ACH50 is a measurement that looks at the number of air volume changes per hour in a home. Currently, this is measured and tested using an apparatus and methodology collectively called a blower-door test. Simply put, the tester installs a temporary plastic front door that contains a large fan. The fan depressurizes the house to 50 Pascals of pressure, (hence where the 50 in ACH50 comes from). The tester then reads how hard the fan has to work to keep the house at 50 Pascals to determine the rate of air leakage through the walls, foundation, windows, etc. The lower the number of the test, the less air leak, or "tighter" the home is. A PH level of air-tightness is considered anything less than 0.6 ACH50. However, current code in Tennessee only requires a house to have a 5 ACH50 which is much "leakier" than a PH. In simple terms, a 5 ACH50 means that the volume of air in the house is turning over 5 times an hour from leaks.
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