A condition arising when more air is exhausted from a building than is supplied, resulting in lower air pressure inside compared to the exterior. This pressure differential causes outside air to be drawn in through any available openings, such as gaps around doors and windows, or through chimneys. For example, running multiple exhaust fans simultaneously without adequate fresh air intake can create this imbalance.
Maintaining appropriate air pressure within a dwelling is important for energy efficiency, indoor air quality, and occupant comfort. An imbalance can increase heating and cooling costs as unconditioned air infiltrates the structure. It can also draw in pollutants, such as radon gas or combustion byproducts from improperly vented appliances, impacting health. Historically, buildings relied on natural ventilation, making this less of a concern; however, modern, tightly sealed construction and the use of powerful exhaust systems have increased the likelihood of this condition occurring.
