Duct leakage is not only detrimental to energy efficiency, but also indoor air quality (in terms of lower air change rates and ventilation efficiency in rooms), comfort, and fire protection. It is often accompanied by other problems, such as inferior commissioning and cleaning. In Scandinavia good ductwork airtightness has largely been promoted together with indoor air quality benefits. Note that the Swedish VVS AMA guideline not only deals with energy issues related to duct airtightness but also with safety and indoor environment. 
 G. Guyot, F. R. Carrié and P. Schild, “Project ASIEPI – Stimulation of good building and ductwork airtightness through EPBD,” 2010.