Wind, stack effect and fan imbalances push outdoor air in through gaps around windows, doors, penetrations and joints, while the same amount leaves as exfiltration somewhere else. Infiltration adds heating, cooling and moisture load that no one designed for, and the leakiness behind it is measured with a blower door as air changes per hour at 50 pascals. Air sealing the envelope reduces it and makes deliberate ventilation necessary.
Blower door says 12 ACH50, the infiltration alone is a ton of load in that old house, seal it before we size anything.
Air seal first, then size equipment; a tighter house needs less capacity and a ventilation plan.
Also heard as
- air infiltration
Related terms
Building envelope
The walls, roof, floor, windows and doors that keep conditioned indoor space apart from the outdoors.
ACH
Air changes per hour: how many times a space's full air volume is replaced in one hour.
Stack effect
The buoyancy-driven airflow that pushes warm air out the top of a building and pulls outdoor air in at the bottom.
Cooling load
How much heat and moisture a cooling system has to pull out of a space each hour to keep it at design conditions.