The source presents typical compressive-strength ranges of 10–14 psi for Type I, 15–21 psi for Type II and 25–33 psi for Type IX [S2].
Type I weighs 1.0 lb/ft³ nominally, Type II weighs 1.5 lb/ft³ and Type IX weighs 2.0 lb/ft³, giving specifiers three distinct load and cost levels rather than interchangeable labels [S5].
Strength and Density Decision Matrix
ASTM C578 defines each type by minimum physical-property requirements, including density and compressive strength, so specifying only the generic term EPS leaves the critical load class unresolved [S1][S2].
Type I has a 0.90 lb/ft³ minimum density and 10–14 psi compressive-strength range in the manufacturer comparison, Type VIII has 1.15 lb/ft³ and 13–18 psi, Type II has 1.35 lb/ft³ and 15–21 psi, and Type IX has 1.80 lb/ft³ and 25–33 psi [S2]. These are typical product values, not a substitute for the ASTM C578 classification thresholds or the project specification [S2].
Type IX has 2.5 times the 10 psi Type I minimum strength and 1.67 times the 15 psi Type II minimum strength, while its 2.0 lb/ft³ nominal density is twice Type I's 1.0 lb/ft³ [S2][S5]. That increase is justified where design loads, overburden or installation stress justify it; it is not justified merely to improve thermal performance at the same thickness.
Thermal Resistance Gain at Equal Thickness
EPS typically provides about R-3.6 to R-4.2 per inch, while XPS is around R-5 per inch; product formulation and test conditions affect the reported value [S1].
At 1 inch, the manufacturer sheet lists R-values of 3.85 for Type I, 4.17 for Type II and 4.35 for Type IX, with R-values calculated based on 75°F [S5]. At 2 inches, the corresponding values are R-7.70, R-8.34 and R-8.70, calculated at 75°F without an aging adjustment [S5].
Type II has a 1-inch R-value of 4.17 compared with 3.85 for Type I and 4.35 for Type IX [S5]. At 1 inch, Type II has an R-value of 4.17 compared with 3.85 for Type I, while Type IX has an R-value of 4.35 [S5].
For an EPS board specified primarily for wall or general sheathing insulation, increasing from Type I to Type II may be economical where the added R-value avoids increasing thickness. If the design load is the controlling criterion, strength and density should determine the type.
Where Each EPS Type Fits
ASTM C578 classifies rigid polystyrene insulation by strength and density for use across roofs, walls and foundations [S1].
Type II is identified as one of the two grades most widely used in buildings, alongside Type IX, with typical values of R-4.0 and R-4.2 per inch respectively [S7]. Type II's 15 psi minimum strength is 50% greater than Type I's 10 psi minimum, so it provides a meaningful reserve when the board must resist roof membrane loads, installation traffic or limited overburden without moving to Type IX [S2].
Type IX is the correct selection for slabs, driveways and heavy floors where higher compressive resistance is required [S2].
Type I is not the right answer for a high-load application simply because its nominal density is familiar, and Type IX is not automatically economical for a lightly loaded wall. Specify the grade from calculated load, exposure, thickness and installed cost.
Load Interpretation and Design Limits
ASTM C578 compressive strength is the pressure required to deform the foam by 10%, so 25 psi corresponds to 3,600 psf on the board area at the stated test condition [S1][S2].
This laboratory value is not the same as allowable installed design pressure. The engineer must account for dead load, construction loads, long-term creep, moisture, workmanship and the applicable building code before assigning the board. ASTM C578 addresses insulation properties, not the complete capacity of the roof, slab or wall assembly [S1].
The specified service range for ASTM C578 insulation is −65°F to 165°F, while the standard covers molded or extruded foam boards and excludes laminated or faced products that alter base performance [S1]. ASTM C578 establishes measurable thresholds for thermal resistance, water absorption, vapor permeance and dimensional stability [S1].
Moisture, Flame and Installation Constraints
The guide gives typical product properties of 1.35 lb/ft³ for Type II and 1.80 lb/ft³ for Type IX, with compressive-strength ranges of 15–21 psi and 25–33 psi, respectively [S2].
Type I remains the economical choice where the board has light framing support and no sustained high load. It is less suitable for exposed or mechanically vulnerable installations because its lower density provides less stiffness and resistance to crushing than Type II or Type IX [S1][S2].
Protect EPS from prolonged ultraviolet exposure, direct flame exposure and water accumulation within the assembly. Do not infer fire performance from the C578 type letter; where flame and smoke indices are required, verify them through ASTM E84 testing and the project specification [S1].
Procurement and Submittal Requirements
ASTM C578 compliance requires controlled property testing before a board carries a specific type designation, and procurement should state the type, nominal density, required thickness and applicable exposure conditions [S1][S2].
For Type I, the minimum density is 0.90 lb/ft³; for Type II it is 1.35 lb/ft³; and for Type IX it is 1.80 lb/ft³ [S2][S5]. At the reference temperature of 75°F, one-inch R-values are 3.85, 4.17 and 4.35 for Type I, Type II and Type IX respectively, with no R-value aging adjustment included in the published sheet [S5].
Request certification that the delivered product matches the specified ASTM C578 type rather than accepting a generic EPS description. Confirm board thickness independently because a 100 mm panel cut to 98 mm provides about 2% less thermal resistance and may fail the design target [S3].
For a documented project decision, record the calculated design load, select Type I at 10 psi, Type II at 15 psi or Type IX at 25 psi only when that class is justified, then verify density, thickness and moisture exposure in the submittal before ordering.