What is the insulation value (Rd value)?
The Rd value (thermal resistance) indicates how well a material or construction resists heat transfer. The higher the Rd value, the better the insulation. The value is expressed in m²·K/W (square metre Kelvin per Watt). It is the key metric for comparing insulation materials.
Rd value vs. lambda value
| Term | Meaning | Unit |
|---|---|---|
| Lambda value (λ) | Thermal conductivity coefficient of the material itself | W/m·K |
| Rd value | Insulation value of a specific thickness of material | m²·K/W |
The Rd value is calculated as: Rd = thickness (in metres) / lambda value
Example: PIR insulation with λ = 0.023 and thickness 100 mm: Rd = 0.100 / 0.023 = 4.35 m²·K/W
Requirements (Bouwbesluit new build)
| Building element | Minimum Rc value |
|---|---|
| Wall | 4.7 m²·K/W |
| Roof | 6.3 m²·K/W |
| Floor | 3.7 m²·K/W |
> Rc is the total thermal resistance of the entire construction (all layers added together). Rd is for a single material layer.
For existing buildings and DIY projects these requirements do not strictly apply, but it is wise to aim close to them.
Insulation materials compared
| Material | Lambda (λ) | Rd at 100 mm | Application |
|---|---|---|---|
| PIR | 0.023 | 4.35 | Roof, floor, wall |
| PUR | 0.025 | 4.00 | Spray foam insulation |
| EPS (expanded polystyrene) | 0.032 | 3.13 | Floor, cavity |
| XPS | 0.035 | 2.86 | Floor, frost apron (compression resistant) |
| Mineral wool | 0.035 | 2.86 | Wall, roof, cavity |
| Glass wool | 0.035 | 2.86 | Wall, roof |
| Wood fibre insulation | 0.040 | 2.50 | Ecological construction |
PIR delivers the highest Rd value per centimetre — ideal when space is limited.
Insulation for DIY projects
Garden house as workspace
- Walls: minimum Rd 2.5 (90 mm mineral wool)
- Roof: minimum Rd 3.5 (80 mm PIR)
- Floor: minimum Rd 2.5
Garage (heated)
- Walls: Rd 2.5-3.5
- Roof: Rd 3.5-4.5
- Floor: difficult to retrofit, consider underfloor heating with PIR
Knee wall
- Minimum Rd 3.5 (100 mm mineral wool or 80 mm PIR)
Related terms
- Mineral wool
- PIR insulation
- Breathable membrane
- Vapour barrier
- Timber frame construction