Many of the jurisdictions we work in are just now getting building code updates that bring them up to the 2010 or 2013 ASHRAE Standard 90.1 energy code. It’s not surprising that the new energy codes have not yet started affecting how engineers design duct systems. Here’s a spoiler alert – if you still design duct systems around rectangular ductwork, you are not going to meet the new energy codes without taking some extraordinary measures.