**18-25 kg/m³** is the sweet spot. Below that the foam crushes too easily; above that it's needlessly heavy for the strength you get.
For specific applications:
- Park flyer wings (sub-1m span): 18 kg/m³ is plenty
- Aerobatic / pylon racers: 25 kg/m³ for the impact resistance
- Slope soaring (lots of crashes): 30-35 kg/m³ if you can find it
Most builder's-merchant EPS is unmarked — assume 15 kg/m³ unless it specifically says otherwise. "Roofmate" / "Floormate" branded XPS is usually 25-35 kg/m³.
For specific applications:
- Park flyer wings (sub-1m span): 18 kg/m³ is plenty
- Aerobatic / pylon racers: 25 kg/m³ for the impact resistance
- Slope soaring (lots of crashes): 30-35 kg/m³ if you can find it
Most builder's-merchant EPS is unmarked — assume 15 kg/m³ unless it specifically says otherwise. "Roofmate" / "Floormate" branded XPS is usually 25-35 kg/m³.