The **ACH50** a measure used to determine how “leaky” a house is using a blower door by depressurizing or pressurizing the house by **50** Pa (Pascals) with respect to the outside.

In order to get the ACH or **Air Changes per Hour** number you simply multiply the CFM number required to keep it at 50 by 60 and divide that number by the total volume of the conditioned space. For example a 1500 SF ranch house with 8’ ceilings has a volume of 12,000 Cubic Feet of Air. If the CFM reading is 1200 CFM and you multiply that by 60 you get 72,000. Dividing 72,000 into 12,000 would equal 6 ACH.