A man in full protective gear is sitting on the attic floor while he sprays spray foam insulation on the underside of the attic ceiling.

Are you building a new home or planning to upgrade and re-insulate an existing one? 

Home insulation is essential for comfort and energy efficiency. High-quality and durable insulation can reduce your energy bills, minimize your carbon footprint, and keep your home cozy and comfortable throughout the year. 

Even if you don’t know much about the topic, you’ve probably heard of foam insulation. It is a type of modern insulating material made from low-density polymer or elastomer that has been solidified in a lightweight cellular mass.  

The two main types of foam insulation are spray foam and rigid foam board insulation. While both are made from the same basic materials, they have certain significant differences. 

What Is the Difference Between Rigid Foam Board and Spray Foam Insulation?

Spray foam insulation typically has a higher R-value and can be applied in inaccessible and hard-to-reach places. In contrast, rigid foam board insulation has a comparatively lower R-value and can be cut to fit a wide variety of spaces. Additionally, rigid foam is less expensive than spray foam.

In recent years, foam insulation has gained popularity among homeowners, architects, and contractors alike.

So, let’s learn a bit more about the different types of foam insulation—how they work, where they can be installed, and how effectively they insulate your home.  

Foam Insulation: An Overview

Foam is a relatively modern insulating material made from various chemical compounds, including polyurethane, polyisocyanurate, polystyrene, and polyol resin.

Scientists chose these compounds due to their shock absorption, thermal insulation ability, and fire-resistance properties. They also are extremely durable and have relatively low density. 

Foam insulation provides an effective thermal barrier between your living space and the outside world. It can block convective, radiant, and conductive heat transfer, meaning foam insulation has a very high R-value (also known as thermal resistance to heat flow). 

Foam insulation is incredibly lightweight, and depending on the type of foam insulation you’re using, the material may also provide acoustic insulation, air sealing, moisture resistance, and other valuable properties.

Spray foam insulation was first used by the US military in the 1940s to insulate airplanes during World War II. Rigid foam board insulation, on the other hand, wasn’t widely used until the 1960s to insulate cars. 

However, it wasn’t until the 1970s that foam insulation first gained popularity as a viable option for insulating residential properties. 

The two main types of foam insulation currently used in residential buildings are rigid foam panels and spray-on foam.

Spray Foam Insulation

Spray foam is a chemical compound that may contain certain carcinogenic ingredients. Therefore, it should only be installed by professional insulation technicians for optimal safety. 

It is made from a type of polymer known as polyurethane. Spray foam insulation is formed when an isocyanate compound reacts with a polyol to create a solid substance. 

The ratio of polyol to isocyanate will determine the relative flexibility or rigidity of the spray foam. Depending on this, the material will be classified as either open-cell or closed-cell spray foam.

Open and Closed Cell Spray Foam

Open-cell spray foam is lighter and more pliable, while the closed-cell version is denser and more rigid. As a result, closed-cell spray foam is more durable and more expensive than its open-cell counterpart. 

Expansion

Spray foam insulation is installed with a spraying machine, whereby two components (such as a liquid resin and a catalyst) are mixed together during application. 

Spray foam insulation is applied in a home

Mixing causes the resin to expand rapidly, covering much more area than it would have in its original, liquid form. This expansion ensures that the insulating material gets into all the nooks and crannies and effectively seals off most of the holes, gaps, and leaks in the area.

Due to its ability to expand upon application, spray foam insulation is used to seal and insulate the most inaccessible and hard-to-reach parts of the house. 

Air Sealing

The relative flexibility of open-cell spray foam makes it perfect for insulating the spaces around light fixtures, windows, electrical outlets, and ceiling cavities. Spray foam is also used to protect open cavities, attics, rim joists, and crawl spaces. 

Open-cell spray foam is shown in use on a ceiling

Both types of spray foam insulation are durable and resilient. They are not prone to sagging or settling over time. Additionally, they can effectively seal the building envelope to prevent air leakage, thus reducing your energy bills by keeping the conditioned air inside.

Spray foam insulation also keeps the unconditioned air outside from entering your living space. 

Rigid Foam Board Insulation 

Usually made from polystyrene, rigid foam board insulation comes in the form of pre-made panels that can be cut into different shapes and sizes, depending on the homeowner’s needs. 

Vertical stacks of rigid foam board insulation panels

Polystyrene is a lightweight, petroleum-based plastic containing styrene monomer, which traps 95 percent of the air that passes through it. This trapped air is what gives rigid foam boards their insulating capabilities.

Components and Properties

These factory-produced, flat panels or sheets of foam are a good option for insulating larger spaces, although they can’t usually be used to insulate inaccessible and hard-to-reach areas. Rigid foam boards usually come in about 2–4 feet wide and 8–12 feet long panels. 

Rigid foam board insulation is installed on top of a layer of OSB

The two most common types of rigid foam insulation are expanded and extruded foam boards. While both contain polystyrene, they are manufactured in slightly different ways, which results in two distinct products with varying performance and properties. 

Rigid foam board insulation is frequently used in new construction projects rather than retrofit jobs involving existing homes and can also be used to insulate unfinished foundational walls. 

Rigid foam is relatively cheap, and the installation process is immaculate and easy. However, it can only be installed on a wide and exposed part of the house, such as ceilings and walls. Crawl spaces and cavities are not typically insulated with rigid foam boards. 

Thickness and Rigidity

Rigid foam board insulation comes in a range of thicknesses to meet varying R-value requirements. These panels can be easily cut and installed in a weekend-long DIY project without safety hazards or other concerns.

One reason to install rigid foam board insulation in your home is that it can enhance the strength of your walls due to its rigid structure and how it is installed. This will help increase the structural integrity of your home, while also improving the comfort level of your interiors. 

Installation Issues

However, rigid foam boards do not expand upon application. Therefore, they are ineffective at sealing gaps and holes in the building envelope. Furthermore, suppose you are re-insulating an existing house using rigid foam boards. In which case, you will have to first remove the drywall and all existing insulation before the foam panels can be installed. 

Comparing Rigid Foam Board with Spray Foam Insulation 

To make it easier for you to see the differences between spray foam and rigid foam board insulation, we will now compare the two materials regarding their features, costs, and application. 

R-Values

Both spray foam and rigid foam board are highly efficient insulation materials. They have high R-values and can keep your home cool in the summers and warm during winter.

However, spray foam usually has a higher R-value than rigid foam board, at similar density levels. 

For instance, a rigid foam insulation panel will usually have an R-value of 3.8–5 per inch of thickness. Conversely, spray foam insulation typically boasts R-values between 5.5–7 per inch of thickness.

Closed-cell spray foam offers the highest R-values of any insulation material currently available on the market. This rating is partly because it is a very dense material and partially because it essentially forms a series of cells containing trapped insulating gas, helping enhance the R-value by as much as 70 percent compared to rigid foam board insulation.

Application

Rigid foam boards come in the form of rectilinear panels, which can be cut and fit into pre-existing wall assemblies or those currently under construction. 

As a result, foam board insulation cannot be readily adapted to fit irregularities in the building envelope, such as arches, domes, curved walls, and other non-rectilinear shapes. It is also hard to in areas surrounding an electrical outlet or a lighting fixture.

On the other hand, spray foam insulation can be directly sprayed onto any surface that needs to be insulated. The quantity of spray foam applied can also vary from one surface to another, depending on the requirement. 

Therefore, spray foam is the most practical choice in buildings where continuity of insulation is essential. It can conform directly to any surface, regardless of any irregularities in its shape or design, ensuring a continuous and uninterrupted layer of insulation over the underlying construction.

The performance of spray foam remains the same, regardless of the shape of the building. In contrast, rigid foam boards would leave gaps in a curved or irregular wall assembly, leading to air leakage.

Insulation Types

Spray foam insulation and rigid foam board insulation can be further subdivided into four distinct types of material. 

Spray foam can be categorized into open-cell and closed-cell insulation. Rigid foam board insulation can be categorized into expanded foam and extruded foam insulation. 

Open-cell insulation is light and flexible, while closed-cell insulation is dense, impenetrable, and highly durable.

Expanded foam is a rigid foam board insulation frequently used as a non-structural exterior insulating sheathing material. It comes in a wide range of densities and R-values and is installed on roofs, walls, and floors. Expanded foam is manufactured using heat and pressure to expand spherical beads in a mold and fuse them. 

Extruded foam is the other variety of rigid foam board insulation. It comprises a homogenous, closed-cell cross-section produced by a manufacturing process involving continuous extrusion of the component materials. This process makes extruded foam more moisture and humidity-resistant and less likely to settle when exposed to water.  

Air Leakage

If your home has any curved walls or irregularly shaped fixtures, you will need spray foam to achieve truly continuous insulation and air sealing. Spray foam can get into the house’s most inaccessible nooks and crannies that rigid foam board panels couldn’t insulate. 

Moreover, unlike rigid foam board insulation, spray foam expands after application. As it expands, spray foam gets into all the gaps and cracks in its vicinity until the air sealing is nearly perfect. 

By contrast, rigid foam boards can be installed side by side on a wide, exposed wall assembly. If your house is perfectly rectilinear in design, this will not be a problem. But the rigid, rectangular panels are not well-suited for insulating buildings with irregular or curving architectural features. 

Rigid foam boards do not expand upon application, so if even a single gap or hole remains unsealed by the insulating panel, it can lead to uncomfortable draughts and inflated energy bills. 

Installation Process

The installation process of spray foam can be quite different from that of rigid foam board because foam boards can be purchased in precut panels in a wide range of standard sizes.

These panels can be cut into any size or shape with a serrated knife. Furthermore, foam board does not cause a mess or release any harmful emissions during the installation process. It can be handled safely with minimal training and installed at any temperature or during any season. 

The process of installing spray foam is typically messy and time-consuming. Over-spraying often causes layers of foam to be deposited in inconvenient places, from where it must be removed after the installation is over. 

Furthermore, installers must wear masks, respirators, and other protective gear to minimize the risk of inhaling spray foam. If you’re not careful, it can also cling to your skin and cause rashes, irritation, and other problems. 

Spray foam is also quite temperature-sensitive. If the weather outside is frigid, this can cause foaming, curing, and other problems during installation. Even during warm weather, the part of the house being insulated must be isolated for at least two hours. 

Concluding Note

As you can see, rigid foam insulation offers cost-effectiveness and ease of installation, while spray foam can provide exceptionally high R-value and near-perfect air sealing. To figure out which of these foam insulation options is suitable for your home, you should consult an experienced insulation technician or contractor in your area.

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