What is Extruded Polystyrene Insulation?

Extruded polystyrene (XPS) insulation is a closed-cell foam insulation product. It is stronger and more rigid than expanded polystyrene (EPS) insulation. The nominal R-value of XPS is R-5.0 per inch compared to the approximately R-3.6 per inch value of EPS.

Extruded Polystyrene Insulation

How Extruded Polystyrene is Made

All polystyrene rigid foam insulation is 95% – 98% air. The solid parts are derived from benzene and ethylene–which are derivatives of oil. Polystyrene is colorless and transparent, but manufacturers add color to it.

Extruded polystyrene starts life as a molten material that is forced through a die that forms the panels into the desired size and cools each sheet. The result is rigid foam board insulation.

XPS is available in blue, gray, pink, and green –depending on which company manufactures the product. All XPS rigid insulation panels are more expensive than EPS rigid insulation–when comparing equal thicknesses.

Is Extruded Polystyrene a Good Insulator?

XPS is an excellent insulator with a higher R-value per inch than fiberglass because it traps more air. Extruded polystyrene is also a better insulator than porous types of insulation because it eliminates convection. The mixture of innumerable gas pockets with solid material in rigid foam boards reduces thermal conductivity and convection. It also seals gaps, cracks, and holes from leakage.

R-value is a measurement of the thermal resistance of a product. Higher R-value numbers mean more thermal resistance and less heat loss, resulting in better insulation. Gasses–such as air–are poor heat conductors compared to liquids or solids. It’s the air in insulation products that act as the insulator. The same is true for blanket insulations like fiberglass batts.

Extruded Polystyrene Uses

Extruded polystyrene is often called Styrofoam. Many people use the term Styrofoam to describe an array of foam products, from coffee cups to packaging materials to insulation. Much of the product termed Styrofoam is expanded polystyrene.

Styrofoam is a trademarked name for XPS manufactured by Dupont. Dupont recently changed its familiar blue insulation to a gray color to meet low GWP (Global Warming Potential regulations in Canada and some US states.)

Manufacturers produce tons of extruded polystyrene every year for insulating purposes.

Installing Extruded Polystyrene Insulation

Extruded polystyrene does not absorb water or hold moisture, making it an excellent below-grade insulation. XPS also resists insects and rodents and doesn’t provide a growing medium for mold and mildew.

Using XPS for Basement Wall Insulation

To use XPS as a basement wall insulation, glue it to exterior basement walls with a compatible adhesive before backfilling. Any adhesive that contains petroleum-based solvents will dissolve XPS.

Two-inch thick XPS insulation is a standard choice for exterior basement walls. For extra moisture protection, spray the foam with foundation waterproofing. Take caution when backfilling to prevent damage to the XPS.

Insulating the interior of basement walls with 2” of XPS adds R-10.0 and saves space when compared to 2 x 4 stud-framed frost walls. Extruded polystyrene rigid insulation acts as a vapor barrier if it is at least 2” thick. You can seal all joints and holes with closed-cell spray foam insulation or caulking–preferably acoustic caulking.

How to Use Extruded Polystyrene for Above-Grade Insulation

Extruded polystyrene rigid foam insulation is one of the most popular products to provide a heat retention blanket for above-grade walls. One-inch to two-inch thicknesses are the most popular and sensible choices.

You should strap any foam insulation over 2” to hold siding and stucco in place. Some exterior finishing, like cement board siding, is heavy and designed to be attached directly to solid framing.

XPS provides a blanket over the entire building envelope. It solves the thermal bridging problem at each stud. Many products–like Styrofoam SM–have ship-lapped edges, which eliminate gaps. You can spray foam, caulk, or tape the joints to eliminate all gaps and drafts. Tightly sealed XPS may create a double vapor barrier trapping moisture inside the wall.

Using Extruded Polystyrene for Floor Insulation

Contractors extensively use extruded polystyrene under concrete floors. It provides excellent R-value per inch and is available in multiple thicknesses. XPS is also much stronger than expanded polystyrene, making it less susceptible to damage from people walking on it or pouring concrete on it.

XPS prevents the conduction of interior heat into the earth beneath the floor. Poured concrete has a thermal resistance of R-0.08 per inch. Meaning that the average 4” concrete floor has an R-value of R-0.32.

XPS under concrete is even more important for heated floors. Heat rises, but the warmth in concrete is conducted into the earth below. The insulation keeps the heat inside the building–providing a more comfortable environment, saving energy and money.

Extruded Polystyrene Roof Insulation

Contractors use extruded polystyrene to complete the building envelope as roof insulation on flat roofs. Typical commercial roof construction involves steel decking on steel or wood roof joists. Hot air rises, and the steel roof deck has almost zero R-value making insulated roofs essential.

XPS rigid foam provides excellent insulation at R-5.0 per inch. It retains its shape and insulation value when walked on and doesn’t compress under the weight of tar and gravel build-up. Recommended R-values range from R-20.0 – R-41.0. Achieving R-41.0 with XPS rigid foam requires at least 8 inches of foam or the addition of interior insulation to reach the required R-values.