Interior spaces in large buildings must rely on artificial lighting and mechanical ventilation or even air-conditioning. Spaces which are on the perimeter of the building and which can benefit from daylight, solar gains and natural ventilation are said to be in the passive zone. For typical ceiling heights the potential passive zone will be about 5.5m deep, or twice the floor to ceiling height. This is a theoretical maximum and in real cases may be reduced by thepresence of furniture or obstructions caused, for example, by an atrium. Figure 8.1 indicates on the plan the passive and the non-passive zones of a building with an atrium.