Can Leaky Ducts Increase My Energy Bills?

Ductwork in most homes runs through unconditioned spaces — attics, crawl spaces, basements, and wall cavities. Leaks occur most commonly at joints between duct sections, at connections to supply and return registers, at the air handler cabinet itself, and at any point where the duct has been disturbed by renovation work, pest activity, or simple age-related deterioration.

Flexible duct (the corrugated plastic duct used in most modern homes) is particularly prone to leakage at its end connections, where it attaches to metal boots or plenums. These connections are often sealed with duct tape at installation, which dries out and fails within a few years. Metal ductwork is more durable but develops leaks at joints and seams over time, especially if the original installation used tape rather than mastic sealant.

Several symptoms point to significant duct leakage. Rooms that are consistently harder to heat or cool than others — especially rooms far from the air handler or in areas where ducts run through unconditioned spaces — often indicate that conditioned air is escaping before it reaches those rooms. If your system runs for long periods without reaching the thermostat setpoint, duct leakage may be reducing its effective output.

Unusually high energy bills compared to similar homes or compared to your own history are another indicator, particularly if the bills increased after renovations that may have disturbed ductwork. Excessive dust in the home can also point to duct leakage — when ducts leak in unconditioned spaces, they draw in unconditioned air (and whatever particles are in it) through the same gaps. A technician can perform a duct blaster test to measure total duct leakage and identify where the leaks are concentrated.

The financial impact of duct leakage depends on how much leakage exists, where the ducts run, and how much you pay for energy. Leaks in ducts that run through an unconditioned attic are particularly costly in summer — the attic may reach 130°F to 150°F, and any conditioned air leaking into that space is immediately overwhelmed. In winter, the same attic may be near outdoor temperature, so heated air leaking into it is simply lost.

Leaks on the return side of the duct system (the ducts that pull air back to the air handler) cause a different problem: the system draws in unconditioned air from the attic or crawl space, which it then has to condition. This increases the load on the equipment and can cause the system to run longer cycles, increasing wear and energy consumption simultaneously.

Traditional duct sealing involves a technician accessing the ductwork and applying mastic sealant — a thick, flexible compound — to all joints, seams, and connections. Mastic does not dry out or crack the way duct tape does, and a properly applied mastic seal lasts for the life of the duct system. Accessible ducts in basements and crawl spaces can typically be sealed in a day. Ducts in finished walls and ceilings are harder to access and may require a different approach.

Aeroseal is a newer technology that seals ducts from the inside. A technician pressurizes the duct system and injects a fine mist of sealant particles into the airstream. The particles travel through the ducts and adhere to the edges of any gaps or holes, building up a seal from the inside out. Aeroseal can reach leaks in inaccessible locations and typically reduces duct leakage by 70% to 90%. It is more expensive than manual mastic sealing but is the most effective option for homes with significant leakage in hard-to-reach areas.

For most homes with measurable duct leakage, duct sealing is a sound investment. A professional mastic sealing of accessible ductwork typically costs $500 to $1,500 depending on the size of the home and the extent of the leakage. Aeroseal costs $1,500 to $3,500 for a typical home. Energy savings of 15% to 25% on heating and cooling costs are commonly reported after sealing, which translates to a payback period of 3 to 7 years in most cases.

Beyond energy savings, duct sealing improves comfort by delivering more conditioned air to the rooms that need it, reduces the load on the HVAC equipment (which can extend its life), and improves indoor air quality by reducing the infiltration of unconditioned air and its associated particles. If your home has rooms that are consistently uncomfortable despite a functioning HVAC system, duct leakage is one of the first things worth investigating.

Duct sealing and duct cleaning are different services that address different problems. Duct sealing addresses air leakage — gaps and holes that allow conditioned air to escape. Duct cleaning addresses accumulated debris inside the ducts — dust, pet dander, mold, and other contaminants that can be distributed through the home when the system runs.

Both services may be appropriate depending on the condition of your ductwork. If your ducts have significant leakage, sealing should be done first — cleaning leaky ducts is less effective because the system will continue to draw in contaminants through the leaks. If your ducts are well-sealed but have not been cleaned in many years and you have had water intrusion, pets, or renovations, cleaning may be warranted. A technician can assess both conditions and recommend the right sequence.