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There are important differences between the different offerings of XPS and EPS insulations. The former is available in Types IV, V, VI, VII, and X, all with minimum R-values of 5.0 per inch, and minimum compressive strengths ranging from 104 to 690 kPa (15 to 100 psi). EPS, on the other hand, comes in Types I, II, VIII, IV, XIV, and XV, with minimum R-values ranging from 3.6 to 4.3 per inch, and minimum compressive strengths ranging from 69 to 414 kPa (10 to 60 psi).

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ASTM C578 requires XPS insulation allow no more than 0.3 percent water absorption (by volume), whereas EPS must allow no more than two to four percent water absorption (by volume), depending on the material type—this is six to 13 times more than XPS. This is because there are fundamental differences between the properties of XPS and EPS that are critical to understanding which material to specify for applications requiring high resistance to moisture intrusion.

XPS is manufactured through an extrusion process. Essentially, a molten material is extruded through a die where it expands into a uniform closed-cell rigid foam insulation board with no voids or pathways for moisture to enter. EPS is manufactured with small foam beads placed in a mold and steam-expanded into a large form from which foam boards are cut. This method of manufacture can result in interconnected voices between the beads that can provide pathways for water to penetrate into the insulation.