being the wrong approach to designing building assemblies. You and I aren't in any position to do the theoretical work or testing necessary to know what works.

  17. Jon_R | | #17

    It's senseless to have a discussion using terms as vague as "vapor barrier" - and even Class I/II/III isn't sufficient. But start with existing data (there is lots of it).

    Vapor diffusion is driven by vapor pressure (not relative humidity). But given a temperature and a relative humidity, one can calculate vapor pressure.

    What's best is highly dependent on many details - for example, the amount of wetting from different sources (which is highly variable).

    IMO, the best generic answer is to use the most vapor open assembly that follows the recommendations here. With sufficient exterior mineral wool , follow here.

    1. Expert Member
      MALCOLM TAYLOR | | #18

      Jon, I think you are right, but in the context of Canadian construction and practice, what we are really discussing is the presence of poly.

  18. user-7484530 | | #20

    Hello Jon,

    Yes, poly is implied here. Where could I find the data relevant to the questions above?

    Is it not also driven by temp or is temp generally less important than vapor pressure? In any case, just seems counter intuitive for it not be RH. Imperfect analogy, but electric charge is defined not by the amount of electrons, but the amount electrons relative to protons.

    Yeah this is a set of assemblies I've referred to in the past, I usually use membrain in Ottawa. Still I just want to put this debate to rest and as such, I'm looking for as much of an in depth answer as possible.

    Thanks for the comments!

    1. Jon_R | | #21

      Temp matters in that cold wet sheathing is far less likely to mold than warm wet sheathing. There are also many materials where their perms vary with relative humidity.

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