Extruded Polystyrene "XPS" Blue Board
XPS is most often used in below grade applications because of its inherent resistance to absorbing water. XPS falls in the middle of the three types of rigid-foam insulation in R-value (about R-5 per inch). XPS typically comes unfaced. Unfaced 1-in.-thick XPS has a perm rating of around 1, making it semipermeable and is considered a vapor retarder, not a vapor barrier. The traditional blowing agents used to make XPS have a very high global warming potential. US Manufacturers are currently testing and introducing a new generation of blowing agents that dramatically reduce global warming potential. Thermal drift, the gradual lowering of R-value over time, is a consideration with both XPS and ISO.
