Short answer: both work under a slab. Pick by what your plans or engineer call for and the load on the slab. Higher-density EPS is a common choice; XPS is a dense board many builders already know.
What they are
- EPS (expanded polystyrene) is molded beads fused into blocks, then cut into sheets. It comes in several densities, so you can match firmness to the load.
- XPS (extruded polystyrene) is pushed out as a continuous board with a smooth skin.
How they compare under concrete
| EPS | XPS | |
|---|---|---|
| Densities | Several to choose from | Fewer options |
| Thickness | Cut to any thickness | Stock thicknesses |
| Moisture | Drains and dries well over time | Smooth skin sheds surface water |
How to choose
- Follow your plans. If they name a type or density, use it.
- For garage, shop or driveway slabs, go firmer (2 lb EPS or XPS).
- For basements and house slabs, 1.5 or 2 lb EPS is common.
- Lay a vapor barrier per your plans, keep the board flat on compacted base and tape the seams.
Typical R-value
| Density | R per inch |
|---|---|
| 1.0 lb EPS | R-3.9 |
| 1.5 lb EPS | R-4.2 |
| 2.0 lb EPS | R-4.4 |
Typical values at 75°F from the Rapid Insulation EPS data sheet (REV004). Ask us for the full sheet.
Get it cut
We carry both and cut EPS to the thickness you need in American Fork, Utah. See EPS foam sheets and XPS foam board.