The Truth About Foam
Not all foam insulation holds up in the ground. Here's how Lexar EPS compares to the usual alternatives.
The 15-Year Below-Grade Study
XPS (pink and blue foam board) relies on blowing agents trapped in the foam for much of its insulating value. Those gases escape over time, a process called off-gassing. EPS insulates with trapped air, so there's nothing to escape.
In a 15-year field study by the EPS Industry Alliance, researchers dug up EPS and XPS that had been buried below grade in St. Paul, Minnesota. EPS kept 94% of its R-value. XPS kept 52% and held nearly four times as much water. Read the study (PDF)
Long-Term Thermal
EPS kept 94% of its R-value after 15 years in the ground: 81% more than XPS kept.
Water Absorption
In the study, EPS absorbed 4.8% water by volume. XPS absorbed 18.9%.
No Off-Gassing
Lexar relies on trapped air, not volatile blowing agents that escape over time.
Termiticide Built In
Treated with Preventol®. Termiticide is built in and protects your home from termites.
How Lexar Stacks Up Against the Other Options
vs. Polyiso
Polyisocyanurate (polyiso) is made for roofs and walls, not for the ground. Like XPS, it relies on blowing gases that escape over time, and its R-value drops as it gets cold, right when you need it under a slab.
vs. Spray Foam
Spraying closed-cell foam on gravel under a slab costs significantly more, takes a specialty crew, and comes out uneven in thickness. Rigid EPS gives you a known R-value across the whole pad.
vs. Nothing
Skipping it is the most common choice, and the most expensive one over the life of the building. Studies put up to 25% of a building's energy loss on uninsulated foundations and slabs. Once the concrete is poured, there's no adding it later.
The question we hear most
Why does Lexar cost more?
For the same reason a metal roof costs more than three-tab shingles, or good windows cost more than cheap ones. It's a better product, and it lasts.
The difference is that you can upgrade a roof or a window later. You can't upgrade what's under your slab without a jackhammer. That's why it's the one place on the build that's worth getting right the first time.