As of 2010, Standard 90.1 (addendum cq) dictates that ductwork that is designed to operate at a static
pressure greater than 3 in.wg., and all ductwork located outdoors, shall be leak tested, and the maximum
permissible leakage shall be determined using a leakage class of 4 for both round and rectangular duct.

Furthermore, the building owner (or designated representative) shall select representative sections
totaling no less than 25 percent of the total installed ductwork, by duct surface area.

In other words, if the ductwork totals 1000 square feet of surface area, no less than 250 square feet of
that system shall be tested for air leakage IF:

  1. The system is designed to operate at a pressure greater than 3 in.wg., or
  2. The system is located outdoors.

Status of State Energy Codes

Your state may or may not have adopted Standard 90.1-2010. To see the current list of the statewide
code adoption per the U.S. Department of Energy, click here.

Since many Engineers are unaware of their local or state’s code in regard to allowable duct leakage, their
specification may not be current. You may see specifications that refer to leakage classes that vary by
construction type (round or rectangular), and also requirements that testing be done at pressures higher
than the design pressure, which may not be required if your state has adopted one of the recent versions
of Standard 90.1.