When moisture gets into a wall assembly, it can reduce the overall R-value of the building envelope. But properly installed XPS 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 percent water by weight. (This compares to molded, expanded polystyrene foam products, or EPS, whose same property ranges from 2 to 4 percent, depending on the density of the product.) In addition to its closed-cell structure, XPS foam products have a natural “skin” surface which is not conducive 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”
EPS and XPS are used in below-grade applications where moisture resistance is key. Unlike XPS, EPS absorbs more water over time, reducing its thermal performance. Lab tests show short-term effects, but don’t reflect real-world, long-term conditions.
Read the Technical Insights, Below Grade Series ID: TI-BG-02
“Moisture Absorption Mechanisms in Polystyrene Foams Are Keys to Predicting Long-term Performance: XPS and EPS Behave Differently in Moist Below-Grade Applications”
Marketing claims surrounding moisture absorption in polystyrene insulation can be misleading if based on limited tests. Smart specifiers understand the key differences between XPS and EPS that affect long-term R-value. Since short-term tests don’t always capture real-world moisture impact, comprehensive data is essential for accurate evaluation.
Read the Technical Insights, Below Grade Series ID: TI-BG-03
“Simple Experiments Demonstrate How Water Moves More Freely through EPS than XPS”
XPS has a closed-cell structure that prevents liquid from passing through. In contrast, EPS has open pathways that allow water to move through, causing it to absorb moisture quickly in below-grade applications. Since thermal resistance decreases as moisture content increases, XPS offers an advantage in maintaining insulation performance over time.
Read the Technical Insights, Below Grade Series ID: TI-BG-04
“Making Sense of the ASTM C578 Types of Rigid, Cellular Polystyrene Insulations”
Unlike EPS, where density has a strong impact on thermal resistance, XPS shows less correlation between density and R-value. In general, XPS types consistently outperform EPS types in thermal resistance at similar densities, making them a reliable choice for insulation across various temperatures.
Read the Technical Insights, Below Grade Series ID: TI-BG-12