Air Changes per Hour at 50 Pascals (ACH50) is a metric used to measure the air tightness and infiltration rate of a building under a standardized pressure difference of 50 Pascals (Pa). It quantifies the amount of air that infiltrates or leaks into or out of a building when subjected to a pressure differential of 50 Pa.
The ACH50 value indicates how many times the total volume of air within a building would be exchanged with outdoor air per hour at a pressure difference of 50 Pa. It is commonly used in blower door tests, where a calibrated fan is used to pressurise or depressurise the building to create the desired pressure difference.
The ACH50 value provides insight into the level of air leakage in a building’s envelope. It helps evaluate the building’s airtightness and the potential for energy losses due to air infiltration or exfiltration. Lower ACH50 values indicate better air tightness and reduced energy loss.
ACH50 values are typically used in energy efficiency assessments, building performance evaluations, and energy code compliance. They are also useful for identifying areas of air leakage and guiding air sealing efforts to improve energy efficiency and indoor comfort.
It’s important to note that ACH50 represents the building’s air tightness under specific test conditions and does not account for natural ventilation or normal operating conditions. Therefore, it is not directly comparable to ACH values measured under natural conditions.