Retrotec Residential Pressure & Air Leakage - Manual (Page 6)

Page 6 of 75
©Retrotec Inc. 2014
1. Air leaks when there is a pressure difference across a hole
Air leakage is the infiltration or exfiltration of air from a building. In order for leakage to occur, there must be a
hole, and there must be a pressure difference across the hole. Holes, both intentional and unintentional, are
unfortunately all too common in buildings.
There are five common issues that create a pressure imbalance: stack pressure, wind, exhaust or supply flows,
duct leakage and forced air duct systems.
1.1 Pressure differences you will see in a home
Stack pressure is created by warm air rising or cold falling and is the major
force in energy loss
The stack effect, or stack pressure, comes from the process of hot air rising, and cold air dropping. Typically,
warmer inside air tends to rise in a building, and leak out of holes near the top. This air is replaced by colder
outside air leaking into holes around the bottom. In warmer climates, this effect can be reversed, as the interior
air is cooler, and drops to leak out of the bottom.
Stack pressure can be calculated. The pressure difference experienced is a product of a constant (0.0342), the
atmospheric pressure, the building height, and the temperature difference (between the top and bottom of the
stack). Essentially, pressure due to stack is proportional to the height, and temperature difference.
Figure 1: Stack pressure is caused by warm air rising and cool air falling.
Wind pushes air into and pulls air out of buildings
When the wind blows, pressures are created where the stream of air is stopped or slowed down by a wall, or
some other part of the house. Because air has mass and wind has velocity, when the wind stops moving (as it
hits a wall), its momentum turns into pressure. Everyone has felt the pressure on their body when walking on a