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 standard measure for comparing insulation materials.
Rd-value vs. lambda value
| Term | Meaning | Unit |
|---|---|---|
| Lambda value (lambda) | Thermal conductivity 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 with: Rd = thickness (in metres) / lambda value
Example: PIR insulation with lambda = 0.023 and thickness 100 mm:
Rd = 0.100 / 0.023 = 4.35 m2-K/W
Requirements (Dutch Building Code, 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 up). 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 | Rd at 100 mm | Application |
|---|---|---|---|
| PIR | 0.023 | 4.35 | Roof, floor, wall |
| PUR | 0.025 | 4.00 | Spray insulation |
| EPS (expanded polystyrene) | 0.032 | 3.13 | Floor, cavity |
| XPS | 0.035 | 2.86 | Floor, frost apron (pressure-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 building |
PIR delivers the highest Rd-value per centimetre — ideal when space is limited.
Insulation for DIY projects
Garden house as a workshop
- 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 add later, 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