XPS (extruded polystyrene): closed-cell rigid foam. R-value: R-5/inch (R-10 for 2in). Compressive strength: 25 psi (Styrofoam, Type X) to 60 psi (heavy-duty). Brands: Dow Styrofoam (blue), Owens Corning Foamular (pink). Water absorption: <0.3% by volume. Uses: below-grade foundation (exterior), continuous insulation (ci) on walls, protected membrane roof insulation. ASTM C578 Type X (15 psi) or Type IV (25 psi). Temperature: -65F to 165F service. GWP concern: traditional blowing agents (HFC-134a) have high GWP — newer formulations use HFO (low GWP). Cost: $1.50-$3.00/sf for 2in. CSI 07 21 13. Must be covered with thermal barrier (1/2in gypsum) when exposed to interior per IBC 2603.