The EPS Industry Alliance (EPS IA) has released NEW moisture absorption data regarding XPS, moisture absorption and the effects on R-Value through the latest Technical Bulletin, EPS Below Grade Series 105: XPS Insulation Extracted After Field Exposure Confirms High Water Absorption & Diminished R-Value, March 2014 (pdf)
A test program conducted in August 2008 evaluated the field performance of expanded (EPS) and extruded (XPS) polystyrene foam insulation in a side-by-side, below grade application following a continuous 15-year installation period. When water and R-value retention were compared between EPS and XPS, the results demonstrated that EPS insulation outperformed XPS insulation with better R-value retention and a lower moisture absorption.
In 2013, an independent testing laboratory was again commissioned to evaluate the R-value and water absorption from XPS insulation samples extracted from several field locations and applications. A summary of the 2013 test results reconfirm there is no correlation between the results from standardized laboratory test methods and actual field exposure for XPS water absorption. Further, the significant loss of R-value associated with XPS water absorption is shown in the summary.
“Studies show that as much as 25% of energy loss from a structure can be attributed to a lack of insulation. Insulation R-value is directly correlated to maximum energy efficiency in a building envelope; higher R-values translate into increased savings. It is important to understand that in-situ water absorption can diminish the thermal performance of building materials and designers must account for this when evaluating different insulation choices.”
Read more on the technical background expanding on the moisture resistance comparison between EPS and XPS conducted in the 2008 test program: