What is thermal conductivity (lambda value)?
Thermal conductivity, written as the Greek letter lambda (λ) and sometimes called the k-value, indicates how much heat a material conducts. The unit is W/(m·K): the number of watts flowing through a layer 1 m thick and 1 m² in area for a 1 degree temperature difference. The lower the lambda value, the better the material insulates.
Typical values
| Material | λ in W/(m·K) |
|---|---|
| PIR insulation | approx. 0.022 to 0.026 |
| EPS | approx. 0.030 to 0.038 |
| Mineral wool | approx. 0.032 to 0.040 |
| Softwood | approx. 0.13 |
| Brickwork | approx. 0.5 to 0.9 |
| Reinforced concrete | approx. 2 to 2.5 |
| Steel | approx. 50 |
| Aluminium | approx. 160 to 200 |
Always use the manufacturer’s declared value.
From lambda to R-value
The thermal resistance of a layer is R = thickness (m) / λ.
Example: 100 mm EPS with λ = 0.035 gives R ≈ 2.9 m²K/W. With PIR (λ = 0.023) about 67 mm achieves the same R. A lower lambda therefore means a thinner build-up for the same performance, useful where space is tight.
Tips for comparing insulation
- Use the declared value (λD) from the product’s declaration of performance.
- Compare cost per unit of R-value, not per centimetre.
- Workmanship counts: gaps and thermal bridges undo a good lambda on paper.
- Moisture: wet insulation conducts heat much better than dry. Keep it dry on site.
Related terms
- Thermal resistance
- Thermal insulation
- Thermal bridge
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