Expanded polystyrene (EPS) and eXtruded polystyrene (XPS) are both closed-cell foams and similar in appearance, but the manufacturing process and resulting material properties are quite different. We all know XPS from its use in commercial buildings supplied by large corporations in mostly blue, pink, and yellow boards.
And many of us remember XPS for its use of extremely ozone-layer depleting CFC Gases before the worldwide ban starting in the 1990s. Currently XPS and most polyurethane (PU) sprayfoam insulation use HCFC Gases - still with high GWP - which the whole world also agreed to ban. The HCFC gases escape from XPS at a pretty slow rate. In contrast, EPS uses pentanes as the blowing agent which get replaced by 100% Air within the first few months after manufacture.
XPS manufacturers use a specific HCFC with lower thermal conductivity than air, allowing them to claim higher R-Values per inch thickness than EPS. However, that's only because the applicable laboratory tests are relatively short-term at 180 days or 5 years. After 10-15 years the reality is very different for the remainder of a building's long life-cyle: XPS should have the same R-Value per inch thickness as EPS after the Gases escaped from XPS. But many real-life installations of XPS exhibited even lower R-Values than expected.
So the EPS Industry Alliance commissioned additional tests of long-term field performance and released a Technical Bulletin about Polystyrene Foam's Water Absorption & R-Values.
XPS R-value loss reached more than 80% in some cases, which means a 2" thick XPS board originally advertised as R-10 would actually perform at about R-2!
EPS R-Value loss was only about 6% in a 2008 test of side-by-side, below grade application following a continuous 15-year installation period, due to limited EPS water absorption (<5% complying with the Standard specifications). So a 2" thick EPS Board with an original insulation value of about R-8 would still perform better than R-7.
A large part of this difference is a result of the sealed-off nature of XPS foam cells, which the XPS industry touts as being superior because it slows down the exchange of water, vapour, and air - in the short term. The short-term laboratory tests allow XPS to claim superior material properties, but over the long lives of actual buildings, XPS admittedly looses the HCFC Gases getting them replaced with whatever is around it: air, if your installation remained water- and condensation-proof, but in some installations, especially below-grade or in roofs, with water!
The sealed-off XPS foam cells also cause a lack of drying capacity of XPS boards compared to the significant drying potential of EPS, as tested and reported in another EPS Industry Alliance test per ASTM C1512 "Standard Test Method for Characterizing the Effect of Exposure to Environmental Cycling on Thermal Performance of Insulation Products".
Combine the two factors and water-logged XPS non-insulation can result. In short, the XPS industry seems to have cherry-picked both the laboratory tests for allowable moisture absorption AND for R-Value determination based on Greenhouse Gases that slowly escape from XPS. The clear losers are building owners whose XPS carries higher risks of becoming useless, costing them energy and possibly large sums for remediation because XPS won't dry out easily. Long-Term Thermal Resistance (LTTR) and the ability to dry out insulation that was exposed to water are priorities for owners, and designers should advise & specify accordingly.
In response to the EPS Industry Alliance bulletins that report on tests conducted per industry consensus standards (ASTM) by independent testing laboratories, Pink XPS promptly developed a "Science Doesn't Lie" tagline and website where they proceed to do just that - lie - about EPS and XPS. A number of brochures and a video heavily rely on unrecognized, made-up "tests", such as digging a hole in the ground to bury samples for three weeks. And of course they keep repeating the cherry-picked, short-term laboratory tests.
And they want to charge you a lot more for THAT? EPS offers the highest insulation value per dollar spent ontop of its physical advantages.
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Airfoam is a part of Aqua-Pak, the nation's leading manufacturer of expanded polystyrene (EPS) containers for over 30 years. Airfoam works with professionals to construct high-quality building insulation systems, geofoam, and flotation solutions at a minimal cost.