It is not uncommon for ductwork, especially warm-air ducts, to be installed without insulation. When this duct is routed through ceiling spaces or unconditioned zones such as mechanical rooms or basements, the thermal losses can be high. There are even losses when the duct passes in between-floors ceiling space, though the temperature difference is less, and consequently, the thermal losses are not as severe.
In addition to the energy lost from uninsulated ductwork, the quality of the conditioned environment suffers. Take a long duct run through a ceiling space as an example. Even though the thermal losses may be small, they will be enough to increase the temperature of air in a cooling system that reaches the most distant room on the run. The first rooms on the duct line can be overcooled. (The designer will have balanced the cubic feet per minute to each space, not taking into account that the delivery temperature might vary.) Depending on the location of the thermostat, the control problems can cause year-round difficulties in maintaining comfortable conditions. Productivity could also be affected adversely because of unstable or high space temperatures.
The first ducts that should be insulated are those in the roof space and those traversing unconditioned zones. Flexible batts or sprayed-on insulation are the simplest ways to insulate, and they’re easily done as a retrofit. (“Sprayed-on” insulation no longer connotes temporary, durable or anything other than sturdy, long-lasting material.) Ductwork that can carry cold air may have to be provided with a vapor barrier to prevent condensation problems. The appropriate fire codes also have to be followed.
Duct insulation is not required if the temperature difference is less than or equal to 15°F, unless a moisture problem is expected when the equipment is started up each day. This can happen if the duct is exposed in the conditioned space or, in some instances, where the ceiling space is used as a return-air plenum. The amount of duct insulation that can be installed economically is a function of the amount of use the duct sees in the course of a year: if the occupancy is only a few hours each day, less duct insulation is needed. The longer the annual use and the higher the temperature difference, the more insulation is required.
It’s a good idea to seal all the duct carefully before installing the insulation. This is especially important at the beginning of each duct run, where the static pressure is highest, and the potential losses are more extreme. A quality ductboard system should have sheet-metal ductwork for the first 10 or 20 ft on the supply and return side of the air handler.
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Excerpt from Energy Conservation Projects.