XPS Insulation Tests Confirm Diminished R-Value When Exposed

XPS Insulation Tests Confirm Diminished R-Value When Exposed

Originally posted on ConstructionRepair.net

EPS Industry Alliance release new comparative test results

The EPS Industry Alliance (EPS-IA) recently completed a series of tests on extruded polystyrene (XPS) to examine the effects of moisture absorption and R-value in different field applications. Two new technical resources look at the behavior of rigid foam insulation exposed to water, specifically related to the materials drying potential and R-value retention.

When evaluating XPS material samples extracted from roofing and below grade applications, in these long-term installations, XPS did not maintain its initial R-value.

Buildings have been and always will be exposed to moisture. It is not a good thing or a bad thing; it is merely another component of the building design process. When materials are exposed to moisture, the ability to dry is key to maintaining thermal resistance.

This issue is addressed in Drying Potential of Polystyrene Insulations Under Extreme Environmental Cycling Conditions , which evaluates the free-thaw cycling effects on rigid foam plastics as prescribed by ASTM C1512. The test results indicate XPS exceeds the recommended water absorption threshold dictated by ASTM C578 by a factor of 2.4, and, test data rendered by Intertek Testing Services show that in-situ water absorption from XPS samples taken from four different locations is widely variable from 5 – 60% by volume.

Standardized laboratory testing, while not intended to replicate in-situ, real-world conditions, substantiate expanded polystyrene (EPS) performance claims to deliver consistent R-value in building environments that may be exposed to moisture. XPS producers claim its lower moisture absorption rate is a benefit; however, this is based on flawed logic. XPS R-values begin to deteriorate at only 0.03 percent, meaning its tolerance for water absorption is extremely low.

This phenomenon is demonstrated in the test results published in XPS Insulation Extracted After Field Exposure Confirms High Water Absorption Diminished R-value . 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.

Expanded polystyrene exhibits superior moisture-related performance properties over XPS. It has higher vapor permeability, meaning it helps promote drying in a wall system. As shown in the EPS-IA technical bulletins, EPS is inherently more capable of tolerating moisture absorption than XPS.

Even at 3.0 – 4.0% moisture absorption, expanded polystyrene insulation delivers consistent R-value of 3.1 – 4.3 per inch.

When evaluating rigid foam insulation performance properties, non-standardized testing, modified test methods or testing not intended for the materials being evaluated should be viewed with skepticism. EPS-IAs new information comparing EPS and XPS moisture absorption and R-value retention is based on testing conducted by a third-party, certified testing laboratory and relies on industry recognized standards ASTM C1512. ASTM C518 and others.

EPS-IA is confident these new documents will be a valuable resource for architects, contractors and consumers that are seeking the best possible insulation for their construction projects. For more information on expanded polystyrene and the results of EPS-IAs new test results please contact Betsy Steiner, EPS-IA Executive Director, at emsteiner@epsindustry.org or 800-607-3772.


  1. Duane Comeau 2 years ago

    Would it be appropriate to put insulfoam under corrugated metal roof material for insulation in hot climate?? Southern AZ. Thinking of using 1 1/2 inch between 2×6 ceiling joists/rafters under corrugated metal.

    • Insulfoam EPS 2 years ago

      If their intent is to put the insulation directly against the metal I would not recommend it. If there is an airspace yes R-Tech with the metal facing upward. The airspace should be minimum 1.5”. Please let us know if you have any further questions! Thanks for reaching out.

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