It has long been recognised that the control of air flow is a crucial and intrinsic part of heat and moisture control in modern building enclosures.
There are three primary classes of reasons why the control of air flow is important to building performance:
- Moisture control – water vapor in the air can be deposited within the envelope by condensation and cause serious health, durability, and performance problems.
- Energy savings –air leaking out of a building must be replaced with outdoor air which requires energy to condition it. Approximately 30% to 50% of space conditioning energy consumption in many well-insulated buildings is due to air leakage through the building enclosure. Convective circulation and wind washing both reduce the effectiveness of thermal insulation and thus increase energy transfer across the envelope.
- Comfort and health – cold drafts and the excessively dry wintertime air that results from excessive air leakage directly affect human comfort, wind-cooled portions of the interior of the enclosure promote condensation which supports biological growth which in turn affects indoor air quality, airborne sound transmission control requires good airflow control, and odors and gases from outside and adjoining buildings often annoy or cause health problems.
There are three primary mechanisms which generate the pressure differences required for air flow within and through buildings:
- stack effect or bouyancy, and
- mechanical air handling equipment and appliances.
Since, it is widely acknowledged that a perfectly airtight air barrier system is unlikely to be achieved in practise, it is also desirable to control the air pressure differences driving the flow. This typically means reducing the pressure imbalance created by HVAC systems, reducing stack effect pressures by compartmentalizing buildings vertically, and reducing wind pressures by compartmentalizing building plans.
Wind forces act on all buildings, typically creating a positive pressure on the windward face and negative (suction) pressures on the walls.
Stack effect pressures are generated by differences in air density with temperature, i.e. hot air rises and cold air sinks. The air within a building during the wintertime acts like a bubble of hot air in a sea of cold air. In the summertime the situation is reversed, although air temperature differences are usually less.
Fans and blowers cause the movement of air within buildings and through enclosures. By doing so, they can generate large pressures. If more air is exhausted from a building than is supplied, a net negative pressure is generated and vice versa.