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EPS vs XPS Insulation Comparison
Environmental Impacts, Ingredients & Properties

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.

Life-Cycle Product Emissions

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Environmental Product Declarations (EPD)

Life-Cycle Impact Category Results for a Functional Unit of RCPS Insulation
Impact Category Units EPS Total XPS Total 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%
Total Energy MJ 71.4 80.7 13%
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.

  1. Product Category Rules for Preparing an Environmental Product Declaration (EPD) for Product Group: Building Envelope Thermal Insulation, Version 1.0, 23 September 2011
  2. ISO 14025:2006(E), Environmental labels and declarations - Type III environmental declarations - Principles and procedures
  3. Independently Certified Environmental Product Declaration of Expanded Polystyrene (EPS) Insulation per ISO 14025
  4. Independently Certified Environmental Product Declaration of Extruded Polystyrene (XPS) Insulation per ISO 14025, UL Environment Declaration No. 4786077032.101.1 (2013)
  5. Franklin Associates report "Energy and Greenhouse Gas Savings for EPS Foam Insulation Applied to Exterior Walls of Single Family Residential Housing in the U.S. and Canada" 2009
  6. A cost curve for greenhouse gas reduction, McKinsey Quarterly

Comparison to multiple XPS-brands

Energy and Emissions Savings from Added Insulation
per Franklin Associates report (at time of study Base Wall was w/o continuous insulation)  Show R-4 Case

Ingredients & Properties

Property EPS XPS
Material: Foam Plastic ≈2% for EPS closed cell polystyrene
Material: Insulator Air ≈98% mostly HCFC, HFC
Material: Flame retardant polymeric mostly HBCD
Cost $ $$
Environmental Impacts
True Long-Term Thermal Resistance
50-year LTTR
Drying Capacity

* 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.

Effects of Off-Gasing on R-Value**

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).

Estimated R-Value of XPS over Time (dry)***
XPS Estimated R-Value over time

*** per Polystyrene Foam Insulation in Long-Term Building Applications - Effective R-values, EPS Industry Alliance 2019

Effects of Moisture on R-Value

EPS vs XPS Water Absorbtion R-Values In-Situ XPS Water Absorbtion R-Values
Some real-life installations of XPS had much lower R-Values than expected even after considering the off-gasing effects. 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. Major findings:
  • R-value loss of foam insulation is directly related to the % of water absorption by volume
  • In-situ water absorption for XPS varies widely ranging from 5-60% by volume, much higher than XPS advertised water absorption values of <0.7% per CAN/ULC-S701 and <0.3% per ASTM C578 using submersion for only a few days
  • For XPS water absorption, there is NO correlation between the results from standardized laboratory test methods and actual field exposure

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.

Choose the Best for your clients & our environment Build with EPS

Additional References
  1. Measurement of Exterior Foundation Insulation to Assess Durability in Energy-Saving Performance, Oak Ridge National Laboratory 2012
  2. XPS Insulation Extracted After Field Exposure Confirms High Water Absorption & Diminished R‐Value, EPS Industry Alliance 2014
  3. Drying Potential of Polystyrene Insulations Under Extreme Environmental Cycling Conditions, EPS Industry Alliance 2014
  4. XPS & Polyiso Long-Term Thermal Resistance & R-Value Performance, EPS Industry Alliance 2016
  5. Polystyrene Foam Insulation in Long-Term Building Applications - Effective R-values, EPS Industry Alliance 2019
  6. Polystyrene Foam Insulation Environmental Footprint, EPS Industry Alliance 2019
  7. Soprema Comparative Advertising Not Aligned with Canadian Code of Advertising Standards, EPS Industry Alliance 2019

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