Comparing Polystyrenes: Looking at the differences between EPS and XPS

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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 in a wall construction assembly delivers excellent R-values, but not all types of rigid foam offer the same thermal performance.

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

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.

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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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