Where Leaked Air Ends Up
ENERGY STAR estimates that about 20 to 30 percent of the air moving through a typical home's ducts escapes through leaks, holes and poorly connected ducts. The result is higher utility bills and rooms that stay uncomfortable no matter how you set the thermostat. If one room is always the outlier, the uneven room temperature guide walks through the checks.
Supply-Side Leaks
Heated or cooled air escapes into the attic, crawl space or wall cavities before it reaches the room.
Return-Side Leaks
The system pulls in attic, crawl space or garage air, along with its dust and heat.
Virginia's Leakage Limits, in Plain Numbers
Section N1103.3.6 of the 2021 Virginia Residential Code limits total duct leakage in new construction. The test runs at 0.1 inch of water column, which equals 25 pascals.
| When the test happens | Limit per 100 sq ft of conditioned floor area |
|---|---|
| Rough-in, air handler installed | 4.0 cfm |
| Rough-in, air handler not installed | 3.0 cfm |
| Post-construction | 4.0 cfm |
For a 2,400-square-foot home, 4.0 cfm per 100 square feet works out to 96 cfm of total leakage. The code calls for a written report of the results and allows a licensed mechanical contractor to run the test. Buying new? The new home duct guide covers the builder walk-through.
Three Ways Technicians Find Duct Leaks
| Method | What it tells you | Best for |
|---|---|---|
| Duct pressurization test | Total leakage in cfm | Code compliance; before-and-after sealing numbers |
| Smoke or fog | Where air escapes at joints and boots | Pinpointing leaks you can reach |
| Infrared camera | Temperature patterns left by leaking air | Leaks behind drywall or above ceilings |
How Is Ductwork Leakage Tested?
Cover Every Register and Grille
Air now leaves the ducts only through leaks.
Connect the Test Fan
A calibrated fan attaches at the air handler or a large return.
Pressurize to 25 Pascals
The fan speeds up until the ducts hold 0.1 inch of water column.
Read the Airflow
The airflow needed to hold that pressure equals the total leakage.
Compare the Number
Check it against the code limit, or against the reading taken before sealing.
A high reading leads to duct sealing at the worst joints first.
Checks You Can Run at Home
- With the fan running, hold a strip of tissue near reachable joints; movement means escaping air
- Look for dark dirt streaks on duct insulation, a sign of air passing through
- Walk attic and basement runs for flex duct pulled off its fitting, a job for ductwork repair
- Compare rooms: one weak register often traces back to a single disconnected run
These checks catch big leaks. Only a pressure test gives you the total.