If you’re building or renovating a home and aiming for it to be healthy, sustainable, and efficient, you’ve probably come across the terms "air barrier" and "vapor barrier" (or "vapor retarder").
Many people (homeowners, contractors and lots of others in the building industry) assume these terms mean the same thing—but air barriers and vapor retarders are not the same. In fact, they serve two different and essential purposes in your home’s building envelope.
Let’s get into exactly what these terms mean, how they differ, and why it’s important to understand the distinction when designing a home that performs well and stands the test of time.
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Common Misconception: The terms Air Barrier and Vapor Retarder Are Interchangeable
Let’s clear this up from the start: an air barrier is not a vapor retarder.
While it’s true that some materials can serve as both in certain circumstances, their roles are fundamentally different. Confusing the two can lead to costly mistakes in your home’s design—mistakes that may cause moisture damage, uncomfortable drafts, or energy inefficiency.
What Is an Air Barrier?
An air barrier prevents air from leaking into or out of your home. It improves your home’s energy efficiency by keeping conditioned air (heated or cooled) inside. And it keeps you comfortable since you won’t feel drafts.
Since air movement can carry moisture, an air barrier also helps stop humid air from passing between indoors and outdoors. BUT! It is not designed to stop moisture from transferring (moisture diffusion) from inside to outside or vice versa. It is designed to stop AIR. Moisture just happens to be in air in most climates, so that’s how it helps moisture control.
What Is a Vapor Retarder?
A vapor retarder slows the diffusion of water vapor through materials. Water vapor can pass through solid surfaces like drywall and insulation if there is a difference in humidity or temperature. A vapor retarder reduces the amount of moisture that travels through your walls, ceilings, and floors, preventing condensation buildup inside the walls.
Vapor retarders are typically seen in cold climates to keep the interior moisture from diffusing through the walls and condensing in the wall cavities in the winter. However, vapor retarders should be used very carefully because you don’t want to trap moisture inside your walls. We’ll get into that in just a bit.
Vapor Retarder vs Vapor Barrier
A vapor retarder is called that because it slows down (retards) the movement of moisture. It doesn’t completely stop it.
A vapor barrier completely stops the transfer of moisture. Materials like plastic and glass are vapor barriers. Moisture doesn’t diffuse through them. Typically the only place you want a vapor barrier is under your foundation floor slab. You don’t want any ground moisture to get into the house, so a plastic vapor barrier stops moisture from diffusing through the slab into your home.
15 mil. poly vapor barrier below foundation slab
Terminology Recap:
An air barrier stops air from moving between inside and outside.
A vapor retarder slows down the diffusion of moisture through materials that separate the inside from the outside.
A vapor barrier blocks moisture completely, allowing no vapor diffusion.
More on Air Barriers:
What can be pretty confusing about an air barrier is that it isn’t one specific ‘air barrier’ material that you install on your walls and roof. An air barrier is a system of components that keeps air from passing between inside and outside.
Also, you have options as to where the air barrier exists on your walls. It could be on the outside of your exterior walls (where your sheathing is), or the inside of your exterior walls (where your drywall is). You can also have it in both places for extra protection.
As for the materials that make up an air barrier, they can be anything that literally blocks air flow. A piece of exterior sheathing (like plywood) is a great air barrier. So is drywall. But when put together with the rest of the sheathing or drywall boards, you have a bunch of seams. And air will leak through those seams if they aren’t taped with an air tight tape.
So those seams need to be covered either with tape or with a weather resistive barrier (aka house wrap) that has all its seams taped. (Using the weather resistive barrier as your air barrier is typically the best way to go for an exterior air barrier.)
Zip Sheathing is a very effective way to get an air barrier on the outside of the walls. Zip is a sheathing and weather barrier combined, and once installed, all of the seams get taped. Once taped, the plane of sheathing is an effective air barrier.
Zip Sheathing - all seams are taped with black Zip tape
The tricky part is where that sheathing or drywall meets up with windows, doors, floors, foundation walls, and ceilings. All of these joints create pathways for air to slip through. And these joints can be hard to seal.
So an air barrier system typically uses the weather resistive barrier (as mentioned above) or drywall (or interior vapor retarder) of a house combined with air-sealing tapes and liquid sealant to form a complete air-tight system around the entire enclosure.
Remember, a teeny, tiny crack lets in an absurd amount of air:
An air barrier system needs to be installed very well to ensure your house is as air tight as it can be. You should always test the air tightness of a house with a blower door test after the air barrier is installed to make sure it’s tight enough.
To learn more about why you need an air tight home, click here.
Spray foam as an air barrier:
Spray foam is a common and not very earth-friendly insulation material. A lot of people like spray foam because it has high R-value (it insulates well), and it ‘works’ as an air barrier. When a house is fully insulated with spray foam, the foam seals all of the cracks and joints I mentioned above.
The problem with spray foam (on top of its health and environmental impacts) is that it becomes brittle when it cures. And as the studs, joists, and other components of a house expand and contract over time, the spray foam pulls away from the material it originally clung onto, leaving air gaps all over the house.
Don’t use spray foam as an air barrier.
Intello Plus Vapor Retarder Membrane
More on Vapor Retarders:
Historically, vapor retarders have been used in cold climates on the inside face of the exterior walls to stop moisture from traveling into the cold wall cavity and condensing on the studs and insulation*. You don’t want moisture condensing in your wall cavities because that’s how mold and rot happen.
*If you use the right amount of exterior insulation outside of the wall cavity for your climate zone, you won’t have to worry about moisture condensing inside of your walls.
Unlike air barriers, a vapor retarder is an actual product. Typically vapor retarders are plastic sheeting (this is really called a vapor barrier since moisture can’t pass through plastic), kraft-faced insulation, or a smart vapor retarder membrane. They go on the inside face of exterior walls and attics.
The building code requires vapor retarders to be used in most cases if you live in colder climates. If you live in a warm or hot climate, the code doesn’t require anything. But always check the building code before designing because building codes change!
Durable, high performance homes tend to stay away from using vapor retarders in the wall assembly.
Moisture pretty much always finds its way into wall cavities, whether it’s by a water leak or condensation, and if moisture is in wall cavities, it needs to get back out again. The safest way to make a durable house is to allow it to dry.
You don’t want to trap any moisture inside of walls. And if you have a low-permeability vapor retarder or vapor barrier on both sides of the exterior wall, you’ll trap moisture. This is why you need to be careful about what type of materials you have on both sides of your exterior walls. Ideally all materials in the wall assembly are vapor permeable (vapor open).
Should you use a vapor retarder on your walls?
If the building code doesn’t require you to use a vapor retarder, then you likely shouldn’t use one, but always work with a local expert to know exactly what you need.
If the building code does require you to use a vapor retarder because of your climate zone and your wall assembly, then I recommend using a smart vapor retarder like the Pro Clima Intello Plus membrane or the Siga Majrex. This type of membrane changes its permeability based on the surrounding humidity levels. So if it senses moisture, it will open up and allow the moisture to pass through.
Final Confusing Part:
Sometimes vapor retarders are used as part of the air barrier system.
For example, in the house I’m building now, we’re using a vapor retarder at the underside of the attic. (This is where you want your vapor retarder to be if you’re insulating the attic floor.)
In our case, the vapor retarder is also acting as the air barrier between the house and the attic. You can see that all the seams are taped to make sure air can’t slip through between the sheets of the membrane.
Intello at ceiling for air barrier and vapor retarder
Recap:
This is confusing, and there’s a lot that goes into figuring out where air barriers go and where (if anywhere) vapor retarders and vapor barriers go. It’s a very good idea to work with an expert that’s familiar with your local climate to help you determine what you need for your situation.
The most important take away is understanding what each one does:
Air Barriers:
Air barriers block air from traveling between the inside and the outside of your house. This is needed for comfort, energy efficiency, air quality, and moisture control (because air can bring in moisture when it’s humid outside).
A house cannot be too air tight (you’ll have mechanical ventilation bringing fresh, filtered air into your house.)
All seams, joints, cracks, connections etc. need to be completely sealed to have an effective air barrier system.
Vapor Retarders:
Vapor retarders slow down the diffusion of moisture through materials between indoors and outdoors. These are generally only used in cold climates to keep the humid, indoor winter air from migrating into the wall cavities.
You need to be very careful not to make your wall cavities vapor impermeable (vapor closed), otherwise any moisture that gets into the wall won't be able to get out, and you’ll have rot and mold issues.
Vapor Barriers:
Vapor Barriers are needed where the house meets the ground so that ground moisture doesn’t travel through the foundation floor and walls into your house.
Learn More:
In The Sustainable Home Blueprint, I break down the technical aspects of home design, like air barriers and vapor control, in simple steps so you can make confident decisions and work effectively with your builder—without feeling overwhelmed. Doors are opening soon! Learn more here.