What’s the difference between XPS insulation and EPS insulation, other than a single letter? For installation on concrete foundations and under floor slabs, the rigid foam insulation you choose can make a difference of tens of thousands of dollars on a project. A careful evaluation of these materials’ performance attributes against the project’s needs can dramatically shrink labor and material costs. The savings could mean the difference between a profitable job and one you just have to chalk up to experience.

When it comes to concrete and insulation, contractors tend to be most familiar with extruded polystyrene (XPS). Yet, expanded polystyrene (EPS) performs as well or better than XPS, and at a substantially lower cost. Three important factors to consider when comparing these two insulations for any belowgrade or under-slab application are compressive strength, moisture retention, and insulating capability.

Compressive strength

The weight of concrete slabs and foundation backfill may imply that the highest strength insulation makes the most sense. For many jobs, though, products with less compressive strength are more than sufficient and can reduce insulation costs. For example, installing a below-grade insulation rated for 100 psi, when only 40 psi is needed, will nearly double the material cost.

Confirm with your engineer what strength is needed. Many designers mistakenly assume that loads placed on slabs transfer through to the underlying insulation and soil at a 45-degree angle instead of a more uniform distribution. This can result in greatly over-designing the insulation’s strength. For example, with a forklift load on a typical 4-inch-thick slab, one calculation would indicate that 32 psi is applied to the insulation compared to only 2.5 psi under a more accurate calculation. As such, it is not uncommon for under slab insulation to be over-designed by a factor of 10 or more.

Using an unnecessarily high-strength insulation, you end up paying for something you don’t really need. Since EPS is lower cost per inch than XPS, and is available in a range of compressive strengths (from 10 to 60 psi), using it belowgrade and under slabs can save on insulation costs.

Moisture retention

A common question with rigid foam insulations is how well it resists water. A number of studies show that EPS retains less moisture than XPS. A case in point is a side-by-side analysis of the two rigid foam types installed on a commercial building foundation in St. Paul, Minn. When extracted and tested after 15 years in service, the EPS had 4.8% moisture content by volume, compared to 18.9% for the XPS (a four-fold difference). The testing lab also found that XPS holds water longer than EPS. After 30 days of drying time, the XPS still had elevated moisture of 15.7%, while the EPS had dried to 0.7%.

For installations where the insulation will be exposed to large amounts of water or frequent wetting, rigid foam insulation is available with water-resistant facers or pre-cut drainage grooves. Insulation with polymeric laminate facers keep water from entering the insulation and also provide an added barrier to water wicking or diffusing through foundations and slabs.

Additionally, rigid foam insulation drainage boards help reduce the hydrostatic pressure of the backfill on foundation walls. Such boards have regularly spaced channels covered with a filtration facer to keep the channels clear. Such boards are effective at channeling water away from the foundation face, and can drain up to five gallons per minute per foot.

Insulating capability

Moisture resistance is also important for belowgrade and under-slab insulation, since wet products provide much lower thermal resistance. The side-by-side insulation comparison highlighted previously found the EPS retained 94% of its specified R-value, while the XPS lost nearly half of its insulating capability over the 15 years the materials were on the foundation.

In addition to higher moisture resistance, EPS also is not subject to thermal drift. This means that its R-value stays the same over time. By comparison, the manufacturing process for XPS uses blowing agents that diffuse from the foam’s cellular structure over the life of the product, thereby reducing its thermal performance. EPS manufacturers typically warrant 100% of the published R-value for 20 years or more, while common XPS warranties cover just 90% of the published R-value.

Whether selecting EPS or XPS insulation, to ensure performance, confirm that the product was manufactured to meet the requirements of ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. This standard provides a key quality check on rigid insulation.

As insulation becomes increasingly common on foundations and under slabs, understanding the performance and cost factors of the different materials is important for successful bids and profitability. EPS offers a number of advantages over the more commonly installed XPS, including having the highest R-value per dollar among rigid insulations, making it a cost effective choice for many jobs.

Ram Mayilvahanan is product marketing manager for Insulfoam, a U.S. manufacturer of engineered EPS insulation products. Visit www.insulfoam.com for more information.