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 system is an integral part of many mechanical ventilation systems. It can have a significant effect on the ventilation rate and efficiency of a building. Duct leakage is a source of energy loss, ventilation rate loss, and a source of indoor pollution (in return ducts). 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.
Commercial Policy Options
All supply and return air ducts and plenums shall be insulated with a minimum of R-6 insulation when located in unconditioned spaces and with a minimum of R-8 when located outside of the building.
When located within a building envelope assembly, the duct or plenum shall be separated from the building exterior or unconditioned or exempt spaces by a minimum of R-8 insulation.