Work out EPS/XPS board count, volume and thermal resistance (R) based on area, thickness and chosen board size.
| Metric | Formula |
|---|---|
| Volume | Area × Thickness (+ margin %) |
| Board count | (Area + margin %) ÷ board area, rounded up |
| Thermal resistance (R) | Thickness (m) ÷ λ (W/mK) |
The higher the R-value, the better the thermal insulation. Building codes (e.g. for A++ class homes) often require R ≥ 5–7 m²K/W for walls or roofs — check the requirements for your specific project.
EPS (expanded polystyrene foam) is cheaper, used for walls and lightly loaded foundations. XPS (extruded polystyrene) is denser, more moisture- and pressure-resistant, used for foundations, terraces and roofs under higher load.
The graphite additive lowers thermal conductivity (λ) by about 15–20%, so you reach the same R-value with a thinner layer — useful when space is limited.
5–10% is standard — it covers cutting waste and joint overlap. For walls with complex shapes (many corners, windows), up to 15% is recommended.
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