The ductwork in your home is the distribution network for conditioned air. Depending on the age and condition of your house, your ducts may be leaking a significant portion of the air your HVAC system works to condition. The U.S. Department of Energy estimates that duct leakage in a typical home wastes 20 to 30 percent of the energy used for heating and cooling. Identifying and sealing duct leaks is one of the highest-return improvements you can make to a home’s energy efficiency.

How Duct Leakage Works

Duct systems run through unconditioned spaces in most homes: attics, crawlspaces, basements, and wall cavities. When the air handler pushes conditioned air through the supply ducts, any gaps, disconnected joints, or poorly sealed connections let that air escape into unconditioned space before it ever reaches a room. Return ducts, which pull air back to the air handler, have the same problem in reverse: leaky return ducts pull hot attic air or cold crawlspace air into the system instead of pulling air from the living space.

The result is a system that runs longer to achieve the set temperature, rooms that never quite reach the right temperature, and energy bills higher than they should be.

Signs You Have Significant Duct Leakage

High energy bills relative to your neighbors or your own history. If your bills have climbed noticeably without a change in usage habits or equipment, duct leakage is a possible cause worth investigating.

Rooms that are harder to heat or cool than others. A bedroom over an unconditioned garage, or a finished room above an attic space, that never reaches the right temperature despite adequate airflow from the register often indicates duct leakage between the air handler and that room.

Excessive dust accumulation. When return ducts leak and pull air from an attic or crawlspace, they bring dust and debris with them. If you find unusually heavy dust on furniture and surfaces, particularly near supply registers, duct leakage may be pulling particulate-laden air into the system.

A musty or stale smell from registers. Air pulled from a crawlspace or basement through leaky return ducts carries whatever is in that space into your living areas.

Visibly disconnected or damaged ducts. If your attic or crawlspace is accessible, do a visual inspection. Flexible duct that has separated from a fitting, rigid duct with gaps at joints, or insulation that has fallen away from flex duct are obvious indicators.

Difficulty maintaining humidity levels. Duct leakage into or out of unconditioned spaces throws off the humidity balance in the living area.

What to Do About It

Minor leakage at accessible duct joints can be addressed by a homeowner. Use mastic duct sealant, which is a thick paste applied with a brush, to seal joints and gaps. Mastic is preferred over tape because it remains flexible and does not dry out and fail over years the way standard duct tape does. Metal-backed foil tape rated for HVAC use is also acceptable for sheet metal ducts. Never use standard silver fabric duct tape on actual ducts; despite the name, it is not rated for the temperature cycling ductwork experiences and fails quickly.

For larger systems or inaccessible ductwork, aeroseal duct sealing is a professional service where a technician pressurizes the duct system and injects a sealant mist that adheres to and seals leaks from the inside. It can treat leaks that are completely inaccessible from the outside and is highly effective.

A professional duct leakage test, called a duct blaster test, can quantify exactly how much air your system is losing and identify where the worst leaks are. Many energy auditors offer this as part of a whole-home energy assessment.