|Impact Category||Units||EPS||XPS||XPS Excess|
|Ozone Depletion||kg CFC-11 eq||1.6E-8||3.63E-4||2,268,650%|
|Global Warming||kg CO2 eq||2.79||60.8||2,079%|
|Acidification||mol H+ eq||0.46||1.78||287%|
|Water Consumption||kg = liter||9.94||37.9||281%|
|Eutrophication||kg N eq||3.6E-4||9.85E-4||174%|
|Total Solid Waste||kg||0.75||0.857||14%|
|Smog Formation||kg O3 eq||0.20||0.208||4%|
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: Fire 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 for many building applications in mostly pink, blue, and green boards. Many of us also remember XPS from fast-food packaging using 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.
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 escape from the insulation at a slow rate and they 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 relatively short-term at 180 days of age or using simulated ageing to 5 years. Over 50+ year service lives of buildings as the gases escape from XPS, it should approach the R-Values per inch thickness of EPS.[10,11] The warranties of some XPS manufacturers confirm this by only covering 80-90% of the advertised R-Values. The simulated 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 "long-term" for buildings nor necessarily accurate. In reality the R-Value of dry 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).
** R means resistance to heat flow. The higher the R-value, the greater the insulating power.
Some actual installations of XPS in high-moisture conditions had much lower R-Values than expected. So additional tests of long-term field performance were conducted in 2013 for below-grade and roof applications where repeated and/or sustained presence of water or high humidity are more likely. The technical bulletin about Polystyrene Foam's Water Absorption & R-Values reported:
XPS R-Value loss reached more than 50-80% in multiple cases which means a 2" XPS board advertised as R-10 would perform at less than R-5 to R-2.
EPS R-Value loss was limited to about 6% in a 2008 evaluation of side‐by‐side, below grade application following a continuous 15‐year installation period. So a 2½" EPS board advertised as R-10 would still perform better than R-9.
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".
Independent long-term research at the Oak Ridge National Laboratory also reported high moisture contents of XPS used below grade and XPS drying times of 3 to 6 months. Because of the closed cell structures both EPS and XPS are relatively good at keeping liquid water out. Besides the gases used, the other primary material difference stems from the manufacturing process: EPS is made from round, expanded beads that are fused together while XPS is continuously extruded. So EPS has bead-boundaries that XPS lacks and they cause a somewhat higher water absorption rate but also better drying capacity.
The more recent research[9,14] suggests a plausible hypothesis for the high water contents of XPS found in moist, cold-climate environments: since both EPS and XPS allow some water vapor diffusion through the material, that water vapor can condense inside the closed foam cells when dew points are reached within the insulation; then the low drying capacity of XPS would lock in most of the water and keep accumulating it over time in repeated dew point periods. In contrast, EPS would allow it to dry out in periods of improved temperature/humidity levels.
15-year research at the Oak Ridge National Laboratory reported below-grade XPS insulation experiencing a 10-44% loss of thermal resistance. Water-logged XPS insulation with even higher thermal resistance losses was also found in other installations exposed to extended high moisture conditions in colder climates.
This means designers cannot rely solely on product data from short-term laboratory test results as established under ASHRAE 90.1 and the dated ASCE 32-01 (2001) because they don't necessarily address actual site conditions over the intended service life of many decades.Soprema XPS False Advertising Debunked
The recent research about additional product properties, conducted per industry consensus standards by independent testing agencies, suggests that repeated condensation within the insulation combined with the low drying capacity of XPS could explain the high water-accumulation found in those XPS installations. In effect XPS in some applications can leave building owners at higher risk to pay for wasted energy and possibly large sums for remediation because XPS won't dry out easily.
For 50+ year building considerations XPS & polyiso overstate R-Values and the HCFC gases many use are strong pollutants while EPS realizes the GWP reductions of insulation with much lower environmental impacts and stable insulation values over time.[3,4,10,12]
EPS also provides the highest insulation value per dollar spent.