Can Open Cell Spray Foam Create A Vapor Barrier?

If you're planning a home improvement project with spray foam insulation, you may wonder if open cell can create a vapor barrier. Some areas of the home need vapor barriers, especially in Climate Zone 5. Is open cell spray foam the right product for your project?

South Central Services has insulated hundreds of homes with spray foam. We work with customers in southern Pennsylvania, Maryland, Virginia, and West Virginia. The climate of our service area necessitates a vapor barrier for many places in the home. We use open cell spray foam for certain applications when insulating a house.

By the end of this article, you will understand:

• What a vapor barrier is
• When vapor barriers are needed
• If open cell spray foam can create a vapor barrier
• Which type of spray foam to use if you need a vapor barrier

Don't have time to read right now? Check out everything you need to know at a glance.

What Is A Vapor Barrier?

Vapor is water in its gaseous state. Air can hold water vapor as humidity until the air reaches dew point. Dew point is when air becomes saturated with vapor and the water vapor condenses into liquid.

The temperature of the air influences dew point. Cold air holds less vapor than warm air. As the air becomes colder, it is more likely to condense.

A vapor barrier stops the passage of vapor into or out of a building through the building envelope. Correctly installing a vapor barrier depends on temperature, climate, season, and building envelope material.

If a vapor barrier is installed in an exterior wall, its job is to prevent vapor from condensing in that wall when temperature differences are more extreme.

Think of a harsh Pennsylvania snowfall in January. The outside temperature would be below freezing, while the inside temperature would be at least 65 degrees Fahrenheit.

Without a vapor barrier, any air passing through the exterior walls would have a potential of condensing. The warm air inside the home would hold more humidity than the cold outside air could manage.

When Are Vapor Barriers Necessary?

You may not live in southern Pennsylvania. So, how can you know when vapor barriers are needed in your home?

Regardless of climate zone, vapor barriers are necessary for below-grade applications. Below-grade refers to any area of the home that is below ground. Basements and crawl spaces are two prime examples.

Above-grade applications may benefit from vapor barriers, but this depends on the climate zone. Above-grade means areas of the home that are above the ground. In our service area, exterior walls often need vapor barriers. Pennsylvania has distinct seasons, with more extreme temperatures in summer and winter.

Can Open Cell Spray Foam Create A Vapor Barrier?

Open cell spray foam insulation is one of two types of spray foam. The other type is called closed cell. The varieties of spray foam get their names from their cellular structure.

Open cell spray foam has an open cell structure. If you’ve ever washed your car with a large sponge, you can visualize the cellular structure of open cell spray foam. Those sponges can be compressed in your hand. If you were to cut the sponge open, you would see irregular pockets of space within the sponge.

Because open cell spray foam has this open cell structure, it cannot create a vapor barrier. The larger and irregular pockets within the foam do not lend themselves to stopping vapor transfer.

It doesn’t matter how much open cell spray foam is installed, because the cellular structure does not change.

If You Need A Vapor Barrier, Choose Closed Cell Spray Foam.

Closed cell spray foam can create a vapor barrier at approximately 2 inches of depth. The cellular structure of closed cell is similar to a tri-fold foam presentation board. Unlike a sponge, which can compress and reform in your hand, a foam board does not compress easily. It would take a great deal of pressure to make a dent in the foam board.

If you cut the foam board in half and zoomed in, you would see a tighter cellular structure with more uniform sizes. This closed structure allows closed cell spray foam to create a vapor barrier.

A single inch of closed cell spray foam does not create a vapor barrier due to lack of depth. However, 2 inches is generally sufficient to make a vapor barrier with closed cell spray foam.

No matter where you live, below-grade applications are surrounded by the cold and humidity of the underground. Closed cell spray foam would be the product you need for a basement or crawl space.

Don’t Use Open Cell Spray Foam For Below-Grade Projects.

Open cell spray foam should never be installed in a crawl space or on basement walls. Both of these locations need vapor barriers, which is a feature that open cell spray foam cannot deliver.

Open cell spray foam could be installed in your basement ceiling. Basement ceilings are not technically considered below-grade, so open cell could be used in a basement ceiling application.

The Bottom Line About Open Cell Spray Foam And Vapor Barriers

Open cell spray foam insulation cannot create a vapor barrier. The cellular structure that defines open cell foam makes the technical properties of vapor barriers impossible. If your project needs a vapor barrier, then the spray foam product you need is closed cell spray foam.

Below-grade projects like basement walls and crawl spaces need vapor barriers. Other locations in the home that require vapor barriers will vary based on your location and climate. For homeowners in southern PA and the surrounding states, our more extreme weather patterns call for vapor barriers in the home's exterior walls.

Now that you understand why open cell cannot create a vapor barrier, your next step is to:

• Discover where closed cell spray foam is best installed
• Learn where open cell spray foam can be installed
• Find which type of spray foam is best for your project