At any moment, the building enclosure has a number of simultaneous heat gains and/or losses. During typical daytime hours, solar gain through windows and skylights is quite significant. In addition, heat is given off by lighting equipment, office equipment, and the building occupants. When it is cooler outside the building than inside, heat is lost through windows, walls, roofs, and floors. Losses also result from infiltration or air leakage through the building envelope. During occupied periods, most commercial buildings are pressurized. There is no infiltration, but warming colder outside air that is brought in for ventilation results in a loss. If it is warmer outside than inside, the building envelope, infiltration, and ventilation losses become gains.
The term "building envelope" refers to the parts of a building that enclose heated or cooled areas. When heat gains into the building are greater than losses, the mass of the building absorbs some of the gains so that they do not have to be removed by the air conditioner. Similarly, when losses are greater than gains (for instance at night), the building mass releases heat to the space.
For many commercial buildings, internal gains and solar heat gains through windows are greater than the losses through the building envelope until the outside temperature is about 45°F. The temperature at which the envelope losses are in balance with the total heat gains is called the balance point temperature of the building. The balance point temperature is different for each building and depends on a number of factors, but internal gains from lighting, equipment, and occupants are very significant factors.
If heat gains exceed the losses (with consideration of storage effects), the building has a cooling load and energy must be used by the air conditioner to remove heat from the building. If heat losses exceed the gains, there is a heating load and energy must be used by a furnace or boiler to add heat to the space. Heat gains and losses are constantly changing in buildings as sun light changes, as outside temperatures change, and as changes occur in internal conditions such as thermostat settings, outside air ventilation rates, and equipment usage. The response to changes in temperature is not immediate. Building envelope elements with thermal mass absorb some of the heat and delay its transfer. Mass elements inside the building also absorb and release heat as temperatures change. The best way to evaluate the net effect of these multiple and complex heat transfers is with a computer program that simulates hourly gains, losses, and storage effects for a typical weather year.