(Air changes per hour at a pressure difference of 50 Pascal.)

This metric takes the CFM@50Pa number, multiplies it by 60 (minutes in an hour), and divides that result by the volume of the heated area of the house. This makes comparing houses of different sizes possible and has become the most common metric referenced in codes and energy efficiency programs. A good goal for most builders is anything under 2 ACH@50Pa. meaning that the entire volume of air in the house is replaced twice per hour during this exaggerated test condition.