ACH (Air changes per hour) is the measure of air flowing into or out of a home. Generally, the volume of air in a house should be replaced every two or three hours. That translates to between .33 and .50 ACH (when the house is not being ventilated). Higher rates mean that cracks, holes and other openings are letting to much airflow in and out. Lower rates could mean the house is too tight and could contribute to indoor air pollution. In the latter case “controlled ventilation” such as opening windows will allow a higher rate of air change. A professional contractor can measure the air change rate and look for “uncontrolled ventilation” such as poorly sealed windows and leaky ducts.