Discussion on possibility of polystyrene bias

June 24, 2015

The April 2015 issue of The Construction Specifier featured an article by John Woestman entitled “Selecting Polystyrene Foam Where Moisture Exposure Occurs[1].”

Tyler Merchant, public relations manager at the EPS Industry Alliance, wrote to the magazine, wanting to comment on what he called “several inaccuracies and omissions” he felt slanted the piece in favor of extruded polystyrene (XPS) at the expense of expanded polystyrene (EPS).

On page 76, the article makes the assertion “long-term exposure to moisture makes it imperative the insulation of vegetative roof systems retains R-value,” and therefore XPS is the material of choice for vegetative roofs. If R-value retention is such an important factor for vegetative roofs, it should be mentioned that XPS—as a closed-cell foam insulation with a blowing agent or gas other than air—is known to lose its R-value over time “…due primarily to diffusion. This results in a general reduction of the thermal resistance of the foam due to an increase in the thermal conductivity of the resultant gas mixture.” Conversely, “rigid gas-filled closed-cell foam insulation products produced using blowing agent gases that diffuse very rapidly out of the full-thickness foam product, such as expanded polystyrene…” retains a consistent R-value. A fair and balanced comparison would make mention of these facts.

(In both cases above, Merchant is quoting from ASTM C1303/C1303M-14, Standard Test Method for Predicting Long-term Thermal Resistance of Closed-cell Foam Insulation.)

Several claims are made regarding XPS’ resistance to moisture absorption, and that “industry experience has shown XPS to provide superior performance in the exterior foundation insulation application.” This is hardly an accepted fact, as shown by EPS insulation’s significant market share. Further, an important consideration when comparing XPS and EPS is that when subjected to moisture absorption and drying in accordance with ASTM C1512, Standard Test Method for Characterizing the Effect of Exposure to Environmental Cycling on Thermal Performance of Insulation Product, EPS is better-suited to release moisture than XPS.

Finally, in the third paragraph on page 74, EPS is listed as available in several different types, including Type IV. EPS is not available in Type IV, but is available in Type IX, which is not listed.

The EPS Industry Alliance has always supported informative articles that advance the knowledge, proper use and application of foam insulation, but we feel the bias of this article and its inclusion in The Construction Specifier constitutes an implicit endorsement of XPS over EPS. This piece is more of an advertorial than factual comparison, and it is inappropriate for a publication meant to represent the entire construction industry to present it as such.


  1. Selecting Polystyrene Foam Where Moisture Exposure Occurs: http://www.constructionspecifier.com/selecting-polystyrene-foam-where-moisture-exposure-occurs

Source URL: https://www.constructionspecifier.com/discussion-on-possibility-of-polystyrene-bias/