1. What exactly is an R-value?
The R-value (thermal resistance) indicates how much heat a material retains. Unit: m²·K/W. Higher R = better insulation.
Formula: R = d / λ
- d = thickness in metres
- λ (lambda) = thermal conductivity in W/m·K
Rockwool 12 cm with lambda 0.035: R = 0.12 / 0.035 = 3.43 m²·K/W.
2. Lambda values of the 6 most-used insulation materials
| Material | Lambda (W/m·K) | R at 14 cm |
|---|---|---|
| PUR (rigid foam) | 0.023 | 6.09 |
| PIR (rigid foam) | 0.022 | 6.36 |
| EPS (polystyrene) | 0.036 | 3.89 |
| Rockwool | 0.035 | 4.00 |
| Glass wool | 0.032 | 4.38 |
| Cellulose | 0.038 | 3.68 |
PUR/PIR give highest R per cm — useful for limited space. Mineral wool = better fire resistance.
3. Minimum R-values per EPB standards (2026)
For residential new-build and major renovations in Flanders:
- Roof or attic: R ≥ 4.5 m²·K/W
- External wall: R ≥ 2.0 m²·K/W
- Slab on grade: R ≥ 2.0 m²·K/W
For minor renovations: just record actual R in VEKA. E-level will be higher, but no penalty.
4. Composite R-value: multiple layers
A wall has multiple layers. R_total = R1 + R2 + R3 + ... + R_air_inside + R_air_outside.
Example cavity wall: plaster (0.029) + brick (0.519) + PIR 12cm (5.455) + facing brick (0.100) + air (0.17) = R = 6.27 → U = 0.159 W/m²·K.
5. Where to find R-values in practice?
- Product label on insulation board
- Construction file — technical sheets
- Architect's execution plans
- Visual measurement on-site + default lambda from VEKA tables
- VEKA default values when no source available
6. Impact of R on E-level and EPC label
Higher R = less loss = lower E-level (better label).
- Each +1.0 R on wall/roof lowers E-level by roughly 5 to 10 points
- Label C (E180) house with roof insulation upgrade R=2 → R=5 can drop to E140, label B
When in doubt: use most conservative (lowest) R-value.