Under ASTM C578, extruded polystyrene (XPS) Types IV, VI, and VII share the same minimum thermal resistance of 5.0 per inch and the same 0.3 percent maximum water absorption by volume, and they are sorted by minimum compressive strength at 10 percent deformation: 25 psi (Type IV), 40 psi (Type VI), and 60 psi (Type VII) [S3][S4].
Type IV is the workhorse of the building enclosure industry and the most commonly produced XPS board; Type VI and Type VII are the structural grades designed for higher loads under slab, in plaza decks, and on parking decks, where compressive resistance is the deciding parameter, not R-value [S5][S7].
Compressive strength is the only property that separates the three types
Per ASTM C578, minimum compressive resistance for these XPS grades is set at 25 psi for Type IV, 40 psi for Type VI, and 60 psi for Type VII (ASTM D1621, at yield or 10 percent deformation, whichever comes first) [S3][S4]. That step pattern matters: every 15 psi increment roughly doubles the allowable dead-plus-live load on a slab-on-grade insulation, so the choice between IV, VI, and VII is essentially a load calculation, not a thermal one. The full C578 line also includes Type V at 100 psi and Type X at 15 psi, but the 25/40/60 tier is where the bulk of below-grade and roofing specification happens [S4].
Thermal performance does not change between these three types. All three carry a minimum R-value of 5.0 per inch at 75 degrees F mean temperature, which is the FTC-published reference condition for US insulation comparison; the DuPont R-value versus mean temperature chart confirms XPS R-value drops by about 0.5 per inch as mean temperature climbs from 25 degrees F to 110 degrees F, but the curve is identical across all three [S4]. This is why a spec writer who only flips the R-value column is missing the real decision: the type letter is a load code, not a thermal code.
Moisture and water absorption are constant across Types IV, VI, and VII
ASTM C578 caps XPS water absorption at 0.3 percent by volume regardless of whether the board is Type IV, Type VI, or Type VII; by contrast, EPS types in the same standard are allowed 2 to 4 percent water absorption, roughly 6 to 13 times higher than XPS [S3]. That gap is structural, not formulation: XPS is extruded into a closed-cell board with no interstitial pathways, while EPS is steam-expanded from beads and can leave interconnected voids between beads that wick water [S3].
For below-grade, protected membrane roof, and frost-protected shallow foundation (FPSF) assemblies, the 0.3 percent cap is the entire reason XPS is allowed where EPS often is not, and it applies identically to Type IV, Type VI, and Type VII. ASCE 32-01 publishes effective in-service R-values for below-grade insulation that are lower than the lab R-value because they credit moisture pickup over the service life; the type letter does not move that effective R-value by itself, the moisture exposure does [S4].
Selection criteria: which type to use where
Type IV (25 psi minimum) is the default for residential foundation walls, commercial roofing above the deck, and wall cavity continuous insulation. It is the lightest of the three and the lowest cost per board foot, and Owens Corning documents Type IV as the workhorse grade sold under the FOAMULAR trademark [S1].
Type VI (40 psi minimum) is the standard under-slab insulation for light commercial slab-on-grade, plaza deck, and cold-storage floor assemblies, where point loads from pallet jacks, forklifts, or stored goods can push a 25 psi board past its yield. Kingspan markets a Type VI 40 psi board specifically for these mid-load applications [S6].
Type VII (60 psi minimum) goes under industrial slabs, heavy-equipment maintenance facility floors, and parking decks where the design load is 1000 psf or more; the extra 35 psi over Type VI is what lets the board spread concentrated wheel loads without creep over decades of service. Going to Type V (100 psi) is the next step beyond Type VII, used under very heavy rolling loads [S4][S5].
Spec-writing pitfalls when the type letter is wrong
Specifying Type IV where Type VI or VII is required is the most common cost-driven error; the symptom shows up years later as board thickness loss, R-value drift, and local slab settlement where a forklift wheel path forms a rut in the insulation. The IIBEC 2024 cautions that ASTM C578 table values are as-manufactured and do not capture long-term creep under sustained load, so designers should add a safety factor and pick the next type up when sustained loads exceed roughly 60 percent of the minimum compressive strength [S2].
Specifying Type VII where Type IV would do is the opposite mistake: it adds 10 to 30 percent to material cost for zero thermal benefit, since R-value per inch is identical across the three [S3][S4].
A second pitfall is the in-service R-value assumption: ASCE 32-01 effective R-values for below-grade XPS are lower than the lab 5.0 per inch and differ by orientation (vertical versus horizontal), so a spec that uses the lab number on the drawings is overstating thermal performance and may fail energy code compliance review [S4].
Comparison matrix: Type IV vs Type VI vs Type VII at a glance
The three grades align on thermal and moisture performance and diverge only on compressive resistance and density: Type IV is 1.60 lb/ft^3 nominal minimum density, Type VI is 1.80 lb/ft^3, and Type VII is 2.20 lb/ft^3, with all three sharing the 5.0 per inch R-value, 0.3 percent water absorption cap, and ASTM C578 type designation rules [S3][S4][S5].
Cost per board foot rises roughly in step with density, so a spec writer working from a fixed budget should treat the type letter as a load requirement first, then look at board thickness for the actual R-value, rather than picking a higher type to chase R-value that does not exist [S1][S7]. For load-bearing criteria, the decision is binary: 25 psi residential, 40 psi light commercial, 60 psi industrial, with the next step being Type V at 100 psi [S4].
Standards sourcing and procurement checks
ASTM C578 is the governing standard for all three types; the test method for compressive strength is ASTM D1621, and the long-term in-service R-value method for below-grade applications is ASCE 32-01. The XPSA (Extruded Polystyrene Foam Association) publishes a summary table on its Material Standards page that mirrors the C578 limits, with Type IV at 25 psi minimum and Type VII at 60 psi minimum, and serves as a quick cross-check during submittal review [S5].
Acceptable evidence at submittal is a third-party evaluation report (ICC-ES ESR or equivalent) listing the type designation, the certified R-value, and the ASCE 32-01 effective R-value for the orientation specified, vertical or horizontal. A spec that only cites the type letter and the minimum compressive strength, without the third-party certification, leaves the door open for substitution with a product that meets the lab number but not the long-term performance [S2][S4].
For more on material grade decision frameworks, the spec-side thinking behind cast iron pattern numbers vs heat numbers follows a similar logic of chasing the property that actually varies between grades. Pressure sensor output chains share the same pitfall, since the lab signal and the in-service signal are not the same number, as laid out in the raw vs amplified vs compensated pressure sensor output map.
The underlying component specifications are covered under xps board, dry type transformer, and pressure transmitter.
