The figure heard most often about insulation is wall thickness, yet thickness is an input and K is the result. The ATP text defines K for the whole body: the heating or cooling power W needed to hold a steady temperature difference ΔT inside is divided by the mean surface S and by ΔT, so K = W / (S × ΔT), in W/(m²K). Thickness, material, corners, the door surround and joints all end up in that one number.
Lambda is the thermal conductivity of a material, in W/(mK). A panel U value describes the heat transfer of a particular panel. Neither covers the thermal bridges at doors, corners and joints, so a panel U value used in place of the whole-body K makes a body look better than it is. In a report, look for the following:
| Look for in the report | What it describes | Comparison question |
|---|---|---|
| K, W/(m²K) | Stated coefficient for the complete body | Are the equivalent surface and test setup defined? |
| Panel U, W/(m²K) | A particular panel or layer | Are doors and joints included? |
| Lambda, W/(mK) | A material property | Are bridges outside the material addressed separately? |
| Mean surface S, m² | Equivalent surface used in the K calculation | Is the definition given? |
In steady state the conduction heat through the body is Q = K × S × ΔT; in a hypothetical example, 0.40 W/(m²K) × 50 m² × 30 K = 600 W. S is not the floor area: in the ATP text (Annex 1, Appendix 2, section 1.2) it is the geometric mean of the inner and outer surfaces, S = √(Si × Se). The 600 W is neither an energy consumption nor a cooling-unit size; door openings, the product's heat load, sun and air leakage are added separately.
K is measured under laboratory conditions and cannot be estimated by eye in the yard. Logging how long a container takes to cool in the field is still useful: record product quantity, starting temperature, outside conditions, door openings and cold source, and the same operation can be compared with another day. That record does not reproduce the certified K value; it shows how the body declines over time.
K grows over the years, because moisture in the panel, knocked corners, holes drilled later and never sealed, and tired gaskets all increase heat transfer. That is why ATP re-checks equipment in service at least every six years: if the expert inspection is favourable the unit stays in service for up to three more years, and if not, it continues only after passing a new K measurement, for six more years.
In a quote or test report, check which body, model and test date the document belongs to, whether K is stated for the complete body, and whether the equivalent surface and temperature conditions are given. If doors, gaskets and service openings do not match the reported arrangement, the number does not describe the body as it is today.
Use the insulation quote checklist to collect these details, and the body-insulation inspection guide to investigate symptoms on existing equipment.
