January 22, 2014

When it comes to foam insulation for commercial building foundations, the strongest and most expensive material isn’t always the best option.

Most builders are familiar with both XPS (extruded polystyrene) and EPS (expanded polystyrene), but it’s not always obvious which one you should use on a project. In fact, the best way to decide is based on which one has the best qualifications for the building you’re working on.

XPS has a lot of advantages: It has a high comprehensive strength and is able to withstand high stress and tensions, making it great for floor insulation even in warehouse settings. But EPS is less expensive, retains less moisture, and retains its insulating capacity well. These are all important factors to consider when insulating your building’s foundation.

Start by talking with your engineer and designer to confirm what strength is needed. Usually, loads placed on slabs will be distributed relatively uniformly, so make sure engineers haven’t overdesigned assuming that loads distribute at a 45-degree angle. Often, they will have overdesigned to a factor of up to 10 times higher psi than is really needed. This could be the difference between using EPS, which is much less expensive per square inch and XPS.

If you have determined that either EPS or XPS will bear the load of the building, the next thing you should consider is moisture retention. EPS often retains less moisture. A study in St. Paul, Minnesota, determined that EPS held just 4.8% moisture after 15 years, while XPS held 18.9%. In colder climates where the ground is frequently freezing and thawing, moisture retention can be a major issue. This year’s Chicago winter could make a huge difference in moisture retained by insulation.

Retaining moisture can also reduce insulations R-value. To ensure that the building you build maintains proper insulation, first confirm that the insulation was manufactured to meet the requirements of ASTM C578 Standard Specification for Rigid, Cellular and Polystyrene Thermal Insulation, a key quality check on rigid insulation.  Then, consider the climate or area in which the insulation is being installed. Wet products won’t insulate as well over time, so in an area where the ground is frequently wet, it’s best to install EPS, which is shown to maintain 94% of its R-value over a 15-year period, instead of XPS, which loses about half of that value over 15 years.  Warranties also reflect this—EPS manufacturers guarantee 100% R-value after 20 years, while XPS warranties 90%.

While builders often assume that the strongest material will provide the best insulation, that’s just not always true. Finding insulation that will retain is qualities in the climate you build in, and be appropriate for the load of your building, is the way to go. Choose the smartest, not always the strongest, when insulating your next building.