Energy & costs
Duct leakage: the 20% you pay for and never feel
Ducts in an attic or crawl space typically leak a fifth or more of the air the system makes. Sealing them is one of the best paybacks in a house.
How much leaks
ENERGY STAR estimates that in a typical house, about 20 to 30% of the air moving through the duct system is lost to leaks, holes and poorly connected ducts. The leaks are mostly at joints: where branches meet the trunk, where boots meet the floor or ceiling, at the plenums and in the air handler cabinet itself. Flex duct pulled off its collar, or never taped, can leak far more.
Supply leaks push heated or cooled air out before it reaches a room. Return leaks pull in air from the attic, crawl space or garage, which then has to be heated or cooled, and which may bring dust, moisture or fumes with it. Both cost money, and both reduce the airflow the rooms receive.
Where the ducts are decides the cost
Leaked air costs only what is lost from the conditioned space. Ducts inside the house leak into rooms that need the heat anyway; ducts in a vented attic leak to the outdoors. The duct leakage loss calculator applies a factor for the location:
| Where the ducts run | Share of the leaked energy actually lost |
|---|---|
| Vented attic | 100% |
| Vented crawl space or garage | 70% |
| Unconditioned basement | 30% |
| Inside the conditioned space | 10% |
These factors are this site’s estimates, not a standard; a crawl space or basement recovers part of the leaked energy through the floor above, an attic recovers almost none. In a hot climate, attic ducts are the worst case of all, because the attic can reach 130 °F or more and return leaks pull in that air.
What it costs
Attic ducts at typical leakage
A 1,200 CFM system with 25% leakage, ducts in a vented attic, $1,300 heating and $600 cooling a season, $1,200 to seal.
- Share lost: 25% × 1.0 = 25%
- Money lost: $1,900 × 25% = $475 a year
- After sealing to 5%: $95; saving $380 a year
- Payback: $1,200 ÷ $380 = 3.2 years
Three years is one of the best paybacks in home energy. In a crawl space with 18% measured leakage, $1,300 of heating and cooling and $800 to seal, the loss is $164 a year and the payback is 6.8 years, longer because the crawl space recovers part of the leak.
How leakage is measured
A duct blaster test pressurizes the duct system to 25 pascals, with the registers taped over, and measures the airflow needed to hold that pressure. The result, in CFM25, is the leakage. Testing leakage to outside, with the house pressurized to the same level by a blower door, separates leaks to the outdoors from leaks into the house. Many state energy codes require new duct systems to leak no more than 4 CFM25 per 100 sq ft of floor area, which on a 2,000 sq ft house is 80 CFM25. Dividing the measured CFM25 by the system airflow gives a leakage percentage the calculator can use.
Without a test, a visual inspection finds the worst leaks: disconnected flex, open joints at the plenum, unsealed boots and a cabinet with gaps around the filter door.
How to seal
Mastic is a paste brushed onto joints, often with fibreglass mesh tape across larger gaps. It is the standard for sheet metal and lasts for decades. Foil tape rated UL 181 works on flat joints; ordinary cloth “duct tape” does not last and should not be used. Aerosol sealing blows a fine sealant through the pressurized duct system from inside, closing leaks up to about the size of a pencil, including those in ducts hidden inside walls and ceilings. It typically brings leakage down to around 5%.
The priorities are the parts near the air handler, where pressure is highest: the supply and return plenums, the cabinet and the filter slot. These are often the largest leaks and the easiest to reach.
Leaks and airflow
Sealing a leaky supply duct sends more air to the rooms; sealing a leaky return reduces the air pulled from the attic or crawl space. Both can change how the system balances, and rooms that were starved can suddenly get enough. Before replacing a system that “never keeps up”, seal and test the ducts: a 3-ton system losing 25% of its air delivers closer to 2.25 tons to the rooms. The guide to duct sizing basics and the duct size calculator cover whether the ducts are big enough once they are tight, and the guide to return air covers the return side.
Insulation
Ducts outside the conditioned space also lose heat through their walls. Codes typically require R-8 insulation on supply ducts in attics in most climates, and R-6 elsewhere. Sealing comes first, because insulation wrapped over a leaky joint hides the leak and does not stop it.
Replacement or repair
If the ducts are badly damaged, crushed, or far too small, replacement may be the better answer. The ductwork cost calculator gives sourced ranges: flex at $7 to $29 a foot installed and sheet metal at $21 to $62, so a whole house in flex runs $1,650 to $7,000. When a new system goes in, ask for the ducts to be sealed and tested as part of the job, as the guide to what HVAC installations cost recommends. The HVAC running cost calculator shows what each hour of heating and cooling costs, which is what leaks waste.