This post originally appeared on the Green Building Advisor website.
Testing air leakage in ductwork used for forced air heating and cooling systems has been required since the 2012 IRC code. The 2012 through the 2018 code allowed a testing exemption for all ductwork located inside the building envelope. In other words, if you kept all ductwork inside the thermal and air boundary of the building, no testing was required. That has changed for the 2021 energy code.