EPS and XPS Insulation Comparison

Ingredients & Properties

Both EPS and XPS are closed cell polystyrene that's known for addressing moisture issues better than other insulation materials.

XPS is supplied by large corporations in mostly pink, blue and green boards. Many of us remember XPS from fast-food packaging using extremely ozone-layer depleting CFC gases before they were banned in the 1970s to 1990s. Then some XPS used HCFC gases that also had Ozone Depletion Potential (ODP) so they subsequently switched to HFC gases with low ODP but their GWP is still hundreds to thousands of times higher than CO₂ - negating a large portion of the insulation's intended GWP-reductions.

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.

Table 3: Overview of ingredients & properties of EPS and XPS insulation

* per reports referenced below based on limited empirical data for long-term exposure to wet or humid conditions

The HCFC or HFC gases in XPS off-gas from the insulation at a slow rate. As refrigerants with lower thermal conductivity than air, they give XPS slightly higher R-values per inch thickness compared to 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. 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 10 is already well below the 5-year LTTR and it keeps decreasing from there as estimated in Figure 5 based on studies of XPS off-gasing. 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 XPS manufacturers confirm this by only covering 80-90% of the advertised R-values.

In contrast, dry EPS does not loose R-value over time which is why the National Institute of Standards & Technology (NIST) chose it as a Standard Reference Material for Thermal Conductivity (SRM 1453).

Estimated R-value of dry XPS over Time^[11]

** R means resistance to heat flow. The higher the R-value, the greater the insulating power.

Water content in any insulation material causes a deterioration of its thermal performance. Unlike most other insulation materials both EPS and XPS are good at keeping moisture out because of their hydrophobic, non-hygroscopic, and closed cell properties. Besides the gases used, the manufacturing processes also differ significantly: XPS uses continuous extrusion with dies whereas EPS is made from expanded beads that are fused together in molds.

Liquid Water Absorption

So EPS has bead-boundaries that XPS lacks and they cause a somewhat higher absorption of liquid water. The North American industry standards again use short-term tests for water absorption, in effect underreporting the values compared to European standards and results from long-term immersion studies as shown in Table 4.

According to the long-term studies, the absorption rates for both EPS and XPS start to plateau after one to two months of immersion and the water contents remain almost constant after about a year of immersion.^[13] Once the bead-boundaries of EPS are saturated, the closed cells of EPS and XPS resist any additional absorption of liquid water equally well.

Table 4: RCPS Water Absorption by Total Immersion over Time

*** Since EPS is available at lower densities than XPS, results for EPS Types with min. densities of less than 19kg/m³ [1.2PCF] are omitted

It is well established that XPS and EPS are suitable for use in high moisture conditions such as below grade and in inverted (or "protected membrane") roof systems. For use below grade ASCE 32-01 guidelines for insulation of frost-protected shallow foundations (FPSF) and the ASHRAE 90.1 energy standard are commonly used in building codes/design.

ASHRAE 90.1 states "Insulation materials in ground contact shall have a water absorption rate no greater than 0.3% when tested in accordance with ASTM C272," effectively eliminating EPS as a below grade option despite its proven use worldwide.

ASCE 32, last revised in 2001, provides suggested "effective R-values" for XPS and EPS to account for different water contents as observed in the 1970s to 1990s - when mature quality assurance (QA) programs were uncommon.

QA effectiveness only affected EPS because of the different manufacturing processes: some EPS manufacturers didn't adequately assure bead-fusion quality, thereby increasing the sizes of the bead-boundaries and water absorption - while XPS' process can't be influenced in such a manner. The absorption results from field observations of some substandard EPS products are still used in ASCE 32-01 by assuming up to 12% water content by vol. for EPS, reducing its "effective R-value" incorrectly by up to 35% compared to dry R-value while XPS' is reduced by only 10-20%. The assumed water content for EPS of 12% by volume is much higher than the 3% to 7% observed in long-term immersion studies.

Today's ASTM C578 and CAN/ULC-S701 standards for polystyrene insulation require stringent QA programs with periodic third-party inspections so substandard EPS is extremely uncommon now when purchased from certified manufacturers.

The 2008 evaluation of side-by-side EPS and XPS installations measured the water contents upon excavation and again after 4 weeks of laboratory conditioning when the XPS sample still had very high water content while the EPS sample was almost dry again as indicated in Table 5.

The researchers at the Oak Ridge National Laboratory also found XPS drying times of 3 to 6 months.^[7] In 2014 additional tests per ASTM C1512 "Standard Test Method for Characterizing the Effect of Exposure to Environmental Cycling on Thermal Performance of Insulation Products" measured low drying potential of XPS compared to the significant drying potential of EPS as shown in Table 6.^[9]

EPS' higher vapor permeance results in higher water accummulation during pre-conditioning of the samples but its bead-boundaries also seem to contribute to the better drying potential of EPS compared to XPS.

Table 5: Moisture contents of EPS and XPS samples after 15 year, side-by-side service below grade

The more recent research^[9,13] suggests a plausible hypothesis for the high water contents of XPS found in moist, colder climate applications: diffusing 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 liquid water and keep accumulating it over time in repeated dew point periods. In contrast, EPS would allow much of it to dry out in periods of improved temperature/humidity levels.