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.
The Extruded Polystyrene Foam Association (XPSA) is a trade association representing manufacturers of Extruded Polystyrene Foam (XPS) insulation products and the industry's raw material suppliers. XPSA members collectively manufacture more than 95% of all XPS destined for use in the North American market. XPSA promotes the benefits that accrue to society from appropriate use of XPS foam insulation applications.
© Copyright 2016 The Extruded Polystyrene Foam Association
750 National Press Building
529 14th Street, NW
Washington, DC 20045