XPS vs EPS rigid foam?

XPS has higher R-value per inch (R-5 vs R-4) and better moisture resistance. EPS is cheaper and more environmentally friendly. XPS is preferred for below-grade.

Can I use XPS below grade?

Yes, XPS is the preferred rigid foam for below-grade foundation insulation due to its closed-cell moisture resistance.

Does XPS R-value change over time?

XPS loses some R-value as blowing agents diffuse out over 20+ years, settling from R-5 to approximately R-4.5 per inch.

XPS Rigid Foam BoardGenerating image…

Insulation

Rigid Board

$0.50-$1.00 per sq ft per inch of thickness

XPS Rigid Foam Board: Moisture-Resistant Insulation (2026)

Prepared by Lumberama's Building Materials Team

|Last updated May 15, 2026|Sources & methodology

Extruded polystyrene (XPS) rigid foam board is a closed-cell insulation identified by its blue, pink, or green color depending on manufacturer. It provides consistent R-5 per inch with excellent moisture resistance, making it ideal for below-grade and high-moisture applications. Its smooth surface and uniform density simplify cutting and installation.

Key Features

R-5 per inch; Excellent moisture resistance; Color-coded by manufacturer; Smooth cuttable surface; Below-grade rated

Specifications

R-value: R-5 per inch. Density: 1.5 lbs/cu ft. Compressive strength: 15-25 PSI. Water absorption: less than 0.3%. Sizes: 4x8 ft sheets in 1/2 to 3 inch thickness.

Applications

Foundation exterior insulation; Below-grade walls; Under-slab insulation; Exterior wall sheathing; Cold storage floors

Installation Overview

Cut with utility knife and straightedge. Attach to walls with construction adhesive and mechanical fasteners. Tape joints with compatible foam tape. Cover exposed foam with approved ignition barrier or cladding. Protect below-grade installations from backfill damage.

Pros & Cons

Pros

✓ Consistent R-value; Excellent moisture resistance; Easy to cut and install; Below-grade rated; Good compressive strength

Cons

✗ Higher cost than EPS; R-value decreases slightly over time; Must be protected from UV; Combustible without barrier; Not as green as EPS

Top Brands

Dow (blue)

Owens Corning (pink)

Kingspan

Code Requirements

Must meet ASTM C578. Ignition barrier or thermal barrier required per IRC R316 when exposed in occupied spaces. UV protection required for exterior exposure.

Buying Tips

Color indicates manufacturer not quality - blue (Dow), pink (Owens Corning), and green (Kingspan) are all comparable. Use XPS over EPS for any below-grade application.