Research on Expanded Polystyrene
For years we have been told that XPS (Extruded Polystyrene) insulation is better than EPS (Expanded Polystyrene) for below grade insulation, but how does it relate to roofing EPS? New research has revealed new information about water absorption in both types of polystyrene insulation. According to the test results conducted by Intertek Testing Service - “ XPS exceeds the recommended water absorption threshold dictated by ASTM C578 by a factor of 2.4.” Water absorption has always been known to lower R-value which can lead to higher utility costs. In fact, third‐party research published by Oak Ridge National Laboratory in April 2012, “XPS below grade systems can experience a 10‐44% loss of energy savings performance when subjected to moisture accumulation in the range of 8% ‐ 16%.”
On the other hand, EPS demonstrates excellent drying abilities and has a much higher tolerance for moisture exposure while still delivering the same R-value throughout the life of the building. It is very simple, "EPS exhibits superior moisture-related performance properties over XPS."
For roofing, it only makes sense that if EPS has absorb moisture, just like below grade, it may have the ability to have it migrate out better than XPS. After all, EPS does demonstrate excellent drying abilities. The bigger question may be in the system that EPS is used, does it have the design opportunity to migrate out. Hey, maybe another study!
Plymouth Foam can meet any R-value requirement needed by simply changing the thickness or density of the EPS insulation. You need 10, 20 or 50 R - no problem.