Sink or Swim: EPS Moisture Performance

R-TECH Below-Grade EPS Insulation Installation

Expanded polystyrene (EPS) rigid foam insulation is often specified for below-grade applications. While this is a valuable feature, one concern consistently raised by building professionals is moisture accumulation. After all, wet insulation is ineffective—materials that retain high moisture levels can lose up to half of their insulating R-value. So, how does EPS actually perform when it comes to moisture?

Lab and Field Tests

Moisture performance for rigid foam insulations—including XPS, EPS, and polyiso—is typically reported on product data sheets using ASTM C272, the standard test method for water absorption. In this test, insulation samples are submerged in water for 24 hours, then weighed immediately after removal to determine moisture absorption.

ASTM C272 results often show that EPS absorbs small amounts of moisture more quickly than other rigid foam insulations. Because of this, EPS moisture performance can sometimes be misunderstood when compared to XPS and polyiso. However, this test does not reflect real-world conditions—very few buildings experience continuous 24-hour submersion. More importantly, while EPS may absorb moisture faster, it also releases it much more quickly. ASTM C272 does not account for an insulation’s ability to dry between exposure cycles.

To better understand real-world performance, Stork Twin City Testing evaluated EPS and XPS insulation buried side by side for 15 years on a building foundation in St. Paul, Minnesota. When removed, the EPS was significantly drier—measuring 4.8% moisture by volume compared to 18.9% for XPS. After 30 days of drying, the EPS moisture content dropped to just 0.7%, while the XPS still retained 15.7%.

The long-term moisture retention of XPS is further supported by a 2012 report from the U.S. Department of Energy’s Oak Ridge National Laboratory. Researchers found that XPS insulation installed below grade for 15 years had absorbed 67% or more moisture. This resulted in a loss of thermal performance of 10% in deep basement applications and up to 44% in slab-on-grade installations.

Wrapping Up

Laboratory testing doesn’t always tell the full story when it comes to rigid foam insulation performance. While XPS and polyiso may absorb less water during short-term submersion, they tend to retain that moisture much longer than EPS. When evaluating below-grade insulation options, it’s important to look beyond lab data and consider real-world performance over time.

Choose Insulation That Performs in the Real World

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