All three figures describe how easily heat passes through, but at three different scales: lambda describes a material, U a wall or panel of given thickness, and K the whole transport body. Their units differ too, so a lambda in one quote cannot be set against a K in another.
| Figure | What it describes | Unit | Where it comes from |
|---|---|---|---|
| Lambda (λ) | Thermal conductivity of a material | W/(m·K) | Material data sheet |
| U | Heat flow per area through a panel | W/(m²·K) | Calculation to ISO 6946 |
| K | The whole body, doors and joints included | W/(m²·K) | ATP test report |
Thickness is what turns lambda into U. An 80 mm foam layer with a lambda of 0.024 W/(m·K) has a thermal resistance of 0.08 / 0.024 = 3.3 m²·K/W, so before surface resistances are added its U is about 0.30 W/(m²·K). At 60 mm the same foam would give a U of 0.40.
K, by contrast, is measured. ATP takes the heating or cooling power needed to hold a constant temperature difference between inside and outside and divides it by that difference and by the geometric mean of the inner and outer surface areas. Corner joints, door seals and fixings are all inside that measurement and add losses on top of the wall's calculated U. The ATP limit is 0.70 W/(m²·K) for normally insulated (IN) bodies and 0.40 for heavily insulated (IR) ones; in a quote, ask for the body's measured K value and the test report behind it.
