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.