ACH50 stands for air changes per hour at 50 pascals of negative pressure. Pascals are a standard unit measure of pressure comparable to pounds per square inch (PSI).  ACH is a calculation of the cubic volume of space divided by air volume moving in and out per hour. For ACH50, a blower is used to measure the infiltration of outdoor air into a structure to compare the relative “tightness” of that structure. The measurement used is 50 pascals of negative pressure in reference to outside pressure. A higher number means that more air can infiltrate the structure (loose), whereas a low number means less air can infiltrate (tight). This number doesn’t reflect air moving in and out of the space when it isn’t pressurized.