Duct leakage is a significant issue for forced-air heating or air-conditioning systems. When ducts are run outside of conditioned space, leakage may decrease heating and cooling efficiencies by up to 40%. Over time, this will add up to a great deal of lost energy and money.



The distribution of conditioned air (heated or cooled) from HVAC systems to a home is an integral part of many mechanical ventilation systems. It is essential for ducts to maintain a constant temperature
as long as possible so that they can deliver this conditioned air through registers in an efficient and effective manner. The quality of ducts can have a significant effect on the ventilation rate and efficiency of a home. Duct leakage is a source of energy loss, ventilation rate loss, and a source of indoor pollution (in return ducts). Research shows that as much as one third of all conditioned air can escape through leaks in a conventionally sealed duct system. Because of this, builders must seal ducts with approved duct sealants.

When ducts are run through unconditioned spaces like attics, crawlspaces, garages, basements and other locations outside the heated or cooled parts of the house, they not only leak air to and from the outside, but any heat lost through the walls of the duct (by heat conduction) is also lost to the unconditioned space instead of heating and cooling the house.

A 2005 LBNL review cites studies showing an energy loss of 30 to 40% when ductwork is installed in unconditioned spaces. Others demonstrate through modeling and field testing that leakage through the average duct system was 37% greater than infiltration through the building envelope. Houses with leaky ductwork and air handlers located in unconditioned spaces are vulnerable to increased infiltration rates – especially in hot, humid climates. Duct leakage can also prevent effective distribution of the supply air, substantially impacting the actual ventilation rate found in the average house.

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2015 IECC: R403.3 Ducts
Ducts and air handlers shall be in accordance with Sections R403.3.1 through R403.3.5.

2012 IECC: R402.3 Ducts
Ducts and air handlers shall be in accordance with Sections R403.2.1 through R403.2.3.

The 2009 IECC covers duct insulation and sealing in Section 403.2.

Building code officials must be aware of the requirements covered in the IECC and need to ensure that:

  • All ducts outside the conditioned space are insulated
  • Ducts are properly sealed using appropriate sealant
  • Insulation meets statewide requirements
  • All ducts are sealed with mastic or UL-181 A and B tapes
  • They check for poorly installed duct work

Code History

Duct insulation requirements are identical in the 2006 and 2009 IECC. The 2009 IECC presents the first significant change to duct sealing requirements. It requires that the ducts either be (a) designed and installed in the conditioned space or (b) pressure tested to meet the specifications in Section 403.2.2. Another change to the 2009 IECC is the Section 403.7 requirement that builders construct ducts serving multiple dwelling units in accordance with Sections 503 and 504. Ducts must be insulated to R-5 when located within the building envelope and R-8 outside of it, as well as sealed to low, medium or high pressure specifications.

Typical Issues

Standard issues/concerns that arise include ducts getting buried in attic insulation and lack of duct sealing within the dwelling.

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