Quick answer: EPS foam has an R-value of roughly 3.6 to 4.0 per inch (about 0.033–0.038 W/m·K thermal conductivity). That performance, combined with low cost and light weight, makes EPS a standard choice for wall, roof and floor insulation.

What is R-value?

R-value measures thermal resistance — how well a material stops heat from flowing through it. A higher R-value means better insulation. In the US it is usually quoted per inch of thickness; in metric units the equivalent is thermal conductivity (lambda), where a lower number means better insulation.

EPS foam R-value

So a 100 mm EPS board delivers roughly R-14 to R-16. Thicker boards give proportionally more insulation.

What affects EPS R-value?

Density

Higher-density EPS generally insulates slightly better, though the gain is smaller than for strength. Packaging grades at 15 kg/m³ and construction grades at 25–30 kg/m³ serve different purposes.

Moisture

EPS absorbs very little water, so its R-value stays stable in damp conditions — an important advantage over materials that lose performance when wet.

Temperature

Like all foams, EPS has a slightly lower R-value at very low temperatures. Designers account for this in cold-climate specifications.

Graphite enhancement

Graphite-enhanced EPS incorporates carbon particles that reduce radiant heat transfer, raising the R-value by roughly 20% compared with standard EPS of the same density.

How EPS compares

XPS insulates somewhat better per inch, but EPS is usually cheaper per unit of R-value and has comparable moisture performance — see EPS vs XPS for exterior wall insulation.

Where EPS insulation is used

EPS boards and blocks insulate walls, roofs, floors and foundations. Moulded EPS forms the core of ICF blocks and provides lightweight fill. All of these depend on moulded EPS produced from EPS molds.

Frequently asked questions

What is the R-value of 1 inch of EPS?

About R-3.6 to R-4.0 per inch for standard EPS. Graphite-enhanced EPS can reach roughly 20% higher.

Is EPS or XPS better for insulation?

XPS has a higher R-value per inch, but EPS typically costs less per unit of R-value and performs comparably in moist conditions. The best choice depends on thickness constraints and budget.

Does EPS lose R-value when wet?

Very little. EPS is closed-cell and absorbs almost no water, so its thermal performance remains stable even in damp service.

How thick should EPS insulation be?

Required thickness depends on your climate zone and target R-value. Divide the target R-value by the per-inch value (about 3.8) to estimate the thickness needed in inches.

Related products and guides

• EPS Mold Manufacturer
• EPS Construction Molds
• EPS vs XPS Insulation Analysis
• EPS Raw Materials