So, does spray foam insulation need a vapor barrier?

Yes and no. The answer you end up with is determined by the type of spray foam insulation in your home. Hence, expect different outcomes from open and closed-cell spray foam insulation.

Are you curious about what a vapor barrier does? Do you prefer to spray foam insulation? Do you want to know whether spray foam insulation is permeable or not?

This article provides all you need to know about spray foam insulation, its vapor barrier functions, and the other vital duties spray foam performs when installed in houses.

What’s A Vapor Barrier? 

A vapor barrier, as the name implies, is a layer of material that prevents the transfer of water or water vapor from the surroundings of your home into it. It prevents the movement of water to safeguard buildings and home structures from water damage. 

Most people often find it difficult to differentiate between vapor barriers and retarders because they perform similar functions. You no longer have to be confused because you’ll be provided with the minor difference they possess as you read on.

While a vapor barrier is particularly for deterring moisture from entering the structure, a vapor retarder specifically reduces the rate at which water vapor moves through a structure. Usually, vapor barriers are often available as coatings or sheets. 

Hence, these sheets are thin and flexible materials in most cases because there are also thick sheet materials known as the structural vapor diffusion barrier. These materials keep getting updated regularly, with most of them even performing additional functions of other building materials.

Here are some examples of vapor barriers and the places you can use them in the home.

  • Aluminum foil
  • Polyethylene plastic sheet
  • Asphalt-coated kraft paper, which is added to a part of fiberglass batts
  • Exterior grade plywood
  • Elastomeric coatings function as a waterproofing and vapor barrier and can be installed on interior or exterior surfaces.
  • Metalized film
  • Paperback aluminum
  • Vapor barrier paints are specially used for an air-tight drywall system where walls and ceilings can’t be replaced or for dry basements.
  • Glass and metal sheets like in windows and doors
  • Hot-applying asphalt or coal tar pitch to concrete roof decks together with reinforcement felts.

Do You Need a Vapor Barrier For Spray Foam?

This question doesn’t have a “yes or no” answer because it depends on the spray foam insulation you use for your home. Usually, spray foam insulation in itself acts as a vapor barrier. Hence, it is not necessary to use a vapor barrier for closed-cell spray foam insulation because they also function as vapor and air barriers. 

However, the same can’t be said for open-cell spray foam insulation because it is a vapor retarder. While its moisture permeability properties can serve most structures sufficiently in some climates, it can’t be compared to the former.

High-quality closed-cell spray foam insulation is another vapor barrier that does not absorb moisture and helps deter leakage by blocking air gaps. If installed properly, this foam insulation is capable of sealing your home properly and allowing it to breathe at the same time. 

Therefore, there is no need to spend extra money installing an additional vapor barrier after purchasing your spray foam insulation. Typically around 1.5 inches thick, a closed cell spray foam insulation is the perfect choice for locations like crawl spaces or basements that are more damp or humid than others in the home. This is why most professionals recommend closed-cell spray foam for all continuous exterior insulation.

Does Open Cell Spray Foam Need A Vapor Barrier?

Yes, you need to install a vapor barrier to support your open cell spray foam. This is of utmost importance if you want it to last as long as it ought. 

When installed properly, open and closed cell spray foam insulation can fill the cavities where they are put. However, it is essential to note that only the closed cell spray foam can provide the additional benefit of a vapor barrier. Hence, closed-cell spray foam insulation is your go-to option if the application calls for an integrated vapor barrier and insulation in one set.

Open cell spray foam insulation is a vapor retarder that reduces the amount of moisture that comes into a structure rather than deterring it. While open cell spray foam is applicable to use anywhere in the home, it does not offer a vapor barrier the same way a closed cell spray foam does. 

Therefore, installing an additional vapor barrier whenever you settle for an open cell spray foam insulation is necessary to prevent condensation from forming.

Is Spray Foam Vapor Permeable?

Spray foam vapor can hardly be described as permeable because it prevents moisture and air from entering a building rather than passing through. The various types of spray foam you will find perform the duties of a vapor barrier, vapor retarder, and air barrier in addition to its thermal control functions.

For decades, spray foam insulation has been used in buildings in many parts of the world. One of the reasons for its popularity is that it provides an additional layer of security for buildings by acting as a protective barrier in addition to its initial thermal insulation capabilities. As a protective barrier, spray foam insulation helps to reduce air leaks, minimizes bulk water movement, and reduces the transfer of airborne moisture.

The known method of application of this insulation is spraying. Since there is no intrusion of joints or seams, a continuous layer in the roof, wall assemblies, and floor is created. It is also perfect for application in exterior spaces as continuous insulation designs rather than just between the studs of buildings. Therefore, spray foam insulation fulfills the building requirements for weather-resistant barriers and optimizes energy efficiency by limiting heat flow and air leakage. 

From a design perspective, spray foam insulation gives liberty to many architectural prospects, unlike other rigid board insulation products that are barely malleable. The application of spray foam insulation makes it easy to conform easily to buildings of any shape, form, and caliber.


Justin's been in construction for over 20 years in both new build and renovation. With experience in both commercial and residential construction, he specializes in healthier and more energy-efficient homes.

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