White Expanded polystyrene (EPS) and colored eXtruded polystyrene (XPS) are both closed cell foams and similar in appearance but the main ingredient, manufacturing process, and resulting emissions & material properties are very different.
|Impact Category||Units||EPS Total||XPS Total||XPS Excess|
|Ozone Depletion||kg CFC-11 eq||0.000002||0.042||2,268,650%|
|Global Warming||kg CO2 eq||321||6,987||2,079%|
|Acidification||mol H+ eq||52.9||205||287%|
|Water Consumption||kg = liter||1,142||4,355||281%|
|Eutrophication||kg N eq||0.041||0.113||174%|
|Total Solid Waste||kg||86.2||98.5||14%|
|Smog Formation||kg O3 eq||23.0||23.9||4%|
EPS Type 2 equivalent for above, 1.0" thick: 1,739 boardfeet = 144.9 ft³ = 4.10 m³
= 37,124 lbs, extrapolated approximation for Canadian SFR average per Franklin Associates report data
The Functional Unit is 1 m² [10.764 ft²] of rigid cellular polystyrene (RCPS) insulation board with a thickness providing an average thermal resistance of RSI = 1 m²·K/W [R-Value of 5.678 hr·ft²·°F/BTU] and with a building service life of 60 years.
|Material: Foam Plastic ≈2% for EPS||closed cell polystyrene|
|Material: Insulator||Air ≈98%||mostly HCFC, HFC|
|Material: Flame retardant||polymeric||mostly HBCD|
|True Long-Term Thermal Resistance
* per reports referenced below based on limited empirical data for long-term exposure to wet or humid conditions
Both EPS and XPS are closed cell polystyrene that's known for addressing water and moisture issues better than many other insulation materials.
XPS is supplied by large corporations in mostly pink, blue, and green boards for many building applications. And many of us remember XPS for fast-food packaging with extremely ozone-layer depleting CFC Gases before their worldwide ban starting in the 1990s. Since then many insulation materials including Polyisocyanurate and XPS used HCFC Gases which the whole world also agreed to ban due to high GWP.
The HCFC or HFC Gases escape from XPS at a slow rate. In contrast, EPS uses pentanes as the blowing agent which get replaced by air - mostly during manufacture and completely a number of weeks or months later depending on product thickness. Aged EPS is about 98% air with no off-gasing and has always been free of CFC, HCFC, HFC and formaldehyde.
The HCFC or HFC Gases in XPS and polyiso have lower thermal conductivity than air, allowing claims of higher R-Values per inch thickness than EPS. However, that's only because the applicable laboratory tests are very short-term at 180 days or using estimates of ageing to 5 years. Over the 50+ year service life of buildings as the Gases escape from XPS, it should approach the R-Values per inch thickness of EPS. The warranties of some XPS-manufacturers confirm this by only covering 80% of the advertised R-Values. The estimated 5-year values are also often labeled "Long-Term Thermal Resistance (LTTR)" as an approximation of the time-weighted average performance over the first 15 years, which isn't necessarily correct nor 'long-term' for buildings. In reality the R-Value of XPS in year 15 is already well below the 5-year LTTR and it keeps decreasing from there.
In contrast dry EPS does not loose R-Value over time which is why the National Institute of Standards & Technology (NIST) chose it as the Standard Reference Material for Thermal Conductivity (SRM 1453).
*** per Polystyrene Foam Insulation in Long-Term Building Applications - Effective R-values, EPS Industry Alliance 2019
XPS R-value loss reached more than 80% in some cases, which means a 2" XPS board advertised as R-10 would actually perform at less than 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. So a 2½" EPS board with an original insulation value of R-10 would still perform better than R-9.
Below-grade and roof insulation seemed to be most susceptible where repeated and/or sustained presence of water or high humidity are more likely.
In addition low drying capacity of XPS was found compared to the significant drying potential of EPS as reported in another Technical Bulletin about tests per ASTM C1512 "Standard Test Method for Characterizing the Effect of Exposure to Environmental Cycling on Thermal Performance of Insulation Products".
Both EPS and XPS repell liquid water but they do allow some water vapor diffusion through the material. Third-party research at the Oak Ridge National Laboratory confirmed that actual installations of XPS can exceed 90% moisture content and that XPS drying times can range from 3 to 6 months. The researchers also suggests a reason for the elevated water content of XPS: water vapor can condense inside the closed foam cells where dew points are reached within the insulation, then the low drying capacity of XPS would lock in most of the water and keep accummulating it over time in repeated dew point periods. In contrast EPS would allow it to dry out in periods of improved temperature/humidity levels.
Designers cannot rely solely on product data sheets about short-term laboratory test results because they don't necessarily address actual site conditions over the entire building life. The referenced literature reports on additional product data from testing conducted per industry consensus standards (ASTM) by independent testing laboratories and are corroborated by research at the Oak Ridge National Laboratory among others.Soprema XPS False Advertising Debunked
Real-life high moisture conditions were shown to often lead to water-logged XPS insulation with reduced thermal resistance. XPS in some applications leaves building owners at higher risk to pay for wasted energy and possibly large sums for remediation because XPS won't dry out as easily.
For 50+ year building considerations XPS & polyiso with HCFC gases overstate R-Values and they are heavy polluters while EPS realizes the GWP reductions of insulation with much lower environmental impacts and stable insulation values over time.
EPS also offers the highest insulation value per dollar spent.
** R means resistance to heat flow. The higher the R-value, the greater the insulating power.