What the λ-value tells you
Thermal conductivity λ (lambda) describes how much heat flows through a one-metre cube of a material when there is a one-degree temperature difference between two opposite faces. The unit is W/(m·K). In practice a simple rule is enough: the lower λ, the better the material insulates. A material with λ 0.035 insulates twenty times better than one with λ 0.70.
That is why 14 cm of insulation achieves more than 60 cm of brick. The thermal resistance of a layer is R = d / λ. For 14 cm of insulation with λ 0.035 that is 4.0 m²K/W, for 60 cm of solid brick with λ 0.68 only 0.88 m²K/W.
Typical design values
The table below lists typical values according to the German standard DIN 4108-4 and EN ISO 10456. They are also built into the U-value calculator. For a formal proof, the value of the actual product always applies.
| Material | λ [W/(mK)] |
|---|---|
| Reinforced concrete | 2.3 |
| Natural stone (sandstone) | 2.3 |
| Cement screed | 1.4 |
| Lime / lime-cement plaster | 1.0 |
| Calcium silicate brick (density 1800) | 0.99 |
| Solid brick (density 1800 / 1600) | 0.81 / 0.68 |
| Perforated brick (density 1200) | 0.50 |
| Gypsum plaster | 0.51 |
| Plasterboard | 0.25 |
| Aerated concrete (density 500) | 0.16 |
| Softwood, OSB | 0.13 |
| Insulating brick, filled (typical) | 0.07–0.12 |
| Wood fibre board | 0.038–0.050 |
| Mineral wool | 0.032–0.040 |
| EPS board | 0.031–0.040 |
| Cellulose, blown-in | 0.038–0.040 |
| PUR/PIR board | 0.022–0.028 |
Density: heavy means poor insulation
For masonry, λ depends strongly on density. A heavy solid brick at 1,800 kg/m³ conducts heat much better than a light perforated brick, because pores and holes trap air. Modern insulating bricks are also filled with insulation and reach values below 0.10. That is why new single-leaf walls without external insulation can achieve U-values around 0.2, while old solid brick walls lie between 1.2 and 1.8.
Conductivity class and label
On insulation products you usually find a thermal conductivity class or a λ-value on the label. In Germany, class 035 (WLS 035) means λ 0.035 W/(m·K). What matters is the difference between the declared value, which the manufacturer states under the European product standard, and the design value used for calculations in Germany. For products without additional approval a safety margin is added to the declared value; for products with a national technical approval the margin is smaller. For a realistic calculation, use the design value from the product documentation.
Thickness versus λ: what is more economical?
An insulation material with λ 0.024 needs just over two thirds of the thickness of a λ 0.035 material for the same U-value. That is valuable where space is limited, for example when renovating a roof from the inside, at terrace junctions or window reveals. Where space is no issue, the cheaper material in a slightly greater thickness is usually more economical. Fire protection, sound insulation, moisture behaviour and summer heat protection also play a role. Wood fibre and cellulose, for example, store more heat than rigid foam and so delay overheating under the roof.
Moisture worsens λ
All table values apply to dry materials in their normal moisture state. Wet insulation or damp masonry conducts heat much better, because water conducts about twenty times better than still air. A damp basement or a leaking roof membrane therefore worsens the actual U-value considerably. Before any insulation work, clarify where moisture could come from.
How to calculate the U-value of a whole element from λ-values is shown in the pillar article Calculating the U-value. In the calculator you can enter a custom λ-value for products with a different value.