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Which holds the better R-Value after time... EPS vs XPS

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Update time : 2018-12-04 11:05:09

Studies show that as much as 25% of energy loss from a structure can be attributed to a lack of insulation on below-grade foundations,crawl spaces and under slabs. Insulation R-value is directly correlated to maximum energy efficiency in a building envelope; higher R-values translate into increased savings. In below grade applications, foam insulation is exposed to moisture and could lose R-value over time if this moisture is absorbed. As shown in an independent, third-party test program expanded polystyrene (EPS) maintains its R-value even after long-term exposure in northern climates. A competing insulation material, extruded polystyrene (XPS), was shownto have lost R-value over time. The results of this test program demonstrate that EPS insulation is a perfect choice to reduce energy loss.


In August 2008, independent testing evaluated the field performance of EPS and XPS insulation in a side-by-side, below grade application following a continuous 15-year installation period. EPS Type I and XPS Type X testsamples were excavated from the exterior of a commercial building in St. Paul, MN at a depth of approximately 6 feet below grade. Specimens were tested for thermal resistance using ASTM C518 “Standard Test Method forSteady-State Thermal Transmission Properties by Means of the Heat Flow Apparatus” immediately after excavation. Moisture content was determined by measuring the sample weight at the time of removal and again after being oven dried.


The results demonstrate that EPS Type Ioutperforms XPS Type X in both R-valueretention and decreased water absorption.Further, whereas the in-service R-value of theXPS insulation is reduced by half, expandedpolystyrene still delivers 94% of its specifiedR- value of 3.6 per inch after 15 years . Theselong term performance advantages make EPSinsulation a preferred choice when compared itscompetition.

This testing further confirms that waterabsorption results determined using ASTM C272“Standard Test Method for Water Absorptionof Core Materials for Structural SandwichMaterials” cannot be correlated to the inserviceperformance of foam insulation.The main reason is that the laboratory testprocedures call for partial or full submersionconditions which are not encountered in fieldapplications. In fact, laboratory test methodswere not developed for predicting actualperformance, but were intended for usein specifications as a means of comparingrelative physical properties of different cellularplastics and for product evaluations andquality control.


© 2008 EPS Industry Alliance | The EPS Industry Alliance publishes technical bulletins to help inform building professionals on theperformance characteristics of expanded polystyrene (EPS) building products. The information contained herein is provided withoutany express or implied warranty as to its truthfulness or accuracy