Air is a fluid, it flows and it has pressure. The pressure can go up or down, and that’s what drives the flow. Air takes the shape of whatever space it’s in and can be kept there when it’s bounded by materials that are impermeable to air.
The following are the fundamental principles of air movement:
- For each cubic foot of air that leaks in a building, another cubic foot leaks out.
When there’s a hole somewhere in the building envelope conditioned air leaks out and unconditioned air is being pulled in through another hole or the same hole if it is large enough. Each cubic foot that leaves is matched by a cubic foot of unconditioned air leaking in somewhere. This is bad because the heating and air conditioning system has done the work to heat or cool the conditioned air that is being lost. Someone had to pay for that conditioned air which has been replaced with new unconditioned air.
- Air flows from areas of higher pressure to areas of lower pressure.
This is the force behind air movement. Air pressure flows to equalization, so higher pressure airs are drawn towards lower pressure areas until equilibrium is established.
- Air takes the path of least resistance
This is a very simple and obvious statement but provides critical guidance when discussing typical pathways of air-leakage.
- For air to move, it needs both a pressure difference and a pathway.
Because air responds to pressure differences, if there isn’t any, then it’s got nowhere to go. Further if there are no holes or pathways for air leakage to occur then conditioned air isn’t being lost; thus the critical need for properly installed and functioning air barrier systems.
Knowledge of how air flow works, the driving forces of air and air pathways will ensure a properly functioning building envelope is achieved.