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What Is Spray Foam Insulation?

August 11th, 2023 | 5 min read

By Kilian Agha

You've heard of spray foam insulation but aren't sure exactly what it is. What is it used for? What is it made of? How exactly does it insulate?

South Central Services has insulated hundreds of projects with spray foam insulation. We answer this question weekly for curious homeowners living in and around Greencastle, PA.

This article will give you a foundational understanding of spray foam insulation. Specifically, by the end, you will know the following:

  • What spray foam insulation is made of
  • How spray foam insulates
  • The key differences between open cell and closed cell spray foam
  • Where spray foam can be used to insulate

What Is Spray Foam Insulation Made Of?

Spray foam insulation is polyurethane foam. Polyurethane is a type of plastic. Two liquid chemical components undergo a chemical reaction to create this plastic foam. These chemicals are commonly referred to as the A-Side and the B-Side. Their technical names are isocyanate and polyol blend.

To create spray foam, spray foam insulation contractors must acquire these liquid chemicals. A-Side and B-Side components are manufactured by many different companies. Manufacturers test the chemicals and the foam they make for about a year before making them available for purchase by professional contractors.

After the contractors acquire A-Side and B-Side, they must facilitate the chemical reaction to create the spray foam insulation. Each component is portioned and heated separately. The chemicals travel separately through a spray hose to a spray gun. When they exit the spray gun, the chemicals atomize. Atomize means that the chemicals become tiny droplets of a uniform size, which allows them to mix effectively.

The chemical reaction to create spray foam is complex and requires specific temperatures, pressures, and ratios of components. Safe and effective foam is sprayed in a 1:1 ratio following manufacturer guidelines. For the chemicals to transform into inert foam, the chemical reaction must be performed correctly.

Could I Install Spray Foam Insulation Myself?

Some companies offer do-it-yourself spray foam kits. As professional contractors, we cannot recommend installing spray foam yourself. The chemical components can pose health risks in their raw form, which is why contractors wear suits and respirators while spraying.

A spray foam installer at South Central Services installing closed cell spray foam in Chambersburg, PA while wearing a moon suit.

Beyond the health risk, the chemical reaction that creates spray foam is too complicated to replicate successfully using a DIY kit. While foam can technically be made from these kits, there is no guarantee that the foam is stable, on-ratio, or inert.

If spray foam insulation is installed incorrectly, it will not insulate properly. If the ratio of the foam is the source of the problem, the unstable foam could off-gas indefinitely, which is a health risk.

How Does Spray Foam Insulation Work?

Spray foam insulation offers two features as an insulation product:

  1. A high R-value
  2. An airtight seal

Let's examine them in more detail.

1. Spray Foam Insulates Against Conductive Heat Transfer.

Spray foam is an excellent insulating product because of its R-value or resistance value. R-value is the traditional measurement of how well materials resist thermal heat transfer.

Materials with a low R-value conduct heat well. An example would be metal. Insulation products must have higher R-values, indicating a heat transfer resistance.

2. Spray Foam Insulates Against Convective And Radiant Heat Transfer.

Spray polyurethane foam insulation also offers air sealing as a feature. Since spray foam is manufactured on-site through its chemical reaction, it can fill almost any cavity in a building envelope. As much as 1 inch of closed cell spray foam and 3.5 inches of open cell spray foam can create an air barrier.

What Makes Open Cell And Closed Cell Spray Foam Different?

There are two types of spray polyurethane foam: open cell and closed cell. If you are reading about spray polyurethane foam, you may read about either open or closed cell foam.

The difference between open and closed cell spray foam is their cell structure. Their different cell structures dictate their features and physical properties.

A comparison of the cell structure of open cell spray foam and closed cell spray foam.Open cell spray foam has an open cell structure. If you picture a yellow sponge used to wash cars, you can visualize the cell structure of open cell foam. There are large pockets of air, and the sponge can compress if you squeeze it.

Closed cell spray foam has a closed cell structure. Picturing foam boards, like tri-fold boards used for science fair presentations, can help explain this cell structure. The foam board is dense. If you pressed your hand against it, only a faint impression might be left behind.

The cellular structure changes a number of features and physical properties for open and closed cell spray foam, including:

  1. Foam density
  2. Structural support
  3. Vapor barrier potential
  4. Application options
  5. Price

Let's examine each of these in more detail.

1. Open Cell Is Less Dense Than Closed Cell Spray Foam.

Open cell spray foam is a low-density foam, while closed cell spray foam is a higher-density foam. Thinking back to our examples of sponges and foam boards, you can understand the density differences. A foam board is more dense than a sponge.

2. Only Closed Cell Spray Foam Can Offer Some Structural Support.

Closed cell spray foam can provide additional structural support to a building. While it will not hold up a building on its own, the density of closed cell foam can aid the structures already in place.

Open cell spray foam cannot provide any structural support. Think about reinforcing a barricade with sponges or foam boards. No matter the size of the sponge, it does not have the density required to offer any structural reinforcement.

3. Only Closed Cell Spray Foam Has Vapor Barrier Potential.

Closed cell spray foam becomes a vapor barrier at 1.5 inches. A vapor barrier helps to resist moisture content. The possibility of forming a vapor barrier is due to the dense cell structure.

With its open cell structure, open cell spray foam cannot create a vapor barrier no matter how much of it is sprayed. For open cell foam applications requiring a vapor barrier, additional coatings can be added on top of the foam.

4. Closed Cell Spray Foam Has More Versatile Application Options.

Closed cell spray foam is more versatile than open cell spray foam. The structural support and vapor barrier that closed cell foam offers makes it an ideal product below-grade. Below-grade applications are below the ground. Basements and crawl spaces are two examples.

Both closed cell and open cell spray foam can be installed above-grade, or above the ground.

Examples include exterior walls and attics. In southern Pennsylvania, spray foam insulation can be installed on a variety of substrates, including wood and masonry.

Sound attenuation is one application that open cell spray foam surpasses closed cell in. Sound attenuation is the process of sounds being muffled, which can be measured in volume and clarity. Rather than eliminating sounds like soundproofing does, sound attenuation reduces their impact. The open cell structure allows the spray foam to absorb sounds, while closed cell spray foam allows sounds to pass through.

5. Closed Cell Spray Foam Is More Expensive Than Open Cell Spray Foam.


Our rates for spray foam insulation are as follows:

  • Closed cell spray foam: $1.15 - $1.75 per board foot
  • Open cell spray foam: $0.25 - $0.75 per board foot

The price difference is partially due to the broader application options of closed cell foam. However, the main reason is related to density and product yield.

Have you ever made homemade whipped cream? You start with liquid heavy whipping cream. How much whipped cream you'll get depends on how much air you whip into the cream. A denser whipped cream has less air and less yield.

Both closed and open cell foam start with 55-gallon drums of A-Side and B-Side. However, more open cell foam can be sprayed than closed cell because of the difference in density. Open cell spray foam yields more finished product for its raw materials than closed cell.

What Is Spray Foam Insulation Used For?

You may wonder whether spray foam insulation could be used for your project. Depending on the type of foam, spray foam is used in these types of buildings.

  • New construction homes
  • Retrofit projects for existing homes
  • Remodeling projects for existing homes
  • Commercial buildings
  • Health care facilities
  • Pole buildings and barndominiums
  • AG buildings and barns
  • Timber frame constructions

As discussed earlier, closed cell spray foam can be used in below-grade applications like basement walls and crawl spaces.

Open cell spray foam is an option for sound attenuation between the floors of buildings.

Because of its air-sealing ability, spray foam insulation is a premium insulation option. Any building type listed above may use spray foam to create an airtight seal and prevent air leakage. Stopping air leakage improves indoor air quality, building comfort, and energy efficiency.

The Bottom Line About What Spray Foam Insulation Is

Spray polyurethane foam insulation is a plastic foam created from a chemical reaction between isocyanate and polyol resin. Spray foam is a popular insulation choice due to its insulating power and air-sealing ability.

There are two different types of spray foam insulation, and the best type of foam will depend on the application's needs. Between closed cell spray foam and open cell spray foam, there are no applications where spray foam could not be an option.

There is so much more to learn about spray foam insulation. Here are some resources to get you started.

Kilian Agha

Kilian has co-owned and operated South Central Services for 8 years. He is passionate about community involvement. In his spare time, he enjoys being with his family, playing ice hockey, and going fishing with friends.