What You Need to Know About Foam Board Insulation

Updated: Feb. 22, 2024

Foam board insulation is a terrific alternative to fiberglass. Learn why, and where to use it.

foam board insulationPetrychenko Anton/Shutterstock

Foam board insulation is easy to use and a great DIY way to add some insulation. It comes in standard-size 4×8 sheets and smaller sizes, in thicknesses from 1/4-in. to two inches. Some types of thicker foam board insulation are available, but may need to be special ordered.

What is foam board insulation?

In short, it’s a great alternative to the common fiberglass batt insulation in most of our homes. It has a higher R-value per inch of thickness. It’s also resistant to moisture and easy to work with.

There are a few kinds of foam board insulation DIYers should know about:

  • Expanded polystyrene (EPS): This is the same stuff your disposable coffee cup is made of. The cheapest knd of foam board insulation, it’s used to protect fragile items in their packaging. It provides an R-value of 4 per inch of thickness. EPS is used under roofing, over foundation walls and beneath siding.
  • Extruded polystyrene: These rigid pink or blue panels can be found at home centers in many thicknesses. Denser than EPS, it provides more R-value, with a rating of 5 per inch of thickness. It’s available with straight or tongue-and-groove edges to limit air movement, and in all different size configurations to accommodate lots of applications. One common use of extruded polystyrene foam board: foundation insulation panels.
  • Polyiso (Polyisocyanurate): This is an even more dense and expensive kind. It provides an R-value of 6 per inch of thickness, and often comes foil-faced to reflect radiant heat. Polyiso comes in several thicknesses. It’s used over house sheathing underneath siding or in attic spaces where its reflective properties are really effective.

When should I use foam board insulation?

Because of its resistance to moisture, foam board insulation is a great choice whenever and wherever there’s a chance it could get wet. Examples: an exterior foundation, inside a basement against the foundation, and on the outside of a house beneath a house wrap.

Foam board insulation is more expensive than traditional batt insulation, so use it only when moisture is a factor and  it makes sense with your budget.

How do I install foam board insulation?

Foam board insulation is easy to cut with a circular saw and a masonry blade, or scored with a utility knife and snapped like you would drywall.

To attach the insulation, use screws with large washers or a special adhesive made for adhering foam board insulation. For smaller pieces, seal them in with caulk or spray foam.