COP v3.0:internal-moisture; condensation

10.4 Condensation 

Condensation is a natural phenomenon and building materials are capable of withstanding repeated short-term episodes of wetness. Problems arise in ceiling spaces when more water vapour enters than exits, and materials stay wet for long periods, or where standing water accumulates.

The primary purpose of roof cladding is to act as a rain screen so that no water enters the building from the outside. It is, however, equally important to ensure that the building is kept dry from inside. Because metal is a good heat conductor and is not absorbent, condensation forms on metal cladding under conditions of high humidity when surface temperatures drop below the dew point.

The solutions in the Code of Practice apply to cold roofs, where there is a gap between the ceiling insulation and the roofing material. 10.12.2 Warm Roofs require specific design to avoid internal moisture problems. They are discussed in more detail under 15.5 Insulated Panels.