What Is Insulation Board?

Insulation boards are rigid panels that can insulate any part of a house–from basements to roof peaks. Used on the interior and exterior, they provide more R-value per inch than other types of insulation.

Insulation boards have up to two times more thermal resistance than most other insulation materials of the same thickness. Spray foam has an equal R-value because it is manufactured from the same material.

Insulation Board - And Its Uses

Types of Foam Board Insulation

Many people refer to foam board insulation as Styrofoam. Styrofoam is a trademarked brand of Extruded Polystyrene Foam (XPS).

All foam boards are available in multiple thicknesses–beginning at ½”. There are three main types of foam board insulation: EPS, XPS, and ISO.

Expanded Polystyrene (EPS)

Expanded polystyrene foam boards are the most versatile and cost-effective foam board insulation. EPS has an R-value of approximately R-4.0 per inch, depending on the manufacturer. It’s insect and rodent resistant, doesn’t absorb or hold water, and is suitable for below-grade installation. Its R-value won’t deteriorate as it ages.

EPS acts as a vapor barrier if its joints are taped.  It meets or exceeds US building codes. 

Extruded Polystyrene (XPS)

Extruded polystyrene boards are the most recognizable type of foam. They are blue, pink, or green and are common for below-grade installation or installation on walls during retrofitting with siding. XPS is a closed-cell foam board that doesn’t fully absorb moisture, so it’s not a vapor barrier.

Water absorption can degrade the R-value of XPS, but this takes many years to become a problem. XPS has an R-value of R-5.0 per inch and is available in butt-joint or shiplap sheets. 

Polyisocyanurate (Polyiso, ISO)

Polyisocyanurate rigid foam has an R-value of around R-6.0 per inch. It often features a foil face on one or both sides to reflect radiant heat. ISO is a popular external roof insulation. Builders also use it inside attics and under siding to help reflect heat out of the building.

The foil facing of ISO is a better vapor barrier than 6 mil poly when the joints are taped with reflective foil tape. Installing ISO inside the house eliminates the need for an additional vapor barrier. Installing ISO on the exterior may trap moisture in the walls if there is an interior vapor barrier.

Polyisocyanurate is a warm-weather insulation product. When the temperature gets below about 60 degrees F, thermal performance begins to deteriorate. It’s not suitable for exterior use in locations where winter temperatures drop below freezing.

Polyiso is more expensive than other insulation boards. The average cost is around 50% more than XPS and over twice as much as EPS per square foot.

Best Uses of Insulation Board

Foam board insulation is the best choice for wet areas or areas that may get wet. These include the exterior foundation walls, interior basement walls, and on exterior walls. After exterior wall installation, builders cover it with house wrap and siding or stucco. Two-inch foam applied to the interior of basement walls acts as a vapor barrier if all joints, gaps, and holes are filled with spray foam or caulking.

Covering entire framed walls–inside or outside–reduces heat conduction through framing members such as wood or steel studs. You can also cut foam boards and install them between studs.  Afterward, you’ll need to seal the foam to the studs with spray foam or caulking.

Tips for Installing Insulation Board

Foam board insulation is light, easy to cut, and attaches to almost any surface. You can cut it with a circular saw, table saw, or score it with a utility knife and snap it like drywall. The thicker the material, the more difficult it is to cut straight with a utility knife.

You can attach foam board to wooden substrates with large-headed nails like roofing nails, screws with washers, or adhesive. Only use foam board adhesives though, some general adhesives will melt the board.

Foam board is especially effective on concrete basement walls since it’s not affected by moisture or humidity like fiberglass. Use adhesive to fasten it directly to the concrete and then frame a wall over it.