In this blog, you’ll find a clear and simple explanation of what these insulation values are, how to calculate them, and which values are considered good for your roof, wall, or floor.
Overview: insulation values at a glance
| Term | Meaning | Unit | Good to know |
|---|---|---|---|
| λ-Value |
Thermal conductivity of a material (how well a material conducts heat) |
W/m·K |
The lower, the better. PIR ≈ 0.022 W/mK is excellent. |
| Rd-Value |
Thermal resistance of a single insulation layer |
m²K/W |
Determined by thickness and λ‑value: Rd = d / λ |
| U-Value |
Thermal transmittance of the entire construction (all layers combined) |
W/m²K |
The lower, the better. Used in EPB regulations. |
What is the λ‑value (lambda value)?
The λ‑value, or lambda value, indicates how well a material conducts heat. The lower the λ‑value, the better the material insulates. The unit is W/m·K (watts per meter per kelvin).
Examples:
- PIR: λ ≈ 0.022 W/mK (excellent)
- Glass wool: λ ≈ 0.035 W/mK
- Concrete: λ ≈ 1.7 W/mK (poor insulation)
The λ‑value is a material property, meaning it does not change with the thickness of the material.
Tip: the lower the λ‑value, the less thickness you need to achieve good insulation. That’s why PIR performs so well where space is limited.
What is the Rd‑value?
The Rd‑value indicates how much thermal resistance an insulation product provides. It is therefore a product property, dependent on both the thickness and the λ‑value.
Formula: Rd = d / λ (d = thickness in metres, λ = thermal conductivity)
Example:
10 cm (0.10 m) PIR with λ = 0.022 W/mK → Rd = 0.10 / 0.022 ≈ 4.55 m²K/W
The higher the Rd‑value, the better the product insulates.
What is the U‑value and how does it differ from the Rd‑value?
The U‑value represents the insulation performance of the entire construction (insulation + structural elements + finishes). It is calculated from the total thermal resistance (Rtot):
U = 1 / Rtot
Difference:
- Rd = thermal resistance of a single insulation layer (e.g. an insulation board).
- U = overall thermal performance of the complete roof, wall, or floor.
In Flanders, EPB regulations work with minimum Rd‑values for insulation products and maximum U‑values for the full construction. Rc‑values, as used in the Netherlands, do not apply here.
How do you calculate the insulation value yourself?
Do you want to know whether your insulation meets the required standards? Then use the formula Rd = d / λ.
Assume:
- PIR board of 80 mm (0.08 m)
- λ = 0.022 W/mK
Rd = 0.08 / 0.022 ≈ 3.64 m²K/W
This allows you to check whether you meet the minimum Rd‑value and whether the construction as a whole achieves the required U‑value.
Which insulation values apply in Flanders in 2025?
Minimum Rd‑values (insulation material):
- Roof ≥ 4.5 m²K/W
- Façade ≥ 3.0 m²K/W
- Floor ≥ 2.0 m²K/W
Maximum U‑values (construction):
- Roof ≤ 0.24 W/m²K
- Façade ≤ 0.24 W/m²K
- Floor ≤ 0.30 W/m²K
Anyone aiming for an energy‑efficient new build or renovation should target higher Rd‑values (and therefore lower U‑values).
Frequently asked questions
1. What is the difference between Rd and the U‑value?
Rd is the thermal resistance of a single insulation layer. U is the insulation performance of the entire construction.
2. What is a good Rd‑value?
That depends on the application. For a roof, it’s best to aim for Rd ≥ 4.5 m²K/W. The higher, the better.
3. Which λ‑value is considered good?
The lower the λ‑value, the better. Materials with a λ around 0.022 (such as PIR) perform exceptionally well.
4. How do I know if my insulation meets the standards?
Use the formula Rd = d / λ and compare the result with the required Rd‑value for your roof, façade, or floor.
Conclusion: what does this mean for your project?
If you choose an insulation material with a low λ‑value, you need less thickness to achieve a high Rd‑value. PIR insulation from Idelco Insulation has a λ‑value of 0.022 W/mK and therefore offers excellent performance for roof, wall, and floor applications.
Want to know which board is best suited for your project?
Visit our product page or request expert advice with no obligation.
