According to the IECC, (International Energy Conservation Code) a duct leakage test is required when any part of the HVAC system or the ductwork sit outside of the buildings’ thermal envelope. The thermal envelope is any place which is conditioned while places outside the thermal envelope are unconditioned spaces such as basements, attics and crawl spaces.
When the HVAC system or ductwork sits outside of the conditioned space it’s important to keep the system as tight as possible. This way we can ensure that minimal air leakage will be lost in unwanted places.
A duct leakage test is generally conducted before the drywall is installed in the home. This way it is easier to seal up the ducts in case access leakage is found. The test can be conducted with the air handler installed or before the air handler is installed.