Heat takes the easiest path. The thermal conductivity of steel is about 50 W/mK, that of polyurethane foam 0.022 W/mK; in other words steel passes two thousand times more heat through the same cross-section. A bolt as thick as a finger passing right through the insulation can carry more heat than a palm-sized area of the panel around it. However good the U-value of the panel, if the bridges are not taken into account the real heat gain turns out higher than the figure on paper. The real problem is mostly not energy but the condensation and ice that the bridge causes.
Typical bridges in cold rooms: the joints and corner profiles where the inner and outer sheets of the panel meet, the hanger rods carrying the ceiling, steel columns and beams passing through the room, the anchoring of rack feet to the floor, the door frame and threshold, the floor insulation being cut off under the wall, and pipe, cable and drain penetrations. The symptoms are visible to the eye: sweating, dripping and mould at that point on the outer surface; frost flowers and icicles on the inner surface; ice building up at the door threshold in a frozen room.
The precautions are taken at the design stage. Load-bearing steel is left outside the room and the ceiling panels are hung from outside; the hanger rods are divided by a thermally insulating spacer (thermal break). In frozen rooms the inner sheet is cut along its whole length at the wall panel-to-floor junction and at the door threshold to break the metal continuity. Pipe and cable penetrations are filled with polyurethane foam and sealed on the warm side with vapour-tight mastic. Door frames are made from thermally insulated profiles; on frozen room doors the remaining bridge is offset by the frame heater.
The quickest way to find the bridges in an existing store is a thermal camera: every point on the outer surface that appears a few degrees colder than its surroundings is a bridge or an air leak. An air leak is even more harmful, because along with the heat it carries moisture and builds up ice inside the panel; ice growing in the joints separates the panels from one another over time. Every panel drilled afterwards (rack installation, a new pipe) creates the same risk; the hole must not be left without being insulated and sealed tight. In the cooling load calculation the bridges are covered by an allowance added on top of the conduction load.