A picture of a person in a white construction outfit installing rigid foam board insulation on a wood wall.

Rigid foam board insulation is one of a large number of types of insulation that could benefit your home. It has particular uses and is an efficient insulation solution for your home. 

Rigid foam board insulation has enormous benefits, including high R-values, continuous insulation wrapping your home, and moisture control. Other benefits include preventing thermal bridging, controlling moisture, preventing air leaks, and using on both sides of the wall or ceiling.

This article will discuss the different types of rigid foam board insulation and the benefits and challenges of this type of insulation. 

Types of Rigid Foam Board Insulation

There are three types of rigid foam board insulation. They are expanded polystyrene (EPS), extruded polystyrene (XPS), and Polyisocyanurate (Polyiso, ISO). Each has unique properties that are outlined below. 

Expanded Polystyrene (EPS)

EPS foam board insulation is a versatile option for homeowners. You can use it to insulate roofs, walls, floors, below grade, or as a structural GeoFoam. EPS is somewhat water-resistant and does not absorb or hold water, meaning it is approved to touch the ground and used below grade. 

A closeup on the edge of a piece of expanded polystyrene foam board insulation

EPS helps prevent insects and other pests and can act as a vapor barrier. Additionally, the R-value does not deteriorate with time. Therefore, its efficiency is the same on the first day of installation as its last day of use. 

Contractors typically use EPS to insulate concrete forms and structural insulated panels. It meets or exceeds the required building and energy codes across the United States. However, it is thermoplastic, which could deteriorate in very high temperatures. The R-value of EPS is about 4.6 per inch. 

Extruded Polystyrene (XPS)

Extruded polystyrene rigid foam board insulation is likely the type of foam board insulation you would recognize. It comes in three colors, blue, green, and pink. So if you have ever seen construction in process and noticed the exterior is pink, they are using XPS foam board insulation. 

Extruded polystyrene foam board is the most recognizable of the types of rigid foam board insulation and the most common for use in walls and below-grade locations. It has several different finishes, including various plastic facings or no finish. 

Several pieces of XPS foam board insulation are displayed on a table

XPS does not act as a complete vapor barrier because it does not fully absorb moisture. Therefore, the R-value can slowly decrease because it can absorb some moisture over time. However, this issue has a slow onset and will not be present for many years. 

One of the benefits of XPS rigid foam board is it is recyclable. Therefore, this is a more sustainable option than many other types of insulation. Additionally, it has a good R-value of 5 per inch. 

Polyisocyanurate (Polyiso, ISO)

The most common application for polyisocyanurate rigid foam board insulation is roofing because it has an R-value of 5.8 per inch, which is relatively high for home insulation.

However, it is relatively expensive compared to the other rigid foam board insulation types. For example, Polyiso averages $0.70 per square foot, whereas XPS costs around $0.42 per square foot and EPS comes in around $0.30 per square foot. 

Polyiso is not recyclable and is relatively permeable. As a result, it is not a vapor barrier. However, fiberglass or plastic sheathing options help create a vapor barrier for this type of rigid foam board insulation. 

Polyiso is an exciting rigid foam board insulation type because it starts in a liquid form. Then, manufacturers create the boards by solidifying the liquid foam into panels. 

Pros of Rigid Foam Board Insulation

There are several pros of foam board insulation that differentiate it from other standard insulation options.

High R-Value

Rigid foam board insulation has good R-values. The R-value for any type of insulation is the measure of its efficiency.

The higher the R-value, the more effective the insulation. Depending on the type of foam board used, the R-values can range from 3.6 to 8.0 per inch of thickness.

For reference, the average R-value for fiberglass insulation, one of the most common types of insulation, is 2.2 to 4.3 per inch of thickness. 

The high R-value means homeowners can use rigid foam board insulation in nearly any climate. 

A piece of XPS foam board insulation is applied to the exterior of a building

Prevents Thermal Bridging

Thermal bridging can happen where heat loss occurs in a home from heat moving through other, more conductive materials instead of the insulation.

Thermal bridging occurs most often when there is a break in interior insulation when it intersects other building materials like studs. Because contractors install rigid foam board insulation on the structure’s exterior, it inherently prevents thermal bridging from occurring. 

Controls Moisture

While not all three types of rigid foam board insulation can act as a vapor barrier, they all help control moisture. Not only do they protect the interior wood or framing from having contact with exterior rain or humidity, but they also keep the interior sheathing warm enough that moisture does not accumulate. 

EPS can act as a vapor barrier. XPS is not a complete vapor barrier, but professionals consider it vapor retardant. Moisture will only impact XPS insulation in the very long term. Polyiso insulation can be made into a vapor barrier by applying different types of sheathing. 

Prevents Air Leaks

Rigid foam insulation provides a continuous seal on the exterior of your home, helping prevent unwanted air leaks, which could cause inefficiencies.

In addition, this type of insulation can stop air from getting in and out of your home. Most seals only prevent one or the other, but rigid foam insulation does both. 

Effective Above and Below Grade

Not many types of insulation are effective above and below grade. Typically the insulation thrives in one environment or the other. But installers can use rigid foam board for everything from below-grade to the roofing and everything in between. 

Cons of Rigid Foam Board Insulation

Despite all the advantages, homeowners will find some challenges with foam board insulation. These need to be considered when making the final insulation decision for your home. 

Cost

The cost of rigid foam board insulation does tend to be higher than other popular forms. As a result, the upfront costs of foam board insulation may be higher, but in this case, the benefits tend to outweigh this drawback.

If you have the budget to pay for the insulation upfront, there is a good return on investment. Typically, the lower utility bills and moisture damage prevention will more than make up for the initial cost of the materials and installation. 

Proper Installation Required

Without proper installation, the benefits of rigid foam board insulation are minimal. Luckily, no special equipment is needed to install it, but it must be done correctly.

In addition, there are code restrictions on the installation and sealing of rigid foam board insulation. Without it, moisture and air can penetrate the insulation. 

Not Very Structurally Strong

Rigid foam board insulation provides some structural strength. Howbeit, it is not as strong as plywood or OSB sheathing.

Luckily, contractors can use rigid foam board and wood sheathing on top of each other to provide both insulation and structural stability. If you do not plan to have wood sheathing with the insulation, you will need to find another form of bracing to help improve the structural integrity of the insulation. 

Sensitive to UV Light

Rigid foam board insulation is sensitive to UV light, such as sunlight. Howbeit, this doesn’t present an issue after installation.

Therefore, it is critical to store it correctly before installation to prevent damage. Contractors will handle this for you, but it’s something to consider as a homeowner. 

Conclusion

Rigid foam board insulation provides excellent exterior insulation and is a terrific seal for your home. There are three different kinds, expanded polystyrene, extruded polystyrene, and polyisocyanurate. Contractors use each for distinct situations. 

There are several benefits and challenges to this type of insulation. First, it tends to be incredibly efficient while preventing moisture damage, air leakage, and thermal bridging.

On the other hand, it can be more expensive than other types of insulation and is not structurally strong on its own. But, overall, the benefits outweigh the challenges, and rigid foam board is an excellent choice for your home’s insulation projects. 

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