Cold bridging is what happens when a conductive element carries heat straight through an otherwise insulated build-up. In a steel roof there are plenty of candidates, and the consequences are not only wasted heat but localised condensation exactly where you least want it.

Where cold bridges occur in a steel roof

• Fixings. Every screw is a small steel rod passing from the cold outer sheet to the warm structure. Individually trivial, collectively significant on a large roof.
• Purlins and rails. Where the structural member touches both the inner and outer skins it forms a continuous path. This is the largest bridge in most buildings.
• Laps and junctions. Insulation is rarely as continuous at a lap as it is mid-panel, and eaves, ridges and abutments are where continuity is most often lost.
• Penetrations. Rooflights, vents and services all interrupt the insulation layer.

How you notice it

The classic sign is a pattern of damp spots or mould following the line of the purlins, or rust-coloured marks radiating from fixings. In a heated building you may also see clean stripes in dust or condensation that map the structure behind the lining. If the pattern is regular and structural, you are looking at a cold bridge rather than a leak.

Designing it out

• Use a composite panel. An insulated roofing panel has a continuous insulating core bonded between two steel skins, so the bridge at each fixing is short and the field of the panel is unbroken.
• Break the path at the purlin. In a built-up twin skin system, a thermal spacer or rail system between the liner and the outer sheet interrupts the direct metal-to-metal route. Our twin skin pages explain how these systems go together.
• Keep insulation continuous. Most heat loss in practice is through gaps, compression and poorly closed junctions rather than through the insulation itself.
• Do not rely on ventilation to fix it. Ventilation manages moisture, which helps with the symptom. It does nothing about the heat loss.

Overcladding an existing roof

Where a building already has a single skin roof, overcladding adds an insulated layer over the existing covering. It is usually less disruptive than a strip and re-sheet because the building stays weathertight throughout, and it addresses both the thermal performance and the condensation risk in one operation.

If you are weighing up twin skin against a composite panel for a particular building, talk it through with us.