Insulation materials come in many forms, and there is a seemingly endless array of options available for ICF homes and buildings, one of the most popular and effective options is rigid-foam insulation. But, there are still multiple options when it comes to rigid-foam insulation. Two options, however, tend to stand out as the most popular picks for builders: EPS, short for expanded polystyrene, and XPS, short for extruded polystyrene. These two forms of insulation have many things in common: they are both made of polystyrene resin, they both use trapped air as their insulating medium, both have a closed-cell structure, and both fall under the same manufacturing standard of the ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation.

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But, that is where the similarities between EPS and XPS end and the differences begin. While they are both made of the same polystyrene resin material, the way these two types of insulation get made differs significantly. EPS uses steam and a blowing agent—a substance used to produce cellular structures via a foaming process—called Pentane to expand the resin beads and then remold them into blocks, to later cut into the necessary sizes.

The processes for creating XPS insulation involves melting the resin through with an extruder and then expanding it, also through the use of blowing agents. These processes lead to the primary differences between EPS and XPS rigid-foam insulation: moisture resistance, cost per R-value, compressive strength, eco-friendliness, and compatibility.

    Moisture Resistance: EPS vs XPS Insulation

    The level of moisture resistance that these different types of insulations offer is critical, especially when it comes to a structure’s insulation. When the insulation product is not very moisture resistant, it is more likely to retain moisture and become damaged over time, which will decrease the effectiveness of the insulation product.  Additionally, by retaining moisture, an environment where mold and bacteria can grow is formed, which can cause a multitude of different health issues.

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    When comparing the moisture resistance of EPS and XPS, EPS has been shown to retain less moisture than XPS rigid-foam insulations. But, the methods companies use to test the moisture resistance of these products vary considerably, making it somewhat difficult to get a proper comparison between the two.

    Manufacturers of XPS rigid-foam insulation test their products by performing controlled, underwater tests. These tests occur by submerging the insulation material for short periods of time—usually somewhere between 2 and 24 hours at a time, depending on the type of XPS that is going through the testing process. When tested through this method, XPS is seen as the more resistant option, because it resists water absorption more than EPS. But, at the same time, while it takes in moisture at a slower rate, it also releases it more slowly. Meaning, any moisture that gets retained is present longer in XPS insulation.

    A real-life study that compared the two types of insulation was made by installing them in the foundation of a commercial building in St. Paul, MN. After being in place for 15 years, researchers extracted the insulation and began testing them for their moisture content. 

    The EPS insulation was found to have a 4.8 percent moisture content by volume compared to 18.9 percent found in the XPS portion. That equated to a massive four-fold difference in moisture content. Additionally, researchers found that the XPS insulation held the water longer than its EPS counterpart. After 30 days of being left to dry, the XPS still had moisture levels of around 15.7 percent, whereas the EPS insulation had almost entirely dried, retaining just 0.7 percent of its original moisture content.

    For situations where the insulation can face exposure to substantial amounts of water, there are types of rigid-form insulations that come with special facers that offer water resistance or pre-cut drainage grooves to help keep moisture from penetrating the insulation itself. These features can also be useful in reducing the hydrostatic pressure found in the backfill of foundation walls and can help to channel water away from the face of the foundation, draining up to five gallons per minute per foot of insulation.

    Cost per R-Value – EPS vs XPS Insulation

    The R-value of a product refers to the material’s resistance to heat transfer. The higher the R-value of the material, then the better it is at insulating. To test the R-value of rigid-foam insulation, companies use the ASTM C518 Standard Test Method. This method of testing requires a technician to measure the product’s thermal resistance when placed between a cold and hot plate.

    While all rigid-foam insulation offers excellent R-values, not all types provide the same level of thermal performance.

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    EPS insulation is the preferred insulation option by builders for insulated concrete forms, structural insulated panels, and exterior insulation and finishing systems. It has the lowest R-value rating at just R-4 per inch. However, its actual R-value of foam insulations varies, depending on the density of each sheet of insulation. The higher the density of the product, then the higher the R-value will be.

    When you compare EPS and XPS insulation at the same R-values, EPS insulation can be between 10 to 30 percent less expensive and costs about 19 cents per sq. ft. for a single, 1-inch 4×8 sheet.

    Compressive Strength – XPS vs EPS Insulation

    The compressive strength of insulation is the measure of how well it stands up to pressure before it begins to break. Both EPS and XPS come in compressive strength levels ranging between 10 and 60 psi. XPS also comes in 100 psi options. Because going with higher rated psi materials can end up costing more, it is essential to be clear about what you need. In many cases, products with lower compressive strength are sufficient in insulating the building and will, therefore, save more money.

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    Additionally, since EPS insulation has a lower cost per square inch and comes in multiple compressive strengths, it is a much more cost-effective option. However, again, if a project requires 100 psi insulation, then XPS is the only viable option.

    Eco-Friendliness – XPS vs EPS Insulation

    With more focus on green construction, it is essential to consider it in every factor of a building, including the type of insulation. Both EPS and XPS insulation have eco-friendly benefits, but the blowing agent used in the creation process. EPS, for example, is made using a blowing agent known as Pentane. This agent has a very low Global Warming Potential. Moreover, EPS insulation does not contain any dyes and can be made using up to 15 percent of recycled content.

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    XPS, on the other hand, is made using a blowing agent known as hydrochlorofluorocarbons (HCFCs), which have a very high Global Warming Potential. Not only that, but HCFC is fire retardant and burns very quickly, releasing dangerous gasses such as CO2. On top of that, XPS tends to lose its R-value over time, as the gas trapped inside begins to slowly escape, a process known as off-gassing. Because of all of this, manufacturers of XPS insulation have begun making a move to other types of blowing agents that have Zero Ozone Depleting formula. XPS insulation also uses dyes to help distinguish it from their competition, which is not environmentally friendly.

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    Compatibility – XPS vs EPS Insulation

    For contractors working with a building made of concrete, XPS tends to be the preferred form of rigid-foam insulation. However, EPS insulation has proven to perform just as well in such situations.

    Additionally, EPS is usually present in structured insulated panels. In general, both EPS and XPS are excellent options when looking for exterior wall insulation and can work well with a wide range of other building materials, when installed correctly.

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    EPS and XPS both have a lot to offer builders and contractors in regard to dependable insulation. But, the best option will be dependant on the needs and desires of the job in terms of moisture resistance, R-value, compression strength, eco-friendliness, and compatibility. It is also imperative that, regardless of which option a contractor chooses to go with to complete the job, that the insulation meets the manufacturing requirements set forth by the ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation, which provides an essential quality check on rigid insulation before it reaches the market.

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    Matt Greenfield

    Matt Greenfield is an experienced writer specializing in home improvement topics. He has a passion for educating and empowering homeowners to make informed decisions about their properties. Matt's writing focuses on a range of topics, including windows, flooring, HVAC, and construction materials. With a background in construction and home renovation, Matt is well-versed in the latest trends and techniques in the industry. His articles offer practical advice and expert insights that help readers tackle their home improvement projects with confidence. Whether you're a DIY enthusiast or a seasoned professional, Matt's writing is sure to provide valuable guidance and inspiration.

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