When the indoor temperature cools down the air cannot hold as much water vapour as when it was warm. This results in the water vapour condensing to become a liquid. It then becomes visible on cold, hard surfaces such as glass and windows. It can also appear on mirrors, walls and ceilings, as well as penetrating carpets, curtains and any other absorbent surface making them feel cold and damp. High indoor humidity and condensation will promote mould growth on walls, ceilings and materials such as curtains and carpets. This mould growth and moisture is a major cause of deterioration in homes and buildings and can create health problems for people who live there.