How To Compare Moisture Performance of EPS vs. XPS?

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EPS and XPS are two common types of insulation made from the same base materials, resulting in similar polystyrene foam boards — but they are manufactured using two different processes.

When comparing the two, there is very little difference between EPS and XPS insulation when it comes to quality. Both are effective products, so when choosing which insulation is right for a project, the determining factor often comes down to a few important details.

Moisture resistance is one factor that should be considered when determining quality. EPS and XPS insulation are both moisture-resistant by nature, but is moisture performance a necessary aspect to consider?

We’ll look at how EPS and XPS insulation are designed to perform and determine whether moisture resistance is a necessary consideration. But first, let’s look at the composition of polystyrene.

What Is Polystyrene?

A common base material found in insulation, polystyrene is a versatile polymer used in making a range of plastic materials. Polystyrene is very moisture resistant in nature, and regardless of its manufacturing process, its moisture resistance stays the same.


EPS and XPS insulation both use a polystyrene base. The key difference is the process in which these insulation products are made.

Expanded polystyrene insulation (EPS) uses a blowing agent, steam and molds to expand the polystyrene into the final product. Extruded polystyrene insulation (XPS) uses blowing agents, heat, and an extruding machine to create the insulation sheets.

Both are closed-cell foam and have a similar cell structure and base materials. EPS does have small interstitial spaces resulting from the manufacturing process.

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When Does Moisture Matter?

So, when does moisture resistance really matter in terms of insulation?

The honest answer is: rarely, especially when talking about above-grade applications.

Most insulation applications are already heavily protected from moisture because they are under other structural materials, so moisture resistance is rarely a factor.

For walls and roofing applications, in particular, it is not a primary consideration. Roof and wall insulation are always protected by an exterior barrier like cladding or other types of covering. And underneath these exterior barriers often lies an extra interior layer of waterproof protection. Generally, insulation is installed underneath waterproof barriers which makes moisture resistance an unnecessary redundancy.

However, for below-grade applications, moisture resistance is definitely important; especially when working in areas known to have water present on site. When using EPS in below-grade applications, it is best to protect the insulation from constant contact with water to limit its absorption. This is accomplished by surface drainage away from buildings and drain tiles below grade

Moisture Testing

With an industry-standard test lab test measuring water content change by volume, the results are generally less than 2% change for EPS insulation when immersed in water for 24-hours. However, it’s important to note the insulation dries back to less than 0.3% change 24 hours after the insulation is removed from water. In comparison, XPS testing results at a 0.3% moisture absorption after immersion.

While XPS insulation has less moisture absorption than EPS in a test environment, the numbers do not equate to a real-world application difference in performance. Years of real-world research has shown the in-situ moisture performance of EPS and XPS to be very similar.

EPS Is the Right Insulation Choice for Your Next Project

EPS is a great insulation product that is comparable to XPS in real-world applications. For most purposes, including for walls, roofs and below-grade, EPS is an ideal insulation choice for a high R-value at a good price.

Click here to learn more about the different types of insulation offered by Atlas Molded Products and how they can be used in your next project. Or click here to talk to one of our reps about your specific project’s needs.