One of a series of articles focussing on problem areas in buildings. 1) Examines condensation risks in buildings. Treats condensation processes, water vapour input and movement, conditions for surface and interstitial condensation in walls and roofs. 2) Treats condensation avoidance in general, humidity control, controlling vapour flow, adding insulation, heating, mould. Illustrates numerous examples diagrammatically from various building types.