Duct leakage in commercial buildings typically accounts for 5 to 25 percent of the supply airflow. Most of this ducted air escapes into unconditioned spaces, such as cavities above suspended ceilings and returns to the air handler without doing any heating or cooling. The main energy impact of this interior duct leakage is to increase the blower’s energy consumption by 10 to 50 percent. However, when the air handler is located outside the building, as with rooftop air handlers, ducted air escapes outdoors increasing heating and cooling costs.
Ducts can be tested for air leaks during installation, air handler replacement, or major renovation. The most accurate method is to use a pressurization fan, like the Duct Blaster® by the Energy Conservatory. It can measure duct leakage in systems with airflow of up to 5000 cubic feet per minute (CFM). (Unfortunately, there is no simple way to test large duct systems.) Duct leakage is usually expressed as a percentage of the rated airflow. Leakage above 15 percent indicates that duct sealing will be cost-effective. Leakage of 5 to 10 percent is considered tight although it’s possible and desirable to seal ducts tighter.
The best materials for duct sealing are water-based duct mastic applied over fiber mesh tape. This mastic sticks permanently to almost any surface, and the mesh tape reinforces the mastic when it bridges a gap or crack. Common duct tape isn’t an acceptable sealant because its adhesive dries out and loses its grip
The best times to seal ducts are during the original installation, when replacing an air-handler, or during a major retrofit. The ideal method for new installations is to prefabricate and seal sections of ductwork either in the shop or on the job to minimize the work done in awkward and tight places.
Duct sealing work should be prioritized according to the severity of air leakage. The most important leaks are in supply ducts close to the air handler, since these experience the highest pressure. Leaks in air handlers are also important, especially rooftop units. Seal holes for pipes and wires with mastic and mesh tape. Check the seal around access panels on air handlers; this is one place where duct tape is OK because a future technician can easily cut it to remove the panel.
Finally, make a special effort to inspect joints that are difficult to reach. These were often hard to assemble and seal during original installation, and they may even be partially disconnected.