Air Changes per Hour (ACH) at a specified building pressure, typically at 50 Pa (ACH50).
This metric indicates the rate at which the air in a building is replaced with outside air, and as a result, is an important metric in determinations of indoor air quality. This normalizes the airflow at a specified building pressure by the building's volume, which allows for more direct comparison of homes of different sizes and layouts.
The Effective Leakage Area (ELA) assumes that all of the individual leakage areas in the building are combined into a single idealized orifice or hole. This value is typically described to building owners as the area of a window that is open 24/7, 365 in their building. The ELA will change depending on the reference pressure used to calculate it. 4 Pa is typically used in the US, whereas a reference pressure of 10 Pa is used in Canada.