Guide · insulation
R-values explained: insulation per material and thickness
You calculate the R-value of insulation by dividing the thickness in metres by the material's lambda value (R = d / λ). The higher the result, the better a wall, roof or floor insulates. This guide gives the lambda values of the 6 most-used insulation materials, a multi-layer calculation example, and what this means for your VEKA entry and E-level.
Author: Dieter Vanwetter, certified Belgian Type A energy expert (VEKA EP18784) ~8 min read Updated 6 September 2026
Summary
- The R-value (thermal resistance) is calculated as R = thickness divided by lambda, in m²·K/W.
- PUR and PIR give the highest R-value per centimetre, mineral wool insulates slightly less per cm but has better fire resistance.
- Minimum R-values per building element apply to new-build and major renovations in Flanders.
- When in doubt between two R-values, always use the lowest, most conservative one for your VEKA entry.
Contents
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.
What are the 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 the highest R per cm, useful for limited space. Mineral wool: better fire resistance.
What are the minimum R-values under EPB standards?
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.
How do you calculate a composite R-value with 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, so U = 0.159 W/m²·K.
Where do you find R-values in practice?
- Product label on insulation board, the nameplate scanner recognises it automatically
- Construction file, technical sheets
- Architect's execution plans
- Visual measurement on-site + default lambda from VEKA tables
- VEKA default values when no source available
What is the 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 the most conservative (lowest) R-value. EPC-Expert Pro's auto-calculations always show the formula and source used.
FAQ about R-values
What is the difference between an R-value and a U-value?
The R-value describes the thermal resistance of a single material or layer. The U-value is the thermal transmittance of the entire building element, including inside and outside air resistances. R and U are each other's inverse: U = 1 divided by the sum of all R-values.
What insulation thickness do you need for R = 4?
That depends on the material. With PIR (lambda 0.022), about 9 cm is enough. With glass wool (lambda 0.032), you need about 13 cm. Calculate it yourself with R = thickness divided by lambda.
Can you estimate the R-value without a technical sheet?
Yes, with a visual measurement on-site combined with the default lambda from VEKA tables. This generally gives a more conservative, so higher, estimated R-value than a manufacturer's sheet.
Let R-values calculate themselves
In EPC-Expert Pro you enter material + thickness and R-value is calculated automatically. You can always override manually.