ASTM C578 Type IV XPS carries a 25 psi (172 kPa) minimum compressive strength at 10% deformation or yield, while Type VII XPS jumps to 60 psi (414 kPa) at the same test point, per the ASTM C578 density and strength ladder cited in DuPont's comparison chart [S4].
Both grades share the same nominal 5.0 R-value per inch at 75°F mean temperature when measured to ASTM C518, but Type VII's higher density and closed-cell structure make it the default for plaza decks, freezer floors, and high-load commercial roofs, whereas Type IV covers most general roof, wall, and below-grade applications [S2][S4].
ASTM C578 Type IV: the 25 psi workhorse grade
Type IV XPS board is recognised under ICC-ES ESR-4755 (DuPont de Nemours, reissued February 2026) and the older ESR-2142 (The Dow Chemical Company) as a non-structural thermal insulating material for wall, ceiling, floor, door-cavity, roof, foundation, attic, and crawlspace assemblies under the 2024/2021/2018/2015/2012/2009 IBC, IRC, and IECC, with applicability limited to Type V construction under the 2024/2021 IBC for non-DuPont-branded variants [S1][S5].
Type IV's potential heat is 2384 Btu/ft² per inch of thickness (27.1 MJ/m²) per ESR-4755, and ESR-2142 caps board thickness at 3 inches (76 mm) for the recognised STYROFOAM brand family, including DURAMATE Plus, Residential Sheathing, Cavitymate Ultra, Scoreboard, and Square Edge [S1][S5].
Type IV also carries dual recognition as a water-resistive barrier when installed per Section 4.4 of ESR-4755, satisfying CALGreen Section 5.407.1, the 2024/2021 IgCC Section 701.3.1.2, ASHRAE 189.1-2023 Section 7.3.1.2, and ICC 700-2020 Sections 602.1.8/11.602.1.8/1202.6/13.104.1.4 [S1].
For context on how Type IV compares with other rigid-foam grades, see the ASTM C578 standard reference for the full type-by-type compression ladder, and the broader insulation board classification for EPS and GPS equivalencies.
ASTM C578 Type VII: the 60 psi heavy-load grade
Type VII XPS board is recognised under ICC-ES Evaluation Report ESR-1061, covering FOAMULAR 400, 600, and 1000 products manufactured by Owens Corning, with compliance cited against the 2024 IBC family and a thickness envelope that allows heavier load-bearing applications such as protected-membrane roofs, plaza decks, and under-slab installations [S6][S10].</potential.
FOAMULAR 1000 sits at the top of the Type VII density band, marketed for high-compressive-load uses including industrial freezer floors, parking decks, and heavy-equipment traffic roofs, where 60 psi exceeds the 25 psi Type IV threshold by a factor of 2.4× at yield or 10% deformation [S4][S6].
The 60 psi figure is the same benchmark cited across the C578 Type VII row of the DuPont compression-strength comparison chart, which also lists Type X (15 psi), Type IV (25 psi), Type VI (40 psi), and Type V (100 psi) as the surrounding tiers in the compressive-strength ladder [S4].
Reviewers comparing the two grades against alternative envelope assemblies can cross-reference the EPS board class equivalent since EPS Type IX (1.8 pcf min.) tops out near 25 psi, never reaching Type VII performance.
ICC-ES report compliance map (ESR-2142, ESR-4755, ESR-1061)
ESR-4755 is the active DuPont report, reissued February 2026, covering the 2024, 2021, 2018, 2015, 2012, and 2009 IBC and IRC editions, with attributes verified for surface-burning characteristics, attic and crawlspace installations, air barrier, water-resistive barrier, thermal resistance, exterior walls in Types I-IV construction, and wind resistance (ANSI/FS 100) [S1].
ESR-2142 is the prior-generation Dow Chemical report reissued October 2012 (subject to renewal October 2014), with a narrower code edition scope (2012 and 2009 IBC/IRC/IECC), but it remains the historical reference cited when specifiers identify the original STYROFOAM family including Cavitymate Ultra, Ship Lap, Square Edge, Tongue and Groove, and BLUECOR [S5].
ESR-1962 (Atlas Molded Products, reissued July 2026) is the EPS-side counterpart covering ThermalStar E/X/G GPS- and EPS-foam boards for Type I construction, illustrating that ICC-ES does not issue a single umbrella report for all ASTM C578 grades; each report holder and product family carries its own evaluation [S3].
ESR-1061 is the Owens Corning reference for FOAMULAR 400/600/1000, with UL Roof Deck acceptance as a complementary listing under the FOAMULAR NGX product line documentation [S6][S10].
Decision criteria: Type IV vs Type VII side-by-side
Compressive strength: Type IV at 25 psi (172 kPa) is sufficient for most commercial roofs and below-grade walls, while Type VII at 60 psi (414 kPa) is required for plaza decks, parking structures, and high-load freezer floors [S4][S6].
Density: Type IV sits at 1.55 pcf minimum nominal, with Type VI at 1.80 pcf, Type VII at 2.20 pcf minimum, and Type V at 3.00 pcf per the ASTM C578 classification summarised in Rmax's C578 explainer [S2][S4].
Thermal resistance: both grades share 5.0 R-value per inch at 75°F mean temperature per ASTM C518, confirmed in the DuPont R-value comparison table that places "XPS (All Types & Densities)" on a single R-per-inch curve [S4].
Code report: Type IV falls under ESR-4755 (DuPont) and ESR-2142 (Dow), with maximum 3-inch board thickness in ESR-2142; Type VII falls under ESR-1061 (Owens Corning) with no equivalent 3-inch cap stated in the available research excerpt [S1][S5][S6].
Application fit: Type IV suits wall sheathing, roof insulation under membranes, foundation perimeter, and protected-membrane assemblies; Type VII is the right pick for plaza decks, ice-rink floors, and any scenario where the engineer calculates sustained live load above the 25 psi threshold [S2][S4].
Limits, failure modes, and sourcing caveats
Type IV's 25 psi limit becomes the binding constraint when the overburden load, slab dead load, and live load combined exceed roughly 3600 psf (psf = psi × 144 in²/ft²) without a safety factor, after which Type VII or higher is required [S4].
R-value degrades with water absorption: any moisture taken up in service reduces thermal performance proportionally, per the DuPont note that "any water absorbed by insulation in an application will directly reduce the insulation's R-value" [S4].
EPS vs XPS still matters for cost-sensitive specifiers: ESR-1962 limits use of EPS Type I/II boards on the outside faces of exterior walls only when the underlying ICC-ES evaluation supports that specific application, and R-Shield's competitive comparison notes that "XPS is not stable" referring to long-term R-value retention under moisture cycling [S3][S7].
ICC-ES reports are time-bound: ESR-4755 is subject to renewal February 2027, ESR-1962 is under continuous compliance through end of July 2027, and ESR-2142 has lapsed past its October 2014 renewal date, so the current authoritative DuPont reference for new submittals is ESR-4755, not ESR-2142 [S1][S3][S5].
Verbatim standard requirement and sourcing
ASTM C578 itself "defines how rigid polystyrene insulation is manufactured, tested, and classified by strength and density" with property thresholds verified by ASTM D1621 (compressive), C518 (thermal), C272 (water absorption), E96 (vapor permeance), and E84 (flame/smoke when required by code) [S2].
ICC-ES ESR-1061, the report covering FOAMULAR 400/600/1000, is published on the ICC Evaluation Service report directory and is the document a code official will ask for when Type VII is specified on a commercial roof or plaza deck [S6].
ICC-ES ESR-4755, the active DuPont report reissued February 2026, identifies Styrofoam brand boards as "grey extruded polystyrene foam plastic boards used as nonstructural thermal insulating material," and the report is renewable annually under ICC-ES continuous compliance [S1].
For a parallel spec decision on adjacent building-envelope systems, see ASTM A996 rail and axle steel rebar grade selection when below-grade XPS wraps a structural slab on grade, and Cantilever vs Selective Pallet Rack for Long Bar Stock for warehouse-construction Type IV under-slab applications.
Next reference nodes: confirm the FOAMULAR NGX datasheet (ESR-1061) is the current submittal for Type VII on the project, verify ESR-4755 is the cited DuPont report (not the lapsed ESR-2142), and check the local AHJ for any Type I-IV construction fire-assembly requirements that may force a flame-spread index under 25 per ASTM E84.