Outdoor air enters and leaves a house by: infiltration, natural
ventilation, and mechanical ventilation. In a process known as
infiltration, outdoor air flows into the house through openings,
joints, and cracks in walls, floors, and ceilings, and around
windows and doors. In natural ventilation, air moves through
opened windows and doors. Air movement associated with
infiltration and natural ventilation is caused by air
temperature differences between indoors and outdoors and by
wind. Finally, there are a number of mechanical ventilation
devices, from outdoor-vented fans that intermittently remove air
from a single room, such as bathrooms and kitchen, to air
handling systems that use fans and duct work to continuously
remove indoor air and distribute filtered and conditioned
outdoor air to strategic points throughout the house. The rate
at which outdoor air replaces indoor air is described as the air
exchange rate. Why does this matter? When there is little
infiltration, natural ventilation, or mechanical ven