Water-Resistive Barriers: When and Why Does Vapor Permeance Matter?

Dr. John Straube is a highly regarded building science expert, well known for his research, his consulting work, and his leadership as an educator. He is a Principal at RDH Building Science and a faculty member at the University of Waterloo. This past year, Dr. Straube discussed key building enclosure concepts (including moisture control, airtightness, and energy efficiency) as part of the DELTA® Academy Seminar Series. We caught up with him to ask some follow-up questions on those topics—here, we get his take on WRBs and vapor permeance.

Q: Let’s start with a basic question: what is a WRB?

Well, depending who you ask, it’s a “water-resistive barrier” or a “weather-resistive barrier.” But if you ask me “weather-resistive” is just plain wrong[1] and misleading. Sometimes people think of weather-resistive barriers as resisting both water and air penetration. But there’s a fair bit of confusion out there. To be clear, we should call them water and air resistive, if they fulfill both functions.

Partly the problem is that the term WRB is used to refer to materials—but different WRB materials can serve different functions, depending on how they’re detailed, like a housewrap (which can control both air and water) or asphalt felt (which only controls water). The important thing is to know what function or functions each material in your assembly is serving, and detail it accordingly.

This is why it’s so important for architects and designers to understand and clearly indicate function. If you look at a building covered in a polymeric membrane, you really can’t tell whether it’s detailed correctly until you know the design intent—is it meant to control water, vapor, air, or some combination of the three? Designers need to decide and communicate which layers are intended to control what.

[1] Essentially no WRBs resist the weather: that is the job of the entire building enclosure, not just one layer and one product.

Q: Do WRBs typically control all three?

They can, but they don’t have to. That’s the thing. We say “WRB” but it doesn’t tell us much. In some situations, a WRB material is acting only as the water control layer, and the label “water-resistive barrier” makes some good sense. Yet the same material, properly detailed at joints and transitions with compatible tapes and so on, can also serve to control airflow, and there is no indication of that important role in the name.

Q: Okay, so if you are using a WRB product to control rainwater penetration, should it also control vapor? What if you’re also using it as an air barrier? Is there one ideal vapor permeance?

From a building science perspective, liquid water and water vapor are two different things. When we talk about vapor control, we’re talking about diffusion, and that’s what vapor permeance addresses. You can have a high vapor permeance, i.e., allowing vapor to pass through at a relatively high rate, and still shed liquid water effectively: many products like felt and specialty polymers do that. Air leaks can carry vapor with them—but again, that’s a different issue from vapor diffusion. You stop vapor carried in air by controlling airtightness, not vapor permeance.

So the short answer is no. There is not one vapor permeance that’s ideal for an air or water barrier. It really depends on your enclosure design—where are you installing this in the assembly, relative to insulation? Where are you installing the assembly in the world? Are we in New Orleans? Are we in Maine? Are we in Edmonton? And what is the inside of the building like? Is it a high-humidity environment or a drier environment? Is it a hot environment or a cold environment?

The designer needs to think through these choices when picking a vapor permeance. However, it can be said that if I have a highly vapor permeable air and water barrier, I can relatively safely put it almost anywhere in the enclosure, because it doesn’t stop water vapor flow. And then I could add another layer for vapor tightness somewhere else if I needed it. Traditionally, in very cold climates with high interior humidities, we would put a vapor barrier near the inside. By having a vapor permeable air and water barrier, I have the option now to place it anywhere downstream in that wall system where it can work best to both stop airflow and to effectively shed rainwater

Now sometimes we want to use a vapor barrier/air-and-water barrier. There are certain wall designs where we’re putting all the insulation on the outside, we have very high interior humidities, very cold exteriors, and we design assemblies where a vapor barrier is needed at the same location as the air-and-water barrier. In that situation, you buy a product that does all three. But having a vapor permeable air-and-water barrier actually gives you a lot of freedom, because you can put it right on the outside just behind the cladding or you can put it three-quarters of the way in towards the inside of the insulation and that won’t affect the performance because it doesn’t affect the paper permeance of the whole system or clog it up.

For a quick summary about the benefits of using a highly vapor permeable air barrier, watch the video below:

Caption: A water-and-air control layer that is vapour permeable can safely be placed in multiple locations within a wall assembly. © RDH Building Science

Q: So there’s no cut-off point? For example, is there a concern about a vapor permeance above 10 perms?

So, materials over 10 perms are considered vapor permeable. That means you wouldn’t use them in situations where you need a vapor barrier.

Q: But can you use them where you need an air-and-water barrier?

Well, because there’s no magic vapor permeance for an air-and-water barrier, clearly it’s not bad above 10 and good below 10 or vice versa. It does depend on the wall assembly, but one of the benefits of a very high-permeance membrane is that it plays no role positive or negative in vapor flow through that assembly, so you get to put your air-and-water barrier wherever, without having to worry about “does it do something bad to the vapor flow?”

Now that said, there are times where we do want to have a lower vapor permeance, and there are certain cladding systems like adhered masonry veneers that drive water vapor in, so we want to intercept that vapor with vapor-resistant layers. And those types of systems mean that, well, you need to do that – you need to have a vapor-resistant system. But that doesn’t mean that an air-and-water barrier with a vapor permeance of 20 perms is bad. Oftentimes having a vapor permeance of 12, 20, even 30 would be very beneficial to encourage drying out if you get accidental moisture in the system.

For the video summary of the points discussed above, check out the video below where Dr. John Straube discusses ideal vapor permeance for air barriers:

So there’s nothing magic about 10, other than it being a round number…it’s just a number and most wall systems would either benefit from, say, 20 perms plus, or if you need to control the vapor then one perm or less, and if you have seven or nine it’s probably not that useful.

Q: Okay, thanks for clarifying. Earlier you mentioned that airtightness can also help prevent vapor condensation problems. Can you say more about that? What does airtightness have to do with vapor?

Sure. As I mentioned, vapor can travel through an assembly by diffusion or can be carried through via airflow. Vapor permeance has to do with diffusion, which occurs when water vapor is pushed through a material due to vapor pressure (a measure of air moisture content) differences. Some materials allow this movement more readily than others; materials with a perm rating of 10 or higher are classified as vapor permeable. A semi-permeable material (greater than 1.0 perms but less than 10) will slow diffusion down. A vapor impermeable material (0.1 perm or less) will essentially stop it. Polyethylene sheeting and aluminum foil are good examples of vapor impermeable material.

But even if you stopped all diffusion, you could still end up with condensation. That’s because water vapor doesn’t just diffuse. It can and will—happily!—travel through your assembly carried by air leaks. This is called convective vapor movement, and under some conditions it will carry quite a lot of vapor.

Actually, air leaks are one reason why more vapor permeable membranes make sense in some assemblies. If you do get an air leak and get condensation accumulating, the ability to diffuse vapor out of the assembly can help it to dry.

Q: Thanks for sharing your insights today, John. Let’s close with a fun question. We’re talking in the AIA headquarters in Washington DC. Can you speculate about how a building like this would be built today? What considerations would there be as far as vapor permeance and air and water control?

How would we likely build a building like this in Washington, D.C. today? We would probably use steel stud framing with exterior gypsum sheathing. And we’d put some insulation in the stud cavity, and we’d put some insulation outside, so the air-and-water barrier would go kind of in the middle, in that split wall, partway through the total insulation value. And that way we can have a firm support for that air-and-water barrier, make sure it covers all the structural components, and still provide some protection of that air-and water barrier from wind and rain and so on by having that exterior insulation layer.

Let’s say the AIA was being very future-thinking and they decided to build the walls of the new headquarters out of wood studs, because they wanted a carbon-neutral building. Now it becomes even more obvious that we’re going to put insulation between the studs, a board sheathing outside the studs, cover that board sheathing with an air-and-water barrier that’s vapor permeable and fully adhered, put a big chunk of insulation outside that to make sure that the energy efficiency is high, and then add whatever cladding they like on the outside and a gypsum board finish on the inside.

That’s not the only way they could build it, but it’s a good way! Like other good solutions, it considers the different control layers required for the enclosure, and arranges them so that they work together to provide efficiency and durability.

Dr. John Straube discusses the role of an air barrier in modern building design using the AIA Headquarters as an example: