Comparing Polystyrenes: Looking at the differences between EPS and XPS

by Katie Daniel | September 3, 2016 10:00 am

by Jason Burgess
Insulation is a critical component to specify when designing a functional, cost-effective, and energy-efficient building. One method to insulate a building is by installing 50 to 152 mm (2 to 6 in.) of rigid foam insulation on the exterior side of the wall framing. Two of the most frequently installed types of rigid foam insulation are expanded and extruded polystyrene (EPS and XPS). Both serve the same basic function: providing a means to manage the passage of heat in a building system. However, they differ in important ways.

The primary responsibility of any insulating construction material is to offer positive thermal performance. However, this is not the only factor to account when specifying a rigid foam insulation material. It is also critical to know how it will perform under several situations.

XPS is manufactured in a continuous extrusion process that produces a closed cell form of foam insulation. EPS, on the other hand, is manufactured by expanding spherical beads in a mold and then using heat and pressure to fuse the beads together.

Each product has proponents claiming one out performs the other. However, it is key to understand each product may be more suited for a particular use than the other. This can be made clearer by examining each product’s thermal and moisture protection, fire and water resistance, and implications for sustainably designed projects.

Thermal and moisture protection
R-value is a measure of a material’s resistance to heat transfer. The higher the R-value, the better the material can insulate. The usual procedure for testing a material’s R-value is ASTM C518, Standard Test Method for Steady-state Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. This test method requires a technician to measure the thermal resistance of a specimen placed between a cold plate and a hot plate.

Rigid foam insulation delivers excellent R-values for such a thin product, but not all rigid foam offers the same thermal performance. The choice of  the insulation should be made after considering the effect its characteristics will have on the performance of the walls.

EPS is the insulation used most widely in insulated concrete forms (ICFs), structural insulated panels (SIPs), and exterior insulation and finishing systems (EIFS). It has the lowest average R-value of rigid foam insulation, typically R-4 per 25 mm (1 in.). The actual R-value of EPS depends on its density, with higher-density foams having higher R-values ranging from about 3.6 to 4.2 per 25 mm. Less-expensive EPS—typically sold at home improvement warehouse stores—is 0.4 kg (1 lb) density per 0.02 m³ (1 cf), appropriately called Type-I density EPS. Type-I products typically offer about R-3.9 per 25 mm or R-7.8 at 50 mm (2 in.).

However, Type-II EPS, rated at 0.6 kg (1.5 lb) nominal density, has an R-value between R-4.15 and R-4.2 per 25 mm. A 50-mm thick sheet would be R-8.3 to R-8.4. Type-II EPS is what most distributors will ship unless otherwise specified. In fact, many contractors refer to Type-II EPS as ‘standard density,’ not ‘high-density.’ (This information comes from the Green Building Advisor, 2015 edition in the Forum and can be found at www.greenbuildingadvisor.com.[1])

XPS, at about R-5 per 25 mm, has only a slightly better thermal performance than EPS. The thermal insulation performance of EPS and XPS in identical densities is quite close. However, EPS with the same level of density is less expensive. XPS is usually avoided in areas where materials with less density are needed or where the material, which is not produced below a certain density, is not applicable. In such a construction case, use of EPS as a less-dense material would provide the required insulation at a much lower cost.

Perm rating comparison
A ‘perm rating’—short for ‘permeance’—is a standard measure of the water vapor permeability of a material. The higher the number, the more easily gaseous water can diffuse through the material. When using XPS insulation in wall assemblies, the perm rating drops from 1.1 to 0.7 to 0.6, as the thickness goes from 25 to 50 to 75 mm (1 to 2 to 3 in.). A material with a lower perm rating is better at retarding movement of water vapor. If the perm rating is low, the material is considered a vapor retarder. If it has a very low perm rating, it is labeled a ‘vapor barrier.’ It all ties in with substrate longevity.

The general rule is the better the vapor barrier and the drier the conditions the less venting required. In colder regions, vapor barriers should be installed on the warm-in-winter side of walls, while in humid areas, such as the Gulf Coast and Florida, it should be placed on the exterior walls. A vapor barrier on the warm side should be constructed with a venting path on the cold side of the insulation because no vapor barrier can keep all water out of a structure.

A perm rating of less than 0.1 is considered a Class I impermeable vapor retarder and is classified as a ‘vapor barrier.’ A rating between 0.1 and 1 is a Class II semi-permeable vapor retarder, and a perm rating between 1 and 10 is a Class III permeable vapor retarder. Any product with a perm rating greater than 10 is highly permeable and is not considered a vapor retarder. Unfaced 25-mm (1-in.) thick XPS has a perm rating around 1, and is measured as semipermeable. The perm rating for EPS is 5. More information is available from the U. S. Department of Energy (DOE) about vapor barriers and vapor retarders.

XPS is manufactured in both an unfaced form or with different plastic facings. However, XPS is considered a vapor retarder, not a vapor barrier.

Although higher densities of EPS have a greater compressive strength than lower densities, EPS is never as strong as XPS and is more susceptible to crumbling at the edges and to other jobsite damage, which is why EPS is rarely used for wall sheathing.

When applied as exterior wall insulation over sheathing, EPS should be installed over a water resistive barrier (WRB), such as house wrap. This type of rigid foam is usually not made with a facer, meaning workers must handle it with extra care.

Insulation and fire performance
Reduced thermal capability at elevated temperatures is one example of how these insulations differ. EPS will soften at a temperature of only 73 C (165 F), which will reduce its thermal performance capability. At 100 C (212 F), EPS begins to melt and drip, which can result in the complete loss of the insulation’s thermal efficiency. According to the EPS Industry Alliance (EPS-IA), under certain fire conditions, the material ignites when exposed to an open flame. The temperature for transfer ignition is usually around 360 C (680 F).

Although foam insulation is fairly difficult to ignite, the burning would readily spread over the exposed surface of EPS and continue to burn until the material is consumed. EPS is an oil-based product and its burning generates a heavy, black smoke that results in the production of harmful gases, including carbon monoxide (CO), monostyrene, hydrogen bromide (a corrosive acid), and other aromatic compounds.

This reaction to flame is also noted on the EPS industry organization website[2]:

When burning, expanded polystyrene behaves like other hydrocarbons such as wood and paper. If EPS is exposed to temperatures above 100 C (212 F), it begins to soften, contract, and finally to melt. At higher temperatures, gaseous combustible products are formed by the decomposition of the melt. Whether these can be ignited by a flame or spark depends largely on the temperature, duration of exposure, and air flow around the material (i.e. oxygen availability).

Conversely, XPS, an insulating foam category called thermoplastics, is formed from non-cross-linked polymers and is able to be reheated and remolded. This makes XPS less rigid and pliable when exposed to a temperature of about 73 C. XPS insulation meets its melting point typically between 93 and 98 C (200 and 210 F). However, in extreme infernos, it too will be consumed by fire and emit noxious fumes.

As of last year, the European Union (EU) has banned hexabromocyclododecane (HBCD)—the brominated flame retardant used in all polystyrene building insulation, including EPS and XPS.

Considerable funds have been invested in developing a next-generation flame retardant for polystyrene insulation. The big question is whether the replacement flame retardants being considered are halogenated compounds (i.e. containing bromine or chlorine). Chemist and Environmental Building News advisory board member Arlene Blum, PhD, a leading expert on health and environmental hazards of halogenated flame retardants, is pleased with the ruling.

Sticking with a halogenated compound “Could mean we’re moving from one toxic to another,” Blum said. She suggests we should look at the bigger questions about flame resistance and safety. “It’s time to ask what the fire safety benefits of these flame retardants are.”

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 www.buildinggreen.com[3].) 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 www.espindustry.org[4].)

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 marketing@gms.com[5].

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