Comparing Polystyrenes: Looking at the differences between EPS and XPS

The choice of insulation should be made after considering the effect its characteristics will have on the performance of the walls.

Water resistance
There are fundamental differences between the properties of XPS and EPS that are essential to determine in deciding which is the better choice for wall applications requiring high resistance to moisture intrusion. The water absorption rate by total immersion for XPS is listed at 0.3 percent maximum by volume, compared to 2.0 to 4.0 percent for EPS, depending on product density. That is a substantial difference to be taken into account when specifying a durable project.

The closed-cell structure of XPS makes it more resistant to water. Although EPS beads are closed-cell and hydrophobic, they are also surrounded by voids. These voids are responsible for the higher water absorption volume found in finished EPS board. The blowing agent used in EPS is quickly replaced by air and paired with these voids. The result is an EPS insulation product with lower thermal resistance capability when compared to XPS. An EPS product with higher density would have fewer voids and thus less potential for water absorption and an increase in thermal resistance.

A higher potential for water absorption means a higher potential for the growth of mold. Again, most EPS products sold at home improvement warehouses absorb much more water than XPS products.

The ‘green’ factor
A building’s ‘green’ rating is also of concern for both owners and those in the design-build sector. While both XPS and EPS have green features, it is important to consider the blowing agent used to create the foam.

EPS is often produced with pentane, which has a very low global warming potential (GWP). XPS uses hydrochlorofluorocarbons (HCFCs) as the blowing agent, which has a high GWP, and has shown to lose some of its R-value over time as this gas slowly escapes. (See Building Green vol. 2, No.1, which can be found on However, this may soon no longer be a negative factor, as XPS manufacturers have begun to shift to newer blowing agents with a zero-ozone depleting formula. To know if the selected product is the newer type, one should consult the manufacturer’s safety data sheet (MSDS).

EPS can be considered a suitable choice for green building designs because it offers the environmental advantages of energy efficiency, recyclable content, resistance to mold, and indoor environmental quality. With hundreds of plant locations in North America, EPS and XPS can help meet green building localized manufacturing goals, which, in turn, helps to reduce the impact of transportation.

Its use in innovative applications improves the performance of a building’s envelope. Further, EPS applications have shown to consistently reduce both jobsite waste and labor costs during installation. Environmental benefits last through the service life of a building with higher insulating properties that result in measurable savings. In addition to the environmental benefits of EPS, the energy needed to make it can be less than what is used to produce non-foam insulating materials. In one such study, conducted by EPS-IA, the energy required to produce foam insulation is 24 percent less than what is needed to make the amount of fiberglass necessary for an equivalent R-value at a representative volume. (For more information, see “EPS Industry Alliance, 2012 Report,” which can be accessed online by visiting

When specifying a rigid foam insulation material, it is critical to know how it will perform in the intended application.

For more than 60 years, EPS has been free of both chlorofluorocarbons (CFCs) and HCFCs. It provides stable R-values that do not need to be adjusted as years pass. EPS offers mold resistance and receives a favorable rating under ASTM C1338, Standard Test Method for Determining Fungi Resistance of Insulation Materials and Facings. This test method is used to determine the ability of an insulation and its facing to resist fungal growth.

Green building credits can be earned by specifying EPS foam insulation in many point categories such as recycled and recyclable content, energy efficiency, indoor air quality (IAQ), sustainable jobsites, and general innovation. Leadership in Energy and Environmental Design (LEED), and other green building accreditation programs, recognize numerous system applications, including SIPS and ICFs that use EPS to provide significant environmental advantages.

To keep an even perspective on both types of insulation, it must be noted XPS has many favorable attributes, including its unique properties that separate it from other types of foam insulation. XPS has many applications, including below slabs, grade foundations, and walls. It is often specified when higher compressive strength, greater moisture resistance, and elevated thermal resistance in the presence of water are required.

For green builders, XPS has two major strikes against it. The material currently contains the flame retardant HBCD, and its blowing agents have a very high global warming potential. However, recycled XPS is widely available and is cheaper than new EPS or XPS. The CO2 impact of recycled XPS has already taken place once, making reclaimed foam potentially the ‘greenest’ of all foam insulation products.

Making the choice
Both EPS and XPS are effective forms of exterior wall insulation and a large amount of contrasting information has been given in this article. However, a solid grasp of these differences is essential to make the best specification choice. The selected insulation will most likely be in service for decades as replacing wall sheathing can be quite an expensive project. (These comparisons are for estimates only and should not be used exclusively for design purposes. It is important to refer to each manufacturer’s specific performance specs for more details on each of these products.)

Jason Burgess has been the products manager for Colonial Materials for more than 10 years. He specializes in insulation, lumber, and non-core products. Burges earned a business degree from Belmont Abbey College in 2006, with a concentration in management. He has been involved in the industry since he was 14, when he started working during summers and school breaks at Dellinger’s Building Supply. He can be reached at

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10 comments on “Comparing Polystyrenes: Looking at the differences between EPS and XPS”

  1. Good article with regard to comparisons between the XPS and EPS. However, may have fallen bit short in the fact that there was no mention of NFPA 285 and the effect that it has on using these insulating materials. Such as added cost in material and labor at each and every window opening.

  2. You say “A higher potential for water absorption means a higher potential for the growth of mold. Again, most EPS products sold at home improvement warehouses absorb much more water than XPS products.” and later say “EPS offers mold resistance and receives a favorable rating under ASTM C1338, Standard Test Method for Determining Fungi Resistance of Insulation Materials and Facings.” These statements seem contradictory.

    1. Xps and eps both hold water it’s a mater of time. Eps will absorb less during a shorter time but it also will not give up the water as fast as Xps. This is why XPS is used under crawlspace because if it gets moisture build up it will release it in a timely manner. Both will get 100% saturated but at different times. Considering you own a home or build a home its life cycle is around 50ys or more. I would go with the one that does not hold the water content.

  3. In the article you mentioned that a typical EPS-panel has an R-value of on average 3.9 for 25 mm.
    However if I use R=d/lambda with a thickness of 0.025 m and a lambda of 0.033 W/m*K I get to an R-value of only 0.8 m2K/W
    Could someone tell me what I do wrong here?

    1. Imperial vs metric! Imperial R-Values in ft²•hr•°F/BTU for 25.4mm EPS range from 3.85 (low density foam @ 24°C =75°F) to R-4.8 (high density foam @ -4°C =25°F).
      Metric R-Values (called Rsi in Canada) in m²•K/W equivalent range for 25.4mm EPS is 0.66 to 0.85.
      If you use Windows/PC, I use this unit converter every day:

    2. I think you are correct here. The R value in this article for EPS seems wrong. EPS in my calculation too would have to be 125mm to be an R value of 3.0

  4. Where this articale is accurate is in discussion of the moisture resistance. Most EPS material characteristics are determined with blanks of material in the as-manufactured state, this includes a “shrink wrap” of thin mil plastic. Once the insulation is cut in the field or in the creation of taper packages, the resistance and absorption change dramatically. the EPS manufacturer’s of course won’t publish this inforation, wherever you cut XPS, the performance stays the same.

  5. What is an XPS board?
    XPS board is an insulating board manufactured using completely biodegradable material. It is produced from foamed polystyrene, a material that does not depreciate. The cell structure in the board is constructed with very fine closed cells that give the product its outstanding physical and mechanical features.

    The advantage of XPS Board:
    Little thermal conductivity due to the air hole group structure. Avoids the spread of air.
    Because 97%-98% of foam insulation board space is crammed with air, have adequate capacity to safeguard the impact of the outside world and by altering the shape, good anti impact ability;
    Low water absorption. Studies have shown that moisture will influence the thermal and mechanical properties, low water absorption materials help to maintain these properties;
    Biodegradable. The recuperation degree is the highest in plastics;
    The manufacture process does not apply Freon;
    The full life cycle power consumption of plastics in the lowest possible.

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