LBNL Report Number
Ducts that carry heated or cooled air are often internally lined with fiberglass for acoustic control and thermal insulation. If the inner face of the fiberglass liner is permeable to air, air flow in the duct may induce convection in the fiberglass and thereby increase the liner's thermal conductance (the reciprocal of its thermal resistance). In fiberglass-insulated flexible ducts with air-permeable and impermeable inner cores, the "temperature-drop" method was used to measure the variation of the thermal conductances with duct air velocity. The thermal conductance of the fiberglass blanket lining a permeable-core flexible duct increased by 36% as the duct air velocity rose from 2 to 9 m s-1. The permeably-lined duct's total thermal conductance (insulation plus air films) increased with air speed at a rate approximately half that reported in a prior study. If a permeably-lined duct is located within the conditioned space of a building, the fraction of the duct air's sensible heat capacity can be saved by rendering the liner impermeable is typically 1-3%. Savings will be typically 1.5 to 2 times greater if the duct lies outside the conditioned space. Savings increase with duct length and air velocity, and decrease with duct diameter.