How does the R-value per inch of high-density EPS compare to XPS and closed-cell spray foam?

High-density expanded polystyrene delivers an R-value of roughly R-4.2 to R-4.5 per inch, trailing extruded polystyrene at R-5.0 per inch and closed-cell spray foam at R-6.0 to R-6.5 per inch.

Building assemblies across climate zones in the United States and Canada must meet strict thermal performance requirements under ASHRAE 90.1 and the 2024 International Energy Conservation Code (IECC). Designers and contractors face strict cavity depth limits when retrofitting concrete basements, framed exterior walls, or shipping containers, making thickness-to-performance ratios critical.

If you only do one thing: Select closed-cell spray foam or high-density EPS based on whether your primary constraint is maximum R-value in a tight cavity depth or long-term dimensional stability and stable R-value per dollar.

• High-density EPS performance: High-density expanded polystyrene (EPS Type II to Type IX) provides R-4.2 to R-4.5 per inch using trapped atmospheric air, meaning its thermal resistance never degrades over time or off-gases.
• XPS thermal drift: Extruded polystyrene (XPS) offers an initial rating of R-5.0 per inch, but blowing agents gradually escape through thermal drift, reducing long-term thermal resistance (LTTR) closer to high-density EPS over a 15- to 20-year span.
• Closed-cell spray foam density and yield: Closed-cell polyurethane spray foam delivers R-6.0 to R-6.5 per inch, providing the highest thermal resistance per inch of depth alongside an integrated vapor barrier at a 1.5-inch minimum thickness.
• Moisture absorption and drying potential: High-density EPS resists moisture retention and dries quickly if exposed, whereas XPS can retain absorbed water longer in below-grade assemblies, dropping its field thermal resistance.
• Cavity depth requirements: To achieve a continuous R-13 basement wall, you need 3.0 inches of high-density EPS, 2.6 inches of XPS, or 2.0 to 2.2 inches of closed-cell spray foam.

• Watch out for: Using XPS below grade without accounting for long-term gas loss, which diminishes the advertised R-5.0 per inch advantage over stable Type II EPS.
• Watch out for: Applying closed-cell spray foam without specialized personal protective equipment and strict temperature controls during installation, leading to curing failures and off-ratio shrinkage.
• Watch out for: Overpaying for high R-value-per-inch materials in wide stud cavities where high-density continuous panels achieve the code target at lower installed material costs.

Measure your available wall depth and target continuous R-value; if cavity depth is under 2.5 inches, use closed-cell spray foam, but if you have 2.5 inches or more, use continuous high-density EPS panels.