Most homes with forced-air heating and cooling have leaky ducts. According to ENERGY STAR and the Department of Energy, the typical house loses 20 to 30 percent of the air moving through its duct system before that air ever reaches a register. That is not a small inefficiency. It means your furnace or heat pump is doing real work — and roughly a quarter of the output is escaping into your crawl space or attic instead of heating your rooms.
The frustrating part is that duct leaks are largely invisible. You cannot see them from inside the house. Most homeowners have no idea they have them. But there are signs, and if several of them apply to your home, it is worth paying attention.
Sign 1: Rooms That Are Hard to Heat or Cool
This is the most common complaint I hear, and duct leakage is often the cause. If certain rooms never quite reach the temperature you set, or if upper floors are consistently colder than the main floor in winter, the duct system is the first place to look.
Supply ducts have to travel farther to reach rooms at the ends of the system. Any air lost along the way means less volume arriving at the register. The room gets some heat, but not as much as the system is supposed to be delivering. The thermostat on the main floor may be satisfied while bedrooms at the far end of the house are not.
This problem often gets worse over time as duct connections deteriorate or flex duct develops small tears at connections.
Sign 2: Energy Bills That Are Higher Than They Should Be
Duct leaks force your system to run longer to make up for the conditioned air that escaped. The equipment does not know the air leaked out — it just keeps running until the thermostat is satisfied, and it takes longer to get there.
The DOE states that leaky ducts can add hundreds of dollars per year to your heating and cooling bills and reduce your system’s efficiency by as much as 20 percent. That is not a precise figure I can attach to your house specifically, because it depends on where your ducts run, how leaky they are, and how much you pay for energy. But if your bills feel out of proportion to what your neighbors are paying for similar square footage, duct leakage is a reasonable suspect.
Sign 3: Excessive Dust
Return ducts pull air back to the air handler to be reconditioned and recirculated. If return ducts leak, they are not just leaking air out — they are pulling air in from wherever they are leaking. That means drawing in unfiltered air from your crawl space, attic, or wall cavities.
Crawl spaces in Western Washington accumulate insulation fibers, soil particles, and debris. If you are noticing more dust on surfaces than usual, or if you are replacing your air filter more frequently than you used to, leaky return ducts are worth investigating. The air entering through those leaks bypasses your filter entirely.
Sign 4: The System Runs Longer Than It Should
A properly functioning forced-air system should cycle on, run for a while, reach the set temperature, and cycle off. When ducts leak significantly, the system never quite delivers what it is supposed to, and run times stretch out.
You can learn your system’s normal behavior by paying attention to how often it cycles during mild weather when there is not much demand. If the system seems to run almost continuously in moderate temperatures, something is reducing its effectiveness. Duct leakage is one possibility; undersized equipment and poor duct insulation are others. These problems sometimes exist together.
Sign 5: Problems You Can Actually See
If you have access to your ductwork — in a crawl space, unfinished basement, or attic — it is worth taking a look. You do not need any equipment. Common visible problems include:
Disconnected flex duct. Flexible duct is held to metal collars and take-offs with clamps or tape. These connections can slip over time, especially if the duct was not installed with enough support. A disconnected flex duct section can dump your entire supply into the crawl space. This is not rare — I find it on service calls more often than you would think.
Gaps at register boots. The metal boot connects the duct to the register in your floor or ceiling. The gap between the boot and the framing is almost never sealed in older construction. Air leaks around the outside of the boot into the wall cavity or subfloor space rather than coming out the register.
Loose joints and visible gaps. Sheet metal duct sections are connected at joints. If you can see light through a joint, or feel air escaping when the system is running, that joint is leaking.
Duct tape that has failed. Standard cloth-backed duct tape — the kind that used to be used for duct sealing — has a short lifespan. Once it dries out and loses adhesion, it peels away and the joint it was covering is open again. Ironically, despite its name, conventional duct tape is not an acceptable material for sealing ducts.
Where Leaks Most Commonly Occur
Based on DOE and Building America research, the most common leak points in a residential duct system are:
- Register boots — where the duct meets the floor or ceiling framing, rarely sealed
- Air handler cabinet — the blower cabinet itself can account for 10 to 15 percent of system leakage
- Plenum connections — where the main supply plenum attaches to the air handler
- Flex duct connections — at take-offs, collars, and boots where flex attaches to rigid duct
- Return air chases — framed-in return plenums using wall cavities or joist bays instead of actual ductwork almost always leak significantly
How Leaks Are Actually Measured
If you want to know precisely how leaky your duct system is, there is a standardized test for it: the duct blaster test (technically a duct pressurization test per ASTM E1554).
All supply and return registers are temporarily sealed with tape or foam plugs. A calibrated fan — the duct blaster — is connected to the system at the air handler or a register opening. The fan pressurizes the duct system to 25 Pascals. The airflow required to maintain that pressure is measured in CFM (cubic feet per minute). That number — typically expressed as CFM25 — tells you the system’s total leakage.
Results can also be measured as leakage to the outside, meaning air escaping into unconditioned spaces rather than just between duct sections inside the conditioned envelope. This is the more meaningful number from an energy standpoint.
For reference, ENERGY STAR’s threshold for new-home certification is leakage to outdoors of no more than 4 CFM25 per 100 square feet of conditioned floor area. An older home with original ductwork often comes in well above that.
The test is performed by technicians with BPI Infiltration and Duct Leakage (IDL) certification or HERS Raters certified through RESNET. In Washington, third-party testing is required for new construction under the 2021 WSEC. For existing homes, it is typically done as part of an energy audit. Expect to pay $150 to $500 depending on home size and who performs it.
How Leaks Are Fixed
Mastic sealant is the preferred material — it is a paste-like compound applied by brush or by hand over joints and connections. Unlike tape, mastic handles the expansion, contraction, and vibration that ductwork experiences over its life without cracking or peeling. For gaps larger than about a quarter inch, fiberglass mesh tape is applied first and the mastic is spread over it. Mastic is a durable, code-appropriate repair.
Metal-backed foil tape is acceptable for smaller joints and situations where mastic is not practical. The specific requirement is that it carries a UL 181A-P or UL 181B-FX listing — this is foil tape designed specifically for duct sealing, not generic silver tape from the hardware store.
Cloth-backed “duct tape” should not be used. This is explicitly called out by ENERGY STAR and the DOE. Despite the name, conventional duct tape fails quickly — it dries out, loses adhesion, and peels away. It is not code-compliant for duct sealing, and any joint sealed with it will need to be redone.
Under the 2021 Washington State Energy Code, metallic ducts in unconditioned spaces must have all joints sealed with mastic. Tape alone does not satisfy the code requirement.
What You Can Do Yourself vs. What Requires a Professional
If you have access to your ductwork and can identify obvious problems — a disconnected flex section, visible gaps at joints, register boots that were never sealed — you can address those yourself with mastic and mesh tape. It is not complicated work. Put on gloves, brush or spread the mastic over joints, let it cure.
The parts that are worth having a professional handle:
- Leaks inside the air handler cabinet and blower compartment
- Return air chases built into wall cavities (these require access that may not be easy)
- Ducts buried in walls or inaccessible ceiling spaces
- Any situation where you want a quantitative baseline rather than just guessing at problem areas
If you want to know the actual state of your duct system, start with a duct blaster test. That gives you a number and tells you whether duct sealing is worth prioritizing.
If you are in the Kelso or Longview area and want someone to take a look, call me. Duct problems are often part of what I am already evaluating on a service call, and it is worth knowing what you are dealing with before something fails.