Rigid Foam Insulation: What Is It And What Are Its Uses?

stack of rigid foam board insulation

As one alternative to the more common blanket or roll batt insulation, rigid foam insulation adds layers of thermal resistance to your home’s exterior walls, roof and foundation. Rather than loose, soft mats of fiberglass particles, rigid foam board insulation consists of sheets of polyurethane, polystyrene or polyiso foam that is closed cell as opposed to open cell. Closed cell foam board insulation is strong and possesses a higher R-value than open cell insulation. R-values indicate the level of thermal resistance of an insulating material. Open cell insulations, like soft batt insulation, possess of lower R-value than rigid foam board. Rigid foam also better resists water and air leaks. Due to its density, it is more expensive and usually requires more material per square foot.

What Rigid Foam Insulation Is

This type of insulation is closed cell, meaning it is solid and produced in varying degrees of hardness. It can be molded to fit any shape. In homes, this is advantageous as it can be form fit to go between studs, or anywhere else. Rigid foam insulation is typically made from three different types of plastic with slightly different R-values. They are known as MEPS, XEPS and polyiso.

The first, MEPS, stands for molded expanded polystyrene. It has an R-value of 3.8 to 4.4 per inch of board thickness. The second type, XEPS, stands for extruded expanded polystyrene. It is formed in a similar way, but it is more resistant than MEPS and has a higher R-value on average.

The other types, polyurethane and polyiso rigid foam insulation have higher R-values of between 5.6 and 8 per thickness-inch. Thinner boards will often provide as much insulation as thicker pieces of other materials.

How Rigid Foam Insulation is Used

Rigid foam insulation is used as an alternative to open cell insulation, and it used for many types of insulation, from the foundation to the roof. While MEPS is not as resistant to water damage, XEPS, polyurethane and polyiso rigid foam make effective barriers against moisture, although sometimes a facing made of aluminum foil may have be needed. Because of their density, XEPS, polyurethane and polyiso rigid foam board make good roofing and exterior wall insulation in structures framed in the traditional manner.

ICFs and SIPs

ICFs are Insulating Concrete Forms. In this design, insulation is built into the walls. This gives the insulation an extremely high R-value and incredible thermal resistance to heat loss. SIPs are Structurally Insulated Panels and are an alternative to stud-built frames. Rather than construct walls, ceilings, roofs and floors with individual components, SIPs are pre-built sections. They are isolated in production and joined in the process of putting the home or building together. Like ICFs, insulation is installed during their prefabrication. For both of these alternatives to framing MEPS is often used as the rigid foam insulation, although XEPS, polyiso and polyurethane is also used in their construction.

Proper installation is very important when using rigid foam insulation. The rigid foam insulation must be equipped to prevent seepage of moisture, burrowing insects and fire damage. Even with retrofits, rigid foam insulation is a stronger alternative to traditional open cell batt insulation. Although the cost per square foot will be higher, it is suitable for insulating your entire home.