The insulation calculation for a refrigerated cabinet, an enclosure or a building component is usually done as area times conductivity. The real loss is almost always higher, and the difference comes from thermal bridges.
What a thermal bridge is
Any high-conductivity path that pierces or bypasses the insulation layer: structural profiles, fasteners, hinges, pipe and cable penetrations, corner junctions.
The area these paths occupy is small, but their conductivity is hundreds of times that of the insulation. The result is a total loss markedly higher than expected — and a one-dimensional resistance calculation cannot see it at all, because the heat flow there is two- or three-dimensional.
The second consequence: surface temperature
A thermal bridge does not only lose energy; it creates a local cold spot on the inner surface. If that spot falls below the dew point, condensation begins, followed by mould or corrosion.
In refrigerated cabinets the same mechanism appears as frost around the door seal. The acceptance criterion in that case is not the total loss but the lowest surface temperature.
Modelling
The problem is pure conduction and can be treated without solving the flow, but the boundary conditions have to be right. On the outer surface, if there is wind it is forced convection; if not, it is natural convection and radiation together.
Take care when simplifying the geometry: discarding screws, profiles and fittings as "insignificant detail" erases exactly the effect you are looking for. See the geometry preparation note.
What belongs in the report
The total loss, the ideal insulation loss, and which bridges account for the difference. That breakdown shows directly where improvement should be made.
References
- Modest, M. F. & Mazumder, S. (2021). Radiative Heat Transfer, 4th ed. Academic Press. ISBN 978-0-323-98406-5.
- Churchill, S. W. & Chu, H. H. S. (1975). Correlating equations for laminar and turbulent free convection from a vertical plate. International Journal of Heat and Mass Transfer, 18(11), 1323–1329. DOI: 10.1016/0017-9310(75)90243-4