What Is A Vapor Barrier?

Who knew picking insulation was so complicated? You're trying to research your options and keep finding terms you need help understanding. The definitions that pop up are complex and scientific.

What's the difference between a vapor barrier and a vapor retarder? Do you even need one? Must you understand what a vapor barrier is to pick an insulation? Or can you trust that the contractors you work with know what they're doing?

South Central Services has installed spray foam insulation in hundreds of projects. In our service area of southern Pennsylvania, vapor barriers are relevant to the insulation discussion. We want you to know what vapor barriers are, how they function, and if they are relevant to your project.

By the end of this article, you will understand:

• What a vapor barrier is and is not
• The differences between air barriers, vapor barriers, and vapor retarders
• How vapor barriers work
• Whether or not you need a vapor barrier
• Products that can create a vapor barrier

What Is A Vapor Barrier, Anyway?

To understand what a vapor barrier is, we first have to define vapor. Vapor is water in its gaseous state. Air can only hold so much vapor. When the air becomes saturated with water vapor, the vapor condenses and returns to a liquid state. Warm air holds more vapor than cold air. So, as the air becomes colder, the likelihood that vapor will condense and become liquid increases.

This is exactly why you see dew on the grass on summer mornings. The hot summer air here in southern Pennsylvania will hold humidity, or vapor, and then overnight as temperatures drop this settles onto the ground. You can see why you wouldn’t want this to happen inside your walls!

The role of a vapor barrier is to stop the passage of vapor into a building through its envelope. Stopping vapor is complicated by climate, temperature, building envelope material, and the season. If a vapor barrier is installed incorrectly, it could trap water in the building assembly rather than preventing vapor from entering.

For example, let's say you have an exterior wall in the summertime. A vapor barrier installed on the outside of the wall would keep vapor from permeating the wall assembly.

But if the vapor barrier was installed on the inside of the wall, the vapor could permeate into the wall assembly. The vapor barrier prevents vapor from traveling through the wall and into the home. However, the wall assembly may be cooler than outside. Water would be trapped inside the wall if the vapor condensed due to temperature differences.

What’s the Difference Between An Air Barrier and A Vapor Barrier?

An air barrier stops air from permeating, while a vapor barrier stops vapor from permeating. While air contains some moisture as vapor, the role of an air barrier is to keep air from permeating. Air barriers are not complicated by temperature and climate in the same way that vapor barriers are.

What’s the Difference Between A Vapor Barrier and A Waterproofer?

Waterproofing products repel water in its liquid state. Waterproofers are usually installed on the outside of a building, such as the roof. Sometimes leaky basements also require waterproofing products to be installed.

Vapor barriers prevent water from permeating in its gaseous state. A vapor barrier is not a waterproofing product. While these barriers can stop water in its gaseous form, they cannot stop water in its liquid form.

What’s the Difference Between A Vapor Barrier and A Vapor Retarder?

You may see the terms vapor barrier and vapor retarder used interchangeably. These two terms are technically different but are related.

Vapor retarders are designed to prevent vapor from diffusing and are classified by how much vapor they allow to permeate. Vapor permeability is measured in perms.

Class I Vapor Retarders allow 0.1 perms or less. Class II Vapor Retarders allow between 0.1 and 1 perms. Class III Vapor Retarders allow between 1 and 10 perms. Anything greater than 10 perms is not considered a vapor retarder.

A vapor barrier is a Class I Vapor Retarder, meaning that vapor barriers allow 0.1 perms or fewer. The relationship between barriers and retarders can be confusing, so we have summarized this information in the following chart.

How Vapor Barriers Work

Vapor generally moves from warm to cold, just like air. Depending on temperature, climate, and building materials, a vapor barrier may be useful to keep water out and get out any water that finds its way inside.

Vapor barriers are most useful in climates with more extreme temperatures, like areas of Pennsylvania, Maryland, Virginia, and West Virginia. The most significant concern with vapor is the potential to condense and become liquid water. More extreme temperatures increase the chance of vapor condensing, as vapor travels from warm to cold and reaches saturation in colder air.

Vapor barriers can go on the inside or outside of a wall assembly. Where a vapor barrier is installed in a wall assembly depends on what the wall assembly is made of, what the vapor barrier is accomplishing, and what climate zone the building is in.

Do I Need A Vapor Barrier?

Homes located in Climate Zone 5 usually need vapor barriers in some assemblies. Our service area of southern Pennsylvania and counties in Maryland, Virginia, and West Virginia are in this climate zone. If you live in Climate Zone 5, you may benefit from a vapor barrier. If you’re wondering what climate zone you live in, check out this climate zone map from the U.S. Department of Energy.

What Products Create Vapor Barriers?

While we at South Central Services distinguish between the terms vapor barrier and vapor retarder, the construction world often uses the terms interchangeably. The following products can create some vapor retardation, but their exact classification will depend on their permeability.

• Closed cell spray foam
• Polyethylene sheeting
• Fluid-applied vapor barriers
• Peel-and-stick membranes

Closed Cell Spray Foam

Closed cell spray foam insulation can create a vapor barrier at a depth of 1.5 inches. Closed cell spray foam is also an insulation product with other benefits, such as an air barrier and an R-value of approximately R-7 per inch. South Central Services uses Carlisle Spray Foam Insulation’s closed cell product.

Polyethylene Sheeting

Polyethylene sheeting is a type of plastic film. The thickness of the sheeting will impact its permeability. Polyethylene sheeting can be used in construction as a house wrap or to cover insulation in wall cavities.

Fluid-Applied Vapor Barriers

Fluid-applied vapor barriers create a membrane through liquid application. Fluid-applied vapor barriers are usually installed on substrates like roofs, concrete, and sheathing. This liquid product from Prosoco is an example of a vapor barrier that meets the permeability expectations.

Peel-and-Stick Membranes

Like fluid-applied vapor barriers, peel-and-stick membranes form a membrane cover wherever installed. Unlike fluid-applied products, peel-and-stick products come manufactured in rolls. One of these products that qualifies as a vapor barrier by permeability is Blueskin Self-Adhering Membrane.

The Bottom Line About Vapor Barriers

Vapor barriers are one of the most complex topics in construction. Climate, temperature, and application determine the need for a vapor barrier. As a homeowner, you do not need to understand vapor barriers entirely. It is the duty of your insulation installer to determine the best options for your unique needs.

However, understanding the role of keeping water out and allowing moisture to escape can be vital to your project and the longevity of your home. Talk to your contractors about vapor barriers if you live in a climate zone with extreme temperature differences.

Want to learn something else important about insulation that isn’t so confusing? Check out this article about air barriers, a vital aspect of insulation that does not depend on variables like vapor.

Wondering what a vapor barrier would look like for your project? Download our guide to choosing spray foam insulation contractors and ensure your contractor is educated about the building codes relevant to your climate zone.