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A Specific Comparison: EPS And XPS

Insulation is a critical component to specify when designing a functional, cost effective, and energy efficient building. One method of insulating a building is by installing 2 to 6 inches 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 Polystyrene (EPS) and Extruded Polystyrene (XPS). Both EPS and XPS serve the same basic function: to provide a means to manage the passage of heat in a building system.

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

What Are the Differences?

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

Each product has proponents who will claim that one out performs the other. The key to understanding these claims is the realization that in certain instances, one may be more suited for a particular use than the other. With detailed examination it is much clearer to see how each product performs in terms of thermal and moisture protection, fire resistance and its value as a “green” building material.

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.

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 1” to 2” to 3”. Material with a lower perm rating is better at retarding the movement of water vapor. If the perm rating is low, the material is considered a vapor retarder. If it has very low perm rating, it is labeled a vapor barrier. It all ties with substrate longevity.

There are fundamental differences between the properties of extruded and expanded polystyrene (XPS and EPS). Knowing these are essential to determining which is a better choice for wall applications facing moisture.

The general rule is the better the vapor barrier and the drier the conditions, less venting is 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. This is because no vapor barrier can keep all water out of a structure, most commonly from rain.

A perm rating of less than 0.1 is considered a Class I impermeable vapor retarder and considered a vapor barrier. A rating between 0.1 and 1 is considered 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 not for use as a vapor retarder. Unfaced 1-inch-thick XPS has a perm rating around 1, and is rated it semipermeable. The perm rating for EPS is 5. More information is available from the U. S. Department of Energy about vapor barriers and vapor retarders. XPS is manufactured in both an unfaced form or with different plastic facings. However, either way 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 job-site damage. That’s 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, which means workers must handle it with care.

Water Resistance

There are fundamental differences between the properties of XPS and EPS, which are essential to determining which is a better choice for wall applications that require 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 the job.

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 together 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 noting that most EPS products sold at home improvement warehouses do absorb much more water than XPS products.

The Green Factor

A building’s “green” rating is also of concern for both owners as well as those in the design-build sector. While both XPS and EPS have green features, one point to consider is the blowing agent used to create the foam.

EPS is often produced with Pentane, which has a very low Global Warming Potential. XPS uses hydrochlorofluorocarbons (HCFCs) as the blowing agent, which has a high Global Warming Potential. XPS has shown to lose some of its R-value over time, as this gas slowly escapes.2 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 of the newer type, consult the manufacturer’s Safety Data Sheet (SDS).

EPS is considered an excellent choice for green building designs because it offers the environmental advantages of energy efficiency, recyclable content, resistance to mold, and indoor environmental quality. Both EPS and XPS can help meet green building localized manufacturing goals, which helps to reduce the impact of transportation. With hundreds of plant locations in North America, they are usually available from a local manufacturer.

Its use in innovative applications improves the performance of a building’s envelope. Furthermore, during installation EPS applications have shown to consistently reduce both jobsite waste and labor costs. 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 used to produce non-foam insulating materials. In one such study, conducted by the EPS Industry Alliance, when compared to fiberglass insulation, the energy required to produce foam insulation is 24 percent less than what is needed to make the amount of fiberglass needed to achieve an equivalent R-value at a representative volume.

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