Condensation is becoming one of the commonest causes of dampness in buildings and is nearly always associated with black mould growth (Aspergillus niger). The problem arises because of high relative humidities, a lack of natural ventilation and, in most cases, cold external walls. Sometimes it's possible to reduce humidities by regulating activities like drying clothes indoors. Ventilation can be provided via trickle vents and so on but these are usually ineffective and unpopular due to perceived heat losses or cold draughts. Walls can be insulated but not without relatively major disturbance to occupants. Because of these factors a new approach to providing energy efficient humidity control was needed and this has been provided through the concept of positive input ventilation.