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Glossary

Rigid Foam Insulation

Rigid Foam Insulation

This is a solid board made of plastic foam that builders use to keep your home warm in winter and cool in summer. You'll often see these large panels on basement walls or under siding to block heat from moving through the house structure. They resist water much better than fluffy fiberglass batts do.

Reviewed by Jill AshUpdated October 13, 20263 min read

Origin

The word rigid comes from the Latin word rigidus, meaning stiff or hard. Chemical companies invented these solid foam panels in the 1940s to provide lightweight insulation for airplanes before adapting them for houses.

How you'll see it used

  • Your contractor's estimate for the basement finishing project includes a line item for installing two-inch pink rigid foam insulation against the concrete foundation walls.
  • The siding installer told you they need to tape the seams of the rigid foam insulation to block wind before they nail up the new vinyl siding.
  • The home inspector noted that the rigid foam insulation in the crawl space is exposed and needs to be covered with a fire-rated material.

What It Is

Rigid foam insulation is a solid panel made of plastic foam. Builders use these boards to keep heat from moving in and out of your house. You'll usually see them sold in large sheets that measure 4 feet wide and 8 feet long. They come in different thicknesses ranging from half an inch up to several inches. Unlike fluffy pink fiberglass rolls that you can squish with your hands, these panels are completely stiff and hold their shape. They also do a very good job of blocking moisture. This makes them a top choice for wet areas like basements and crawl spaces where fiberglass would just soak up water and grow mold.

Why It Matters To You

A well-insulated home keeps your family comfortable and lowers your monthly energy bills. Rigid foam is special because it stops heat from traveling right through the solid wood framing of your house. Builders call this heat transfer thermal bridging. When wood studs touch both the inside drywall and the outside siding, heat leaks right through the wood. Wrapping the outside of your home in rigid foam stops this leak completely. This takes a lot of stress off your HVAC & Climate Control system. The boards also act as a strong barrier against wind and rain. If you want a dry and warm basement, this material is a huge help. It even helps block outside noise from entering your living space.

Types Of Rigid Foam

You'll see three main types of rigid foam at the local hardware store. They each have different colors, price points, and specific uses around the house.

  • EPS (Expanded Polystyrene): This is the cheapest option available. It looks exactly like the white foam used in cheap disposable coolers. It works well for basic projects but can absorb a little bit of water over a long period of time.
  • XPS (Extruded Polystyrene): You'll easily recognize this type as the smooth pink or blue boards in the lumber aisle. It stops water much better than EPS and offers more insulation power per inch of thickness.
  • Polyiso (Polyisocyanurate): This foam usually has a shiny foil face on both sides. It gives you the highest amount of insulation power but it also costs the most money. It's very popular for roofing projects and attic walls.
Always cover rigid foam insulation with drywall or another approved fire barrier if it is inside your living space. The plastic foam can catch fire easily and release dangerous, toxic smoke.

Where You Run Into It

You might spot rigid foam during a major home remodel or when you are looking closely at your Foundation & Structure. Contractors often glue XPS panels directly to bare concrete basement walls before putting up the interior wood framing. You'll also find it on the outside of your house. Builders frequently nail it to the outside of the wood studs right before they install new siding. Sometimes, roofers lay thick sheets of Polyiso on flat roofs to keep the top of the house warm. Plumbers even use it under concrete slabs when installing heated floors. If you're planning a big exterior upgrade, you can read more about wall layers in our guide to Exterior: Siding, Garage & Decks.

What It Costs

Pricing depends heavily on the type and thickness of the foam board you choose. Keep in mind that material and labor prices vary based on where you live and the season. For the bare boards alone, expect to pay between 15 and 40 dollars for a standard 4 foot by 8 foot sheet. The white EPS foam sits on the lower end of that price range. The foil-faced Polyiso foam sits at the high end. If you hire a professional contractor to insulate your basement walls with rigid foam, the total cost usually runs from 1,000 to 3,000 dollars. This final price includes the special foam tape to seal the seams, the construction glue, and the labor required to cut and fit the boards perfectly.

Frequently asked

Can I leave rigid foam insulation exposed in my basement?

No, building codes require you to cover rigid foam with a fire barrier like drywall if it is inside your house. The plastic foam catches fire very easily and creates toxic smoke. You must cover it to keep your family safe.

Which side of the foil-faced foam board faces out?

The shiny foil side should face an open air space to reflect heat properly. If you press the foil tightly against wood or concrete, it loses its ability to bounce heat away. Your contractor will usually point the foil toward the outside if you live in a hot climate.

Is rigid foam better than fiberglass batts?

Rigid foam is much better at stopping water and blocking air leaks than fluffy fiberglass. However, fiberglass is cheaper and much easier to stuff between wood studs. Many builders use both materials together to get the best results for the lowest price.

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