• Each effect creates pressure differences across the envelope
• Flow is always from high to low pressure
• Pressure differences + cracks and holes = air flow and heat loss

The amount of air flow and the resulting heat loss depends upon the size of the openings and the pressure difference across the envelope.

During the heating season, air infiltration generally occurs in the lower part of the building while air exfiltration occurs in the upper part of a building.

For air flow to occur, there must be both:

• A pressure difference between two points
• A continuous flow path or opening connecting the points

Primary mechanisms that generate the pressure differences required for air flow within and through buildings include:

1. Wind forces act on all buildings, usually positive pressure on the windward face and negative (suction) pressure on the other walls.
2. Stack effect or buoyancy: pressures are generated by differences in air density with temperature (i.e. hot air rises and cold air sinks).
3. Mechanical air handling equipment and appliances: fans and blowers cause the movement of air within buildings and through enclosures. If more air is supplied to a room than is exhausted, the excess air leaks out of the space and the room is said to be under a positive air pressure.
4. The magnitude of the pressure difference can also vary considerably depending on the shape of the building, the exposure, the height and local conditions of outdoor temperature and humidity.