Vapor Barrier vs Air Barrier
Am I right by saying,
Air barrier - prevents air leakage, so prevents air from going through, but we want it to be breathable - so it will allow vapor to pass through
Vapor barrier - prevent vapor and air from passing through
so say for a cold climate, we put vapor barrier on the inside of the insulation, to prevent air and vapor from going through the wall. However, if vapor does get through ( no barrier is 0 perm) it can still escape to the outside by going through the air barrier ( as it should be breathable and allows air to go through)?
Thanks !!!!
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This is very important, and you should remember:
Always use only one layer of moisture barrier, never use 2 or more layers, otherwise, moisture will be trapped between the barriers and create mold and other damages to the structure.
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)
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Where do I put the vapor barrier?
These enclosure questions are by far the most common questions I get from those in practice—and for good reason, because enclosure is the most nuanced area of building science, and among the most complex.
Your building skin will need to (in order of importance)
- Keep out rain: rain control layers
- Keep out outdoor air: air control layer
- Keep out cold/heat: thermal control layer
- Dry out when the assembly gets wet (and throttle the rate of vapor in-migration, but contrary to popular opinion among those in our field, providing for vapor out-migration is more important than preventing vapor in-migration): vapor control layer
Sometimes more than one of these functions may be handled by a single material in an assembly, and sometimes more than one material in an assembly is responsible for a single function.
Let’s start with rain. We’ll keep the rain out with three layers: with a mechanical layer, with a capillary break, and with a raintight layer. Let’s assume a wall with metal panels (but it could be stone, brick, fiber cement board, wood siding, or EFIS as our exterior finish material). Those panels, our first line of defense, will deflect, say, 99% of the rain like a shield; water simply rolls down the panels and to the ground. Then 98% of that small portion that sneaks through the panels, will drop down through an airspace (capillary break) and be redirected through flashing to weep holes that take the water back to the outside. Finally, the two-percent-of-one-percent that makes it past the panels and the airspace capillary break will encounter the raintight layer. In our example, we’ll use a spray-on fluid-applied product impermeable to air, water, and vapor (think of a truck bedliner, it looks like this), but we could use a rolled-on or troweled-on fluid-applied product, a self-adhered membrane product (this), lapped building felt/tar paper this, or lapped polymeric building wrap/Tyvek, this.
The lapped products don’t also serve as air or vapor control layers—just rain control layers—because air and vapor can sneak between the lapped layers; and in the case of the building wrap, vapor can also sneak through the Tyvek itself. Though vapor can pass through, rain, will run down the outside of the lapped layers.
For rain control, we can lap layers on a steep roof, but a low-sloped roof will require a fluid-applied surface or membrane to keep out rain. No three layers on a low-sloped roof . . .just the one, and it has to be perfect, which is much more difficult to deal with than the more-glamourous wall.
Underground, like the wall, the water control layer sits immediately outside the enclosure, between the rigid insulation and the underground enclosure, but unlike the above-ground condition, water only wants to move from the always-wet ground to the always-dryer basement. We have no use underground for loose lapped products or materials like Tyvek that allow vapor to pass through.
You should be able to put a highlighter down on the rain control layer and run that highlighter around the entire perimeter of the building, without ever picking up your pen, and without ever leaving the rain control layer.
Before we move on, we have to clear up something else. While water is water is water, moisture moves through an assembly in three different ways. The first, rain, is already familiar to you, but you likely have conflated the next two. The second is water moving as humidity, and humidity moves with air. Think of every air molecule as having a tiny backpack that may be filled with water. A surprisingly large amount of water can move through an assembly as humidity, piggy-backing with the air leaks, so an airtight assembly, which is important for thermal reasons, is also important for humidity infiltration and in-assembly condensation reasons. The third path for water is vapor migration, which is a different process than the moisture that moves through the air as humidity. Vapor migrates through the molecules of the solid. It is a much slower process than the humidity-with-air mode of water transfer, and it can happen even if the assembly is airtight. If I asked you to build a box of dimensional lumber, you could seal the joints airtight, and it would also be somewhat raintight, but if you spilled water on the top, after some time, there might be a water stain on the inside of the box as the water seeped through the wood. Likewise, if you left a puddle of water inside the box, it would slowly dry out and there might be a water stain on the bottom of the box. If however the box was lined on the inside with vinyl, you can imagine that the water might not dry out for a decade or more. The vinyl-lined box is impervious to rain, air, and vapor, and the wood-only box is impervious to rain (somewhat) and air, but not vapor.
Having dispensed with rain control, we’ll move onto air control. We want our outside fresh air to enter through the mechanical system and be distributed through existing A/C ductwork, not through the envelope: the lungs should function as the lungs and the skin should function as the skin (and not function as the lungs). Lots of common materials are acceptably airtight, including interior and exterior gypsum board, taped (not lapped) Tyvek, CMU, and glass. . . however it is not the panel we are concerned about in air leaks, it is the seam between panels. For this reason, lots of caulk, tape, fluid-applied sealant, and a keen sense of both adhesive chemistry and detailing/construction management is needed to execute a continuous air seal all the way around the building. Air leakage can be measured with a blower door test. The tighter the building, the lower the heating/air conditioning bill, and the lower the likelihood of condensation inside the assembly, because less air leakage begets less humidity. If you are using OSB as your building’s air control layer, be sure to look at the technical specs: many OSB panels are air-leaky through the middle, even if their seams are sealed. Again, you’ll want to run your highlighter around the plan and section at the air control, to ensure a continuous seal. It is not unusual for a poorly-sealed building to leak 20x the air, relative to a similar building that is well-sealed. Remember the rain control layer may also be the air control layer. . . or they may be separate.
The thermal control layer, which in our example sits immediately outboard of the fluid-applied rain/air/vapor control layer and inboard of the air space capillary break, will likely be a rigid or spray-on foam, though fibrous insulation like glass fiber and mineral wool have outdoor-rated versions available too. We want to avoid thermal bridges associated with overhangs, balconies, and other places where the structure of the building extends beyond the thermally controlled interior, but some thermal bridging is likely to happen anyway, and unlike the rain and air control layers, a little bit (but not much) of necessary compromise in thermal bridging is okay.
And finally, we come to vapor control. You may have been taught (and NCARB may still think) that the vapor control layer is a “vapor barrier,” designed to prevent vapor from migrating into a building’s assembly. . . but keeping vapor totally out isn’t really a worthwhile goal because much more moisture will enter the building through air leaks (air with backpacks of water) than will ever enter by way of vapor migration through a solid. Instead we’ll think about vapor control as a system to throttle the flow of vapor inward, but more importantly, to eliminate the possibility that moisture, once infiltrated into an assembly will stay there forever. That’s right: the vapor migration system is intended to let water out, not keep it out.
To do this effectively remember: never have two vapor-impermeable layers inside the same assembly. I took this photo a few weeks ago at a construction project around the corner from where I sit. The fluid-applied (black) product in the photo better be an air-control-layer only, and remain vapor-impermeable. . . otherwise we could trap moisture between two vapor-impermeable layers: the black fluid-applied one and the foil one. because the foil, sealed at the seams, is vapor impermeable.
Foil, vinyl, melamine, plastics like white board material or polyethylene sheet, taped rigid insulation, and certain fluid-applied “bedliner” products all are (practically) vapor-impermeable. Lapped products and Tyvek keep rain out, but allow for vapor migration.
Moisture will always get in, either as humidity, rain leaks, vapor migration, or as a byproduct of the construction process (curing concrete shedding water after the wall is sealed up, for example). Knowing that water will get in, limiting the assembly to no more than one vapor-impermeable layer allows water, once it does enter the assembly, to dry either to the inside or outside. It may even take two months to dry through the process of vapor migration (remember our wooden box?). . . but that’s okay so long as it doesn’t stay wet for more than ½ a year. In our example, the fluid-applied “truck bedliner,” the one that is impervious to rain, is also impervious to vapor migration. Condensation occurs on vapor-impervious layers on the warm humid face when the other side is cold. Because the vapor-impervious layer is inboard of the insulation in our example, humidity (from air leaks) won’t condense on the inside of that vapor-impervious layer because that layer is still warm in the winter—it’s inside of the insulation. In humid summers when there’s air conditioning on inside, there may be condensation on the outside of the fluid-applied layer, just behind the insulation, but that’s okay too, because that layer was made to be waterproof—to protect from rain incursion—so a little bit of condensation shouldn’t be a problem. It will evaporate into the air space capillary break later in the week, or drip down to the flashing and out the weep holes. The assembly I described looks like this. It properly accounts for rain, air, thermal, and vapor control!
This idea of the vapor control layer letting water out is not new, and is widely accepted among building scientists. You can read John Straube’s High Performance Building Enclosures (get it here, it’s not available on Amazon), peruse Building Science Corporation’s website and follow Building Science Fight Club on Instagram to learn more, because that’s a lot of content to summarize in one blogpost here (I did my best, but this will be hard to follow for some of you). Why doesn’t the ARE know about this? Why does NCARB continue to ask you where to locate the “vapor barrier” (but then realize that locating it on the warm side of the insulation is not that clear in a mixed climate, so they have to let you know that you are either in an obviously all-year warm climate like Puerto Rico or an obviously all-year cold climate like Alaska). That doesn’t really help you if you are in a mixed climate like most of us are, and it relies on 60-year old research that was junk research to begin with and only ever applied to old buildings that leaked lots of air in cold climates anyway. Why does NCARB ask the wrong question? It’s because of their original sin of relying solely on volunteer test question writers. Those volunteers, haven’t been exposed to the building science because they are not experts, and they know the old way (“put the vapor barrier on the warm side of the insulation”) because that’s the way the profession understands it to be. Why does the profession have the misunderstanding? Because that’s how they were tested when they took the tests in the past. Volunteer test item writers are adequate in many topics, but when deep technical exposure to complex processes are needed, as they are here, volunteerism often (though not always) falls short. —Michael Ermann, The Amber Book
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Hello All,
Can someone also give a good definition of "weather barrier"? I could not find one.
Is it a material that combines air-barrier and vapor barrier in one?
Also, I'm not sure what kind of "water-resistive membrane" does IBC 2015 talk about in 1404.2 (below)?
In wall sections we only show these 3 general types of "barriers": air barrier, vapor barrier and waterproofing/damp proofing with latter installed to portions in contact with walls. So I understand this is not waterproofing that 1404.2 talks about? Than what is it?
1404.2 Water-resistive barrier. Not fewer than one layer of
No.15 asphalt felt, complying with ASTM D226 for Type 1
felt or other approved materials, shall be attached to the studs
or sheathing, with flashing as described in Section 1405.4, in
such a manner as to provide a continuous water-resistive barrier
behind the exterior wall veneer. -
Is it a material that combines air-barrier and vapor barrier in one?
No, a weather barrier is just a general term that includes air-barrier and vapor barrier and any other barriers that keep the “weather” out. Air-barrier is one kind of weather barrier, and vapor barrier is another kind of weather barrier.
“Weather barriers keep outside weather out and conditioned interior air in. A weather barrier is part of the wall assembly; it prevents the passage of moisture, rain and wind through critical areas of the walls and roof, and protects vulnerable building components from deterioration.”
See link:
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)
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I think water-resistive membrane in IBC 1404.2 refers to the felt paper for walls and under the roof (roof underlayment). When you pass by a construction site, the black or yellow paper on the exterior walls (before plastering is applied) are building felt paper or moisture barrier or water-resistive membrane .
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)
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It is ALWAYS called out on wall sections as a moisture barrier. This is a very standard detail, and ARE exams will always test it.
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)
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They are different names for the same thing.
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)
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Great,
Than I will ask the last question I have on this topic:
In the same boof FBC there is a condition @ foundation wall
where a vapor barrier is placed on warm side of insulation of the wall (which makes sense) but on the cold side under slab against the earth (which doesn't make sense). So that insulation remains higher towards interior side. How so?
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There are for different purposes:
A vapor barrier is placed on warm side of insulation of the wall (which makes sense) to prevent condensation that can result in moisture that can create mold within the wall and damage the wall materials.
A vapor barrier is placed on the cold side under slab against the earth because its main purpose is to prevent moisture from the earth to get into the building. In this case, the risk of the moisture from the earth is much bigger.
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)
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