White Expanded polystyrene (EPS) and colored eXtruded polystyrene (XPS) are both closed cell, foamed plastics but the main ingredient, manufacturing process, and resulting emissions & material properties are quite different.
Independently certified EPDs allow direct comparison of products' life-cycle environmental impacts as shown in Table 1.
Table 1: Life-Cycle Impact Category Results for 115 Functional Units of RCPS Insulation [1,2]
|Impact Category||Units||EPS ||XPS ||XPS Excess|
|Ozone Depletion||kg CFC-11 eq||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%|
= 37,124 lbs, extrapolated approximation for Canadian SFR Average
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.
Net savings in Global Warming Potential (GWP) from adding R-4 EPS insulation in Canada are estimated in Table 2. And a study about the costs of reducing greenhouse gas emissions found that improved building insulation provides the highest net income (not cost) over 'business as usual'.
Table 2: GWP Reductions provided
|Total GWP Invested||707||321|
|GWP Reductions compared to Base Wall||B.C.||Alberta||Quebec||NW Terr.||Canada Average|
|Annual GWP Reductions||513||893||790||1,441||742||337|
|Payback Period in Years||1.38||0.79||0.89||0.49||0.95|
|Savings over 50 Years||25,639||44,640||39,505||72,050||37,124||16,839|
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. Since then many insulation materials including Polyisocyanurate and XPS used HCFC gases which the whole world also agreed to ban because they still have some Ozone Depletion Potential (ODP). They are now slowly switching to HFC gases with low ODP but their GWP is still high, negating a large portion of the insulation's 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 and properties of EPS and XPS
|Material: Foam Plastic ≈2% for EPS||closed cell polystyrene|
|Material: Insulator||Air ≈98%||mostly HCFC, HFC|
|Material: Fire Retardant||polymeric||mostly HBCD|
|Cost / Functional Unit||$||$$|
|True Long-Term Thermal Resistance
* 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 and polyisocyanurate off-gas 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. 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).
** 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 non-hygroscopic and good at keeping moisture out because of their closed cell structures. 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.
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. 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
|Region, Method||Immersion Period||Water Absorption max. % by vol.|
|EPS Types 2/II & denser***||XPS most Types|
|USA, ASTM C578||1 day
|Europe, EN13164||28 days (EN12087)||3||1.5|
|Worldwide, Long-Term Studies||2-3 years||3-7||2-5.5|
*** Since EPS can be manufactured 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.
The lack of good QA affected only 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" 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 much less common than decades ago when purchased from certified manufacturers.
ASCE 32-01 assumes water content for XPS below grade to be limited to about 7% by volume.
In 2008 an evaluation of side‐by‐side, below grade vertical application with identical protection systems following a continuous 15‐year installation period in St. Paul, MN found that XPS lost 48% of its thermal resistance due to water content of 18.9% by vol. EPS right next to it lost only 6% of its thermal resistance with 4.8% by vol. water content as indicated in Fig. 1.
In 2012 independent 15-year research at the Oak Ridge National Laboratory also reported below grade XPS insulation experiencing up to 44% loss of thermal resistance due to high water contents. In 2013 additional tests of long-term field performance of below grade and roof installations were conducted and the technical bulletin about Polystyrene Foam's Water Absorption & R-values reported:
XPS' water contents found in three quarters of those installations greatly exceeded the worst-case 7% by vol. assumed in ASCE 32-01.
For polystyrene insulation's water content no correlation can be established between the results of laboratory test methods using total immersion and actual field exposure. Ingress of liquid water can only explain a portion of the in-situ observations and since XPS and well-fused EPS also have zero capillarity/wicking action, the effects of water vapor have to be considered.
Fig. 1. In-situ R-value retention and water absorption of EPS and XPS
Fig. 2. R-value retention of XPS after in-situ water absorption
In contrast to liquid water, water vapor (humidity in air) can slowly diffuse (penetrate) through the cell walls of XPS and EPS. Up to certain thicknesses/densities EPS and XPS can serve as vapor retarders, be it semi-permeable (Class III, 1-10 perm) or semi-impermeable (Class II, 0.1-1 perm). True vapor barriers are less than 0.1 perm (Class I) although Canadians use the term "vapor barrier" also for vapor retarders. EPS' bead-boundaries allow higher vapor permeance of about 1.2 perm versus roughly 0.3 perm for XPS for functional R-12 insulation.
Building designers already use that vapor permeance of insulation materials to reduce vapor getting trapped inside framed walls by specifying vapor barriers on the other side of the wall. However, given a conducive temperature gradient, diffusing water vapor can condense inside polystyrene insulation's closed cells. What happens thereafter with this liquid water inside cells impermeable to liquid water has not been studied in detail but it becomes apparent that the ability to dry out the insulation might play a significant role.
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. 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.
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
|Moisture contents % by volume||EPS||XPS|
|Conditioned 4 weeks @ 72°F, 50%RH||0.7||15.7|
The more recent research[9,14] 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.
To assess full seasonal effects in various climates, the ASTM C1512 test's 20-day environmental cycling period isn't sufficient so other test methods may be needed.
Table 6: Drying Potential of EPS and XPS per ASTM C1512 tests
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 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 (2001) because they don't necessarily address actual site conditions over the intended service life and are based on decades old research that doesn't account for modern quality assurance during manufacture.
Recent research about additional material properties suggests that repeated condensation within the insulation combined with the low drying capacity of XPS could explain the high water contents found in some XPS installations. In effect XPS in some applications can leave building owners at higher risk to pay for wasted energy and possibly remediation because XPS won't dry out easily.
For 50+ year building considerations XPS & polyiso overstate R-values and the HCFC or HFC gases they 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 on foam plastic insulation.