Thermal
Benefits of Double-Wall, Insulated
Duct and Fittings
Control
of Heat Loss and Heat Gain
The insulation in double-wall duct and fittings acts as a thermal barrier,
increasing a duct system’s effectiveness by helping to maintain the
temperature of heated or cooled air being ducted through the system. McGill
AirFlow’s standard 1-inch layer of insulation has a maximum thermal
conductance of 0.27 Btu/hr/ft2/°F. Where lower thermal conductance
values are desired, we also offer double-wall duct and fittings with insulation thicknesses
of 2 inches (0.13 Btu/hr/ft2/°F) and 3 inches (0.09 Btu/hr/ft2/°F).
Integral
Vapor Barrier
The sheet metal outer shell of our double-wall duct and fittings serves
as an integral and permanent vapor barrier to prevent moisture from condensing
in the insulation. The vapor barriers of externally insulated duct systems
usually are thin plastic or metal foil membranes that are difficult to
seal properly, especially where there are penetrations for hanger rods,
straps, and external reinforcement. With double-wall duct, all hangers
and reinforcements are attached outside the vapor barrier, so there are
no penetrations. The integrity of our vapor barrier is checked during
installation when the duct system is pressure tested for leaks. If air
cannot leak out, condensation cannot get in. Vapor barriers for external
insulation are rarely checked.
External
Condensation Control
Condensation can form on the outside of duct systems that convey cold
air, especially at times during the summer when the ambient relative humidity
is high and a large temperature differential exists between the inside
and outside of the duct system. Our standard 1-inch-thick insulation prevents
external condensation in all but the most extreme combinations of relative
humidity and temperature differential. For extreme conditions, we can
supply 2- and 3-inch-thick insulation to prevent condensation and the
water damage it can cause. External insulation does not always prevent
condensation problems because duct hangers and external reinforcements
penetrate the insulation, often resulting in localized external condensation.
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