Rigid Foam Insulation: XPS, EPS, and Polyiso Compared

Three pink, white, and foil-faced boards sit side by side at every lumberyard, and most homeowners grab whichever is cheapest. That is how basements end up with the wrong panel glued to the wall. Rigid foam insulation comes in three distinct chemistries — expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso) — and they behave very differently when it comes to moisture, cold weather, and long-term R-value. Pick by application, not by price tag, and the same $60 spent at the checkout can deliver 30 percent more real-world performance.

The Three Types at a Glance

All three products ship in 4×8 sheets from 1/2 inch to 4 inches thick, and all three resist compression far better than batts. Beyond that, the similarities end.

• EPS (expanded polystyrene): R-3.6 to R-4.2 per inch. White beadboard, the same material as a foam cooler. Cheapest of the three at roughly $0.30 to $0.45 per square foot per inch of thickness.
• XPS (extruded polystyrene): R-5 per inch as labeled. The pink (Owens Corning Foamular), blue (DuPont Styrofoam), or green (Kingspan GreenGuard) boards. Runs $0.45 to $0.70 per square foot per inch.
• Polyiso (polyisocyanurate): R-6 to R-6.5 per inch, foil- or fiberglass-faced. The highest R per inch you can buy in board form, at $0.55 to $0.80 per square foot per inch.

A 2-inch sheet of each tells the cost story clearly: EPS around $22 to $28 per 4×8 sheet, XPS around $35 to $45, and polyiso around $38 to $52 depending on facing and region.

R-Value Per Inch — and How It Drifts Over Time

Labeled R-value is a day-one number, and two of the three foams lose ground. XPS is blown with gases that slowly diffuse out of the cells over 15 to 30 years; its long-term thermal resistance settles closer to R-4.5 per inch, which is why manufacturers now publish “LTTR” figures. Polyiso holds its blowing agent better but has a different quirk: its R-value drops in cold weather. At a mean temperature of 25°F, polyiso can dip from R-6.5 to roughly R-5 per inch — a real issue on a Minnesota roof in January.

EPS is the boring, honest performer. Because its cells contain plain air, R-3.8 per inch on the label is R-3.8 per inch in year 20, and it actually performs slightly better in cold temperatures. When you price boards by delivered R-value over decades rather than sticker R, EPS frequently wins.

Moisture: The Deciding Factor Below Grade

Where the board lives should drive the choice, and moisture exposure is the sharpest dividing line. Polyiso is the one foam that should never touch wet ground. Its facers and cell structure can wick and hold water, and a saturated polyiso board loses a large share of its insulating value. Keep it above grade, full stop.

XPS has long been the default for foundation exteriors and under slabs because it absorbs very little water. High-density EPS (Type II, 1.5 lb per cubic foot or better) has closed the gap, however, and it drains and dries faster than XPS when it does get damp. Both carry code approval for below-grade use; EPS just needs the higher density grade rather than the flimsy 1-lb craft-store variety.

Where Each Board Belongs

EPS

Under concrete slabs, exterior foundation walls, insulated concrete forms, and exterior wall sheathing where budget matters. It is also the standard core in structural insulated panels. Use Type II or Type IX density anywhere it bears load.

XPS

Basement interior walls, frost-protected shallow foundations, and any application where boards get handled roughly — XPS shrugs off jobsite abuse that crumbles EPS edges. Its 15 to 25 psi compressive strength suits patio and slab work.

Polyiso

Above-deck commercial and residential roofing (it dominates that market), interior basement ceilings, and continuous exterior insulation in warm and mixed climates. Foil-faced polyiso doubles as a radiant barrier when installed facing a 3/4-inch air gap, and it is the easiest of the three to leave exposed in some jurisdictions because certain products pass fire testing without a thermal barrier — verify with your local inspector before skipping the drywall.

Vapor Permeance and Wall Assemblies

Foam thickness changes how a wall dries, and this is where DIY projects go sideways. Foil-faced polyiso is a true vapor barrier (under 0.1 perms). XPS at 1 inch runs about 1 perm — a class II retarder. Unfaced EPS is the most vapor-open at 2.5 to 5 perms per inch, which lets sheathing dry outward in mixed climates.

Two rules keep you out of trouble. First, in cold climates, exterior foam must be thick enough to keep the sheathing above the dew point — roughly R-7.5 minimum over a 2×6 wall in climate zone 6. Second, never sandwich framing between two vapor barriers; if you install foil-faced foam outside, skip the interior poly sheet. If you are comparing this approach against attic-floor strategies, our guide to attic insulation types and cost covers where loose fill still makes more sense.

Fire, Codes, and the Thermal Barrier Rule

All polystyrene and polyiso foams are combustible and give off dense smoke. Building code (IRC R316) requires a 15-minute thermal barrier — typically 1/2-inch drywall — between foam and living space. That includes basements and rim joists in finished areas. Crawl spaces and attics with limited access can sometimes use an ignition barrier instead, and a handful of products such as Dow Thermax are approved for exposed installation. Budget for the drywall layer when you price a basement job; it adds about $1.50 to $2.00 per square foot installed.

Environmental Footprint

If green metrics influence your choice, the rankings shifted in 2021. XPS was historically the worst offender because HFC-134a blowing agent carried a global warming potential over 1,400 times CO2. U.S. manufacturers have since reformulated — Owens Corning’s Foamular NGX and DuPont’s low-GWP Styrofoam cut that impact by roughly 90 percent — but older stock still circulates, so check for the NGX or “low GWP” branding. EPS and polyiso both use pentane, a low-GWP agent, and EPS remains the cleanest overall thanks to its low material density.

Installation Tips That Actually Matter

Cutting and sealing determine whether you get the labeled performance. A few field-tested habits:

• Score-and-snap works for 1-inch boards; for 2 inches and up, use a fine-tooth handsaw or a hot knife to avoid ragged, gappy edges.
• Foam every seam. Canned polyurethane (Great Stuff Gaps and Cracks, about $7) plus sheathing tape on the face turns individual boards into an air barrier.
• Use foam-compatible adhesive — Loctite PL300 or similar. Standard construction adhesive melts polystyrene.
• Stagger joints when stacking two layers; offset seams by at least 12 inches so no gap runs straight through.
• Below grade, protect exposed foam above soil with cement board or a parge coat; UV and string trimmers destroy bare foam in a season or two.

One more planning note: two thin layers beat one thick one. A pair of 1-inch boards with staggered, taped seams air-seals better than a single 2-inch sheet, costs about the same, and is far easier to carry up a ladder in wind. Also check the reclaimed market before paying retail — used commercial roofing polyiso sells through salvage yards and Craigslist at $10 to $20 per 2-inch 4×8 sheet, roughly half price, with only cosmetic facer damage. For basements, garage doors, and slab work where looks are irrelevant, reclaimed foam is the best per-R bargain in the entire insulation aisle.

Bottom Line: Match the Board to the Job

Choosing rigid foam insulation gets simple once you sort by exposure. Ground contact or slab work calls for XPS or Type II EPS. Roofs and above-grade walls in most climates favor polyiso for its unmatched R per inch, with the cold-weather derating caveat up north. Tight budget across a big area? EPS delivers the lowest cost per point of R-value and never loses performance with age. Whatever you pick, seal the seams — an air-leaky R-10 wall performs worse than an airtight R-7 one, and the spray can costs less than any of the boards.