Foam Board Insulation:
EPS vs XPS vs Polyiso,
R-Values & Costs

Foam board insulation is rigid sheets of plastic foam, usually 4 by 8 feet, in three families: expanded polystyrene (EPS), extruded polystyrene (XPS) and polyisocyanurate (polyiso). It is the standard way to put continuous insulation on foundation walls, under slabs and over exterior sheathing, because it gives a lot of R-value for its thickness and covers the studs that cavity insulation cannot. We make spray foam, and our applicators install it against foam board all the time; each has a job. This guide covers what separates the three boards, the ASTM grades that matter, real costs, code rules, and two calculators for your project.

Foam board insulation compared

Here are the properties that actually decide which board goes where. Click a heading to sort.

The polyiso cold-weather catch

Every rated R-value is measured at a 75°F mean temperature. Building Science Corporation testing reported by Green Building Advisor found polyiso that delivers about R-6 per inch in summer can fall to roughly R-4.5 per inch on a cold winter day, which is why the roofing industry designs cold-climate roofs at R-5 per inch. Polystyrene goes the other way: FOAMULAR 250 rises from R-5.0 at 75°F to R-5.4 at 40°F. In cold climates a common fix is a layer of EPS or XPS on the cold side with polyiso inside it, where it stays warm.

Thermal drift

XPS and polyiso are blown with gases that slowly diffuse out, so their R-value drops over the first years. Polyiso is rated by long-term thermal resistance (LTTR), a 15-year time-weighted average, and the design value for permeable-faced polyiso roof board was cut from R-6.0 to R-5.7 per inch in 2014. EPS is expanded with pentane that leaves quickly, so its labeled R-value is essentially its long-term value.

ASTM C578 grades and compressive strength

Polystyrene foam board is sold by type under ASTM C578. The type sets minimum density, R-value and compressive strength, which is what matters under a slab, a footing or a parking area. Ask for the type on the label, not just the color of the board.

Installing foam board insulation the right way

Foam board is forgiving to cut and hang, and that is exactly why it gets installed badly. The R-value on the label assumes no gaps. These are the steps we see skipped most often on basement walls, the most common DIY job.

1. Fix water first

   Foam on a wet wall hides the problem. Check grading, gutters and cracks before anything goes up; a damp wall needs drainage, not insulation.

2. Glue boards tight to the concrete

   Use a foam-compatible adhesive (solvents in some construction adhesives melt polystyrene) and keep boards tight so humid basement air cannot circulate behind them.

3. Tape or foam every seam

   Taped seams and canned-foam gaps at the top, bottom and corners turn the board into an air barrier. Two layers with staggered joints do this even better.

4. Treat the rim joist as its own job

   The rim is the leakiest spot in most basements. Cut-and-cobble board sealed with canned foam works but is slow; many contractors spray closed-cell foam there instead. See rim joist insulation.

5. Cover it with a thermal barrier

   Foam plastic inside a home must be separated from living space by an approved thermal barrier, typically 1/2 inch gypsum board, under IRC Section R316.4. Exposed foam board on a finished basement wall is a code violation and a fire hazard.

Exterior walls and roofs

On exterior continuous insulation, the board goes over the sheathing with a water-resistive barrier behind or on it, long fasteners or furring strips to hold the siding, and window and door openings detailed so water drains out. Thicker foam keeps the sheathing warmer, which matters for condensation in cold climates. DOE's recommendations for zone 4 and colder wood-frame walls already assume R-5 to R-15 of continuous insulation. On roofs, polyiso goes over the deck under the membrane; inside a cathedral ceiling, closed-cell foam is usually simpler (see cathedral ceiling insulation).

Foam board vs spray foam

We sell closed-cell spray foam, so here is the straight version. On large flat surfaces, a basement wall, a slab, the outside of a house, foam board is usually cheaper per R-value and a homeowner can install it. Spray foam wins on irregular surfaces: rim joists, stone or block foundations, crawl spaces with piers and pipes, and the underside of roof decks, where cutting and sealing board takes hours and still leaks at the edges. Plenty of good jobs use both, board on the walls and foam at the rim. Our basement wall insulation guide prices both methods side by side.

Foam board insulation: common questions

Sources

1. ASTM C578 EPS type properties (Quad-Lock technical library): quadlock.com
2. Owens Corning, FOAMULAR and FOAMULAR NGX 250 product data: buildsite.com
3. U.S. DOE Building America, Guide to Insulating Sheathing: eere.energy.gov
4. U.S. DOE Energy Saver, Types of Insulation and recommended R-values: energy.gov, energy.gov
5. Green Building Advisor, Cold-weather performance of polyisocyanurate: greenbuildingadvisor.com
6. Building Science Corporation, Info Sheet 502, temperature dependence of R-values: buildingscience.com
7. Roofing Contractor, New LTTR values for polyiso: roofingcontractor.com
8. Airfoam, Long-term R-values of polystyrene foam insulation: airfoam.com
9. International Code Council, 2021 International Residential Code, Chapter 3 (R316 foam plastic, R318.4 termite protection): codes.iccsafe.org
10. HomeGuide, Insulation cost (2026): homeguide.com
11. HomeAdvisor, Insulation cost by type (2026): homeadvisor.com