Moisture Resistance

When moisture intrudes into a wall assembly, it can reduce the overall R-value of the building envelope. But properly installed XPS insulated sheathing provides a continuous layer of protection against inside and outside moisture.

XPS is manufactured in a proprietary process that melts plastic resins and additives into a molten material which is then extruded through a die, where it expands and cools into a uniform closed-cell rigid foam insulation board. This uniform closed-cell structure is inherently moisture resistant.

Per ASTM C 578, without the need for a facer or laminate, XPS products only absorb 0.3% water by weight. (This compares to molded, expanded polystyrene foam products, or EPS, which ranges from 2-4%, depending on the density of the product.) In addition to its closed-cell structure, XPS foam products have a natural “skin” surface which is somewhat resistant to moisture absorbency, including in below-grade applications.

For more information on the XPS benefits related to moisture absorption, visit these Technical Insights links: 

“Limitations of Short-term Testing of XPS and EPS Polystyrene Foam Insulation: An Overview of Moisture Absorption in Polystyrene Foam Insulations” (PDF)

“Moisture Absorption Mechanisms in Polystyrene Foams Are Keys to Predicting Long-term Performance: XPS and EPS Behave Differently in Moist Below-Grade Applications” (PDF)

“Simple Experiments Demonstrate How Water Moves More Freely through EPS than XPS” (PDF) 

“Making Sense of the ASTM C578 Types of Rigid, Cellular Polystyrene Insulations” (PDF)