Understanding your result
The headline is the money lost to duct leakage each year, with the share of the heating and cooling bill and the saving and payback of sealing in the sub-line. The bars compare the loss now with the loss after sealing to about 5%. The stats give the share lost, the saving and the payback. The table shows the leakage as a percentage and in CFM, the location and how much of the leaked energy it loses, the season costs, the loss before and after sealing, and the capacity the system loses on a design day.
The capacity lost row is the comfort side of the same figure. A system leaking 25% into an attic delivers about three-quarters of its rated heating and cooling to the rooms, which is why sealing often fixes a room that never gets comfortable.
The duct system’s size and layout come from the duct size and equivalent length calculators, and the duct velocity calculator checks an existing run; if the ducts are beyond sealing, the ductwork cost calculator prices replacement. The season costs come from your bills or the HVAC running cost calculator.
How we calculate this
The 20 to 30% typical leakage is ENERGY STAR’s figure for duct systems in US houses. The location factors are our judgement of how much of the energy in leaked air is truly lost, set out here so they can be checked: none of it is lost inside the conditioned space, most of it in a vented attic, part of it in a crawl space or basement, where some of the heat warms the floor above. Using CFM25 divided by system airflow as a leakage percentage is the usual shorthand; a test that measures leakage to outside at operating pressure is more accurate. Duct sizes and velocities are on the duct size chart.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Typical leakage | 25% (20 to 30% range) | ENERGY STAR, duct sealing guidance |
| Location factors | 1.0 attic, 0.7 crawl, 0.3 basement, 0.1 inside | Judgement on the share of leaked energy lost |
| Leakage after sealing | About 5% | Typical results of thorough mastic or aerosol sealing |
| Sealing cost | $1,200 | Mid-range: mastic sealing $300 to $1,000; aerosol $1,500 to $3,000 (2024–25 cost surveys) |
| System airflow | 1,200 CFM | A 3-ton system at 400 CFM per ton |
| New-construction limit (reference) | 4 CFM25 per 100 sq ft to outside | IECC duct leakage requirements |
Assumptions last reviewed October 8, 2026.
The calculator does not separate supply and return leakage, does not model the extra infiltration that unbalanced leaks drive through the house, and does not count duct insulation, which matters as much as sealing in an attic. It treats the season costs as entirely flowing through the ducts, which is not true of baseboard or radiant heat. The guide to duct leakage and sealing covers testing, sealing methods and insulation, and the guide on return air covers the return side.
Two worked examples
Attic ducts at typical leakage
1,200 CFM system, 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. The note points to the return plenum and the air handler cabinet, often the largest leaks in an attic system and the cheapest to seal.
A tested crawl-space system
1,000 CFM system, 180 CFM25 measured leakage to outside, ducts in a vented crawl space, $900 heating and $400 cooling, $800 to seal.
- Leakage: 180 ÷ 1,000 = 18%
- Share lost: 18% × 0.7 = 12.6%
- Money lost: $1,300 × 12.6% = $164 a year
- After sealing to 5%: $45; saving $118; payback 6.8 years
A crawl space recovers part of the leaked energy through the floor, so the payback is longer than in an attic. Insulating the crawl-space ducts at the same time adds to the saving, and encapsulating the crawl space would bring the ducts effectively inside the house.
Where to find your inputs
System airflow. 400 CFM per ton of the system, from the model number of the outdoor unit or the air handler.
Leakage. A duct blaster test report gives CFM25 to outside; without one, 25% is a fair estimate for an older system and 10 to 15% for one installed under modern codes.
Duct location. Look where the main trunks and the air handler are. A system with ducts in two places, such as an attic and a basement, can be run twice with each share of the airflow.
Season costs. The heating and cooling parts of the bills, or the running cost calculators.
Common mistakes
- Cloth duct tape. It fails within a few years; use mastic or UL 181 foil tape.
- Sealing only the supply. Return leaks in an attic are often the larger problem.
- Sealing without testing. Test before and after to know what was achieved.
- Ignoring insulation. Sealed ducts in a hot attic still gain heat through the metal.
- Buying a bigger system. A room starved by leaks needs sealed ducts, not more tonnage.
- Counting leakage inside the house as lost. Leaks into conditioned space cost little energy.
Questions people ask
- How much do leaky ducts cost?
- ENERGY STAR estimates that typical duct systems lose 20 to 30% of the air moving through them to leaks, holes and poorly connected joints. With ducts in a vented attic most of the energy in that air is lost, so a house spending $1,900 a year on heating and cooling with 25% leakage loses about $475. The same leakage in ducts inside the conditioned space costs almost nothing in energy, though rooms at the ends of the runs still go short of air.
- How much does duct sealing save?
- Sealing accessible ducts with mastic or sealing the whole system with an aerosol sealant typically brings leakage down to around 5% of airflow. On the $1,900 example with attic ducts that saves about $380 a year, so a $1,200 sealing job pays back in about three years. In a crawl space, where some leaked energy still benefits the house, the saving is smaller and the payback longer, six to seven years in a typical case.
- What is CFM25?
- The air leakage of the duct system measured in cubic feet per minute when the ducts are pressurised to 25 pascals with a duct blaster fan, the standard test in building codes and energy programmes. Leakage to outside (excluding leaks into the conditioned space) is the figure that matters for energy. Many codes cap new systems at 4 CFM25 per 100 sq ft of floor area; existing houses commonly test at 15 to 30 per 100 sq ft.
- Do return duct leaks matter as much as supply leaks?
- In an attic or crawl space, yes, and sometimes more. A supply leak dumps conditioned air outside the house; a return leak pulls unconditioned attic or crawl-space air into the system, at 120 °F in summer or near freezing in winter, along with dust and sometimes moisture or soil gases. Return leaks at the air handler cabinet and the return plenum are often the largest and the cheapest to seal.
- Can I seal ducts myself?
- Accessible joints in an attic, basement or crawl space can be sealed with water-based duct mastic brushed over the seams, or with UL 181-rated foil tape; never use ordinary cloth duct tape, which fails within a few years. Hidden ducts inside walls and floors can only be reached with an aerosol sealing service, which seals the system from the inside. Have the system tested before and after to see what was achieved.
- Does duct leakage affect comfort as well as the bill?
- Yes. A system that loses a quarter of its air loses a quarter of its capacity on the hottest and coldest days, so rooms at the far ends of the duct runs never reach the setpoint, the system runs longer, and leaky returns can depressurise the house and pull in outdoor air. Sealing often fixes "the back bedroom is always hot" problems that people try to solve with a bigger air conditioner.