A properly ventilated structure increases the comfort level for its occupants and eliminates indoor pollutants by exchanging stale indoor air with fresh air from the outdoors. This exchange of air (air circulation) can be accomplished by either natural or mechanical ventilation.
Natural ventilation is the unaided movement of air into and out of a house or building. Unaided air movement from the outdoors into the interior of the structure through cracks in the walls and around windows or doors is called infiltration. The unaided movement of air from the interior to the outdoors is called exfiltration. The temperature differences between the indoor and outdoor air, the wind, the orientation of the structure, and the location of the windows and doors, must all be factored in when determining natural ventilation rates. Because so many variables are involved, it is very difficult to calculate an appropriate natural ventilation rate accurately for a structure.
Before the advent of well-insulated houses and central heating/cooling systems, natural ventilation was an acceptable means of exchanging outdoor and indoor air. Structures were naturally drafty because of the minimal use of insulation. The air could move relatively freely through the wall cavities between the outdoors and the interior of the structure. As fuel prices increased over the years, however, homeowners also increased the levels of insulation in order to reduce energy costs. The higher levels of insulation, tighter construction, and the growing reliance on central space conditioning required the use of centralized mechanical ventilation systems to circulate the air between the outdoors and the indoors. If a central heating/cooling system is properly sized, it can provide the optimal number of air change rates required to maintain fresh, healthy interior air. Ventilation principles and detailed descriptions of centralized mechanical ventilation systems are covered in Volume 3.