At a standard 75°F mean test temperature, a 1-inch rigid foam board measures R-4.0 for EPS, R-5.0 for XPS, and R-6.0 to R-6.5 for foil-faced polyiso [S2][S7]. A 2-inch board therefore reads R-8.0 (EPS), R-10.0 (XPS), and R-12.0 to R-13.0 (polyiso) on the same test basis [S2].
Those three numbers anchor nearly every wall and roof assembly calculation in North American commercial and residential work, but each material has a different aging and temperature behavior that the lab value alone does not show. Specifying a board on per-inch R alone, without reading the test conditions and the long-term drift, is the most common mistake on continuous-insulation details.
Per-Inch R-Value and Board Thickness
Rigid foam boards are sold in standardized thicknesses from 1/2 inch to 4 inches, and R-value scales linearly with thickness on the published charts. The 1/2-inch / 1-inch / 1.5-inch / 2-inch / 3-inch / 4-inch ladder resolves to R-2.0 to R-16.0 for EPS, R-2.5 to R-20.0 for XPS, and R-3.0 to R-26.0 for polyiso, with polyiso carrying a 0.5 spread per thickness because the foil-faced product typically lands at R-6.0 to R-6.5 per inch [S2].
Density adds a smaller secondary lever on EPS: products from roughly 1 to 3 pcf span 10-60 psi compressive strength, and a higher-density board gains a small R-value bump in addition to the structural gain [S1]. For most code-compliance work, thickness is the dominant variable; density is a tie-breaker when a substrate also has to carry a roof live load or vehicle traffic, as detailed in the insulation board reference.
Temperature Behavior: Where Polyiso Loses Its Lead
Polyiso's R-6.0 to R-6.5 per inch is a 75°F rating, and the foam's insulating gas mixture loses performance as the mean temperature through the layer drops. A 2-inch foil-faced polyiso board that reads R-12 to R-13 at 75°F can drop toward R-10 or lower in a cold-climate wall where the average temperature through the foam layer sits closer to 25-40°F, which is a 20-30% per-inch penalty versus the lab number [S3][S5].
EPS and XPS both use air (EPS) or a more stable blowing-agent mix (modern XPS) inside a closed cell, so their R-value stays nearly flat across the temperature range that building envelopes actually see. For a zone 6 or zone 7 above-grade wall, the "real winter" R per inch of polyiso often falls below XPS, even though the lab sticker says the opposite. Designers running the calculation against ASHRAE 90.1 or IRC tables should adjust the polyiso number down using the manufacturer's cold-weather curve before summing assembly R.
Long-Term Drift: Blowing-Agent Loss in XPS
Traditional XPS relies on a trapped blowing agent with lower thermal conductivity than air, so the day-one R-5.0 per inch is partly funded by that gas. As the blowing agent diffuses out and air diffuses in over 20-50 years, R-value drifts downward, and a board originally rated R-5.0 per inch can settle closer to R-4.3 over its service life [S4].
EPS contains about 98% air and 2% polystyrene with no high-conductivity blowing agent to lose, so its R-4.0 per inch is stable on day one and at year 50 [S1][S4]. Newer XPS formulations with alternative blowing agents have narrowed the gap, but the long-term R-value guarantee is the structural reason many spec writers choose EPS over XPS for above-grade walls where moisture exposure is controlled, even though XPS still wins below-grade where water resistance matters more than a 0.7 per-inch delta, as covered in the XPS board reference.
Moisture, Vapor, and Below-Grade Use
XPS absorbs less water than EPS because of its closed-cell extrusion process, which is why it remains the default for foundation perimeter, under-slab, and plaza-deck applications where the board sits in contact with damp soil. EPS has a more open cell structure and higher vapor permeability, so it is rarely used directly against masonry or soil without a separate damp-proofing layer. [S1]
Foil-faced polyiso is essentially a vapor barrier (permeance well under 1 perm), which can be an asset or a liability depending on wall assembly direction. In a cold climate with interior vapor control, foil-polyiso on the exterior sheathing can trap moisture; in a hot-humid climate with exterior vapor control, the same skin is exactly what the assembly needs. Mesh-faced polyiso opens vapor permeability to roughly 2 perms, which gives the designer a knob when the assembly ratio of exterior continuous R to cavity R needs balancing against code-required condensation analysis [S5].
Cost per R and Practical Selection
On raw board cost, EPS is usually the cheapest per square foot at 1-inch nominal, with XPS slightly higher and foil-polyiso varying widely by region. Translated to cost per R, polyiso frequently lands the lowest because so much R-value ships per inch; one 2026 vendor quote put cost per R at $2.26 for EPS, $2.53 for foil-polyiso, and $3.49 for XPS on 2-inch 4x8 boards in a zone 6 mountain climate, which is a real procurement signal even if it is one regional snapshot [S5].
The decision tree that holds up across most projects: pick polyiso when the assembly has the depth budget and the mean temperature through the foam stays above roughly 50°F, such as compact roofs and warm-climate walls; pick XPS when the board is in ground contact, under a slab, or in any detail where compressive strength above 25 psi and water resistance matter; pick EPS when the project is above-grade, budget-driven, and the designer wants a 50-year-stable R-value with the lowest first cost. A continuous-insulation layer behind cladding, in a lamps and light fittings penthouse equipment room, or over a metal deck on a construction machinery and equipment service bay is usually one of those three jobs, and the answer is rarely "all three at once."
Fire, Facing, and Code Considerations
Polyiso carries a Class A fire rating more readily than EPS or XPS because of its isocyanurate chemistry, and most foil-faced polyiso boards carry a UL-listed assembly for direct application to steel roof decks. EPS and XPS require a separate thermal or ignition barrier in most inhabited spaces per the IRC Section 316 and IBC Chapter 26, typically 1/2-inch gypsum or a listed intumescent coating. [S1]
Compressive strength is the other axis that does not show up on an R-value chart. Standard EPS Type I runs about 10 psi, XPS Type IV 25 psi, and high-density polyiso 20-25 psi; higher-density EPS (Type IX at 2.0 pcf) reaches 25-30 psi and is often used under parking decks or plaza pavers where both R and a vehicle load have to be carried by the same board. The EPS board reference and the insulation board reference lay out the ASTM C578 type numbers behind those ratings.
Where the Specs Break Down
Three failure modes show up repeatedly when these boards are mis-specified. First, polyiso at 2 inches or thinner in a cold climate, where the aged cold-weather R can drop the assembly below code R-value if the designer used the warm-temperature 75°F number on the cut sheet. Second, XPS used as the only exterior continuous insulation on a wall with no interior vapor control, where long-term diffusion of the blowing agent degrades R-value faster than the warranty period. Third, EPS in direct contact with soil or a damp foundation wall, where moisture absorption knocks down both R-value and structural performance within a few seasons, and the lighting equipment and electric lamps room adjacent to a wet foundation is a common place to see this. [S2]
The fix in all three is the same discipline: pick the board to match the service environment, adjust the published R per inch for the in-service mean temperature, and reserve the highest per-inch number (polyiso) for assemblies where the foam will actually sit near 75°F average, which in most North American projects means compact roofs and warm-climate walls, not cold-climate above-grade walls.
For 2026 procurement, the trackable signals to watch are the polyiso cold-weather correction curves published by JM, Rmax, and Atlas, the ASTM C578 type table that anchors the EPS and XPS compressive strength numbers, and the IBC Chapter 26 ignition-barrier language that drives whether EPS needs a separate thermal barrier on the wall it is being installed on.
This topic is covered further in Stud Weld vs. Drilled and Tapped Hole: Strength, Cycle Time, and When Each Wins.