Your AC runs all afternoon. The thermostat says 74°F, but the bedrooms upstairs feel like 80°F. Your bill is higher than your neighbor’s. And every time you dust, it is back a few days later.
Before you blame the air conditioner, look at the ducts. According to ENERGY STAR, in a typical home about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes, and poorly connected ducts. That can make even a brand-new AC or furnace feel weak.
This guide explains how much of that loss actually matters in Denver homes, where ducts usually leak, how sealing works, what it costs, and how to tell if it is worth doing in your house.
How much is lost. ENERGY STAR estimates typical homes lose 20 to 30 percent of duct air to leaks, and that leaky ducts can reduce heating and cooling efficiency by as much as 20 percent.
Where it matters most. Leaks in attics, crawlspaces, garages, and outside walls are true losses. Leaks in a conditioned basement mostly stay inside the house, but they still starve upstairs rooms of air.
Signs you have leaks. Hot or cold rooms, a hot upstairs in summer, high bills, extra dust, and ducts you can see in an attic or crawlspace.
The fix. Seal joints and seams with mastic or approved foil tape, reconnect loose ducts, seal register boots, insulate ducts in unconditioned spaces, or use aerosol sealing from the inside for hard-to-reach ducts.
Is it worth it. Usually yes if your ducts run through unconditioned spaces, you have comfort problems, or you are about to buy new equipment.
First, the honest version of the 20 to 30 percent number
You will see “20 to 30 percent” quoted everywhere, so it is worth being precise.
- That figure is about air lost from the ducts, not always energy lost from the house.
- If a duct leaks inside your finished or heated basement, the cold (or warm) air still ends up in your home. It is not wasted money, but it is misdirected: it cools the basement instead of the bedroom you wanted to cool.
- If a duct leaks into your attic, crawlspace, garage, or an outside wall cavity, that air is truly lost. You paid to cool it and it went outside your living space.
- Return-side leaks can be the worst of all. They pull air into the system from wherever the leak is: a 130°F attic in July, a dusty crawlspace, or a garage with car exhaust.
So the real question for your home is: where do your ducts run, and where are they leaking?
Where ducts leak in Denver-area homes
Denver’s housing stock is a mix of 1950s to 1970s ranches, split-levels and bi-levels, and newer two-story homes. Each has its own weak spots.
Homes with basements (common across Denver, Arvada, Lakewood, and Wheat Ridge)
- The furnace and main trunk ducts are usually in the basement ceiling.
- Leaks at trunk seams and branch takeoffs often blow into the basement. That is less wasteful but causes weak airflow upstairs.
- Panned joist returns are common in older homes. Builders nailed sheet metal across the floor joists and used the space as a return duct. These are very leaky and can pull air from wall cavities and rim joists.
Split-levels and bi-levels
- Ducts often run through several short runs and odd transitions, with more joints to leak.
- Rooms over the garage are a frequent problem. Ducts may pass through or near the unconditioned garage.
Two-story homes and bonus rooms
- Upstairs supply ducts often run a long way from the basement furnace, so every leak along the way reduces what reaches the bedrooms.
- Bonus rooms over garages and some upstairs runs pass through attics or knee walls, where summer temperatures are extreme.
- This is a big reason upstairs rooms run hot in summer.
Homes converted from swamp coolers or with attic ducts
- Homes that added central AC later sometimes have flexible ducts in the attic. These can sag, kink, tear, or pull loose from their connections.
- Attic ducts face the hottest air in the house in summer and the coldest in winter.
Everywhere: the connections you can see
- Register boots: the gap between the metal boot and the drywall or floor around each vent
- The furnace and coil connection: where the supply plenum meets the furnace and AC coil
- Filter slots and access panels that do not seal well
- Old cloth duct tape that dried out and fell off years ago
Why duct leaks hurt more in Denver
Two local factors make duct losses more noticeable here.
1. Your AC already loses capacity at altitude
At 5,280 feet, thinner air means an air conditioner moves less heat than the same unit would at sea level. We explain why in how Denver’s altitude affects your AC’s cooling capacity. When your system starts with less capacity, it cannot afford to lose more through leaky ducts.
2. Hot attics and big temperature swings
Denver’s strong summer sun can push attic temperatures far above the outdoor temperature. A supply leak in the attic dumps cold air into that heat. A return leak pulls super-heated air into your system. Both make the AC run longer.
In winter, the same leaks lose heated air, and cold drafts pulled in by return leaks make rooms feel chilly.
Signs your ducts are leaking
- Some rooms are always too hot or too cold, especially rooms farthest from the furnace
- Upstairs is much hotter than downstairs in summer
- Weak airflow from some registers
- Your AC or furnace runs long cycles but the house never feels quite right. See why does my AC run constantly but never cool the house?
- More dust than normal, or dust streaks around registers
- Higher energy bills without a clear reason
- You can see ducts in the attic, crawlspace, or garage, especially flex duct
- Whistling or rattling from the duct system
- Stuffy or musty smells that could come from air pulled in from a crawlspace or attic
A safety issue most people do not know about
Leaky return ducts near the furnace or water heater can lower the air pressure in the mechanical room. If you have a standard (non-sealed) gas water heater or an older furnace that draws combustion air from the room, that negative pressure can pull exhaust gases back into the house instead of up the flue. This is called backdrafting, and it can release carbon monoxide.
Return leaks in or near an attached garage can also pull in car exhaust and fumes.
This is one more reason to have working carbon monoxide alarms, and to have a professional check return ducts near fuel-burning appliances. Read more in how to tell if your furnace is leaking carbon monoxide.
Duct sealing vs. duct cleaning: not the same thing
People often mix these up.
| Duct sealing | Duct cleaning | |
|---|---|---|
| What it does | Closes leaks, holes, and loose joints | Removes dust and debris from inside the ducts |
| Main benefit | Better comfort, efficiency, and airflow to the right rooms | Can help after construction, pests, or visible contamination |
| Effect on energy bills | Can lower them by stopping lost air | Little direct effect in most homes |
If your main complaints are uneven temperatures and high bills, sealing usually matters more. We cover cleaning separately in can dirty air ducts make your furnace less efficient?.
How professionals find duct leaks
Duct leakage testing
The most accurate method is a duct leakage test. A technician seals off the registers, connects a calibrated fan to the duct system, and measures how much air leaks out at a set pressure. The test can measure total leakage and, with an added step, how much leaks to the outside of the home. Testing before and after sealing shows exactly how much was fixed.
Visual and hands-on inspection
- Looking at every reachable joint, seam, and connection
- Checking flex ducts for sagging, crushing, or disconnections
- Checking register boots
- Measuring airflow at registers and system static pressure
Thermal imaging
An infrared camera can show hot or cold spots in ceilings and walls where ducts may be leaking.
How ducts are sealed
1. Mastic sealant
A thick paste brushed onto joints and seams. For larger gaps, it is applied over fiberglass mesh tape. Mastic lasts for decades and is the standard for accessible metal ducts.
2. Approved foil tape
Foil tape rated for ducts (look for UL 181 on the label) works on some joints and flex duct connections. Regular cloth “duct tape” is not a good choice. It dries out and fails, often within a few years. ENERGY STAR’s own guidance says never to use it.
3. Reconnecting and supporting flex ducts
Loose flex ducts are reattached with proper clamps and sealed. Sagging runs get supported so they do not kink or pinch.
4. Sealing register boots
The gap between each register boot and the drywall or floor is sealed from the room side. It is a small job that adds up across a whole house.
5. Aerosol sealing from the inside
For ducts hidden in walls, floors, and ceilings, an aerosol sealing process can seal leaks from the inside. The duct system is pressurized and fed a fine sealant mist that collects at the edges of leaks and closes them. It is measured in real time, so you get a before-and-after leakage number. It is more expensive but reaches leaks no one can get to by hand.
6. Insulating ducts in unconditioned spaces
After sealing, ducts in attics, crawlspaces, and garages should be insulated. Sealing stops air loss. Insulation stops heat moving through the duct walls.
What does duct sealing cost in Denver?
Costs depend on how much of the system is accessible, how leaky it is, and which method is used. As a rough guide:
| Type of work | Typical cost range |
|---|---|
| DIY sealing of accessible basement joints with mastic | Under $100 in materials |
| Professional sealing of accessible ducts (basement, attic) | Often a few hundred dollars to around $1,500 |
| Aerosol sealing of the whole duct system | Often about $1,500 to $3,000 |
| Duct repairs, reconnecting or replacing damaged sections | Varies widely by access and scope |
Get a clear scope in writing, and ask whether before-and-after testing is included.
Are there rebates?
Standalone duct sealing rebates come and go. Xcel Energy currently offers insulation and air sealing rebates for eligible customers through participating contractors. Rules require specific testing and qualifying measures, so ask before you start. The federal energy efficiency tax credit that once covered some of this work ended December 31, 2025. See our 2026 rebates and tax credits guide.
Is duct sealing worth it for your home?
Duct sealing usually pays off when:
- Your ducts run through the attic, crawlspace, or garage
- You have rooms that never get comfortable
- Your home is older and the ducts have never been checked
- You are replacing your furnace, AC, or adding a heat pump. Sealing first means your new system can be sized correctly and actually deliver its rated performance. See how to choose the best HVAC system for a Denver home.
- You are switching to a heat pump. Heat pump air is warm, not hot, so duct losses feel more noticeable in winter. See do heat pumps work in Denver winters?.
It may matter less when:
- All your ducts are inside a finished, conditioned basement and every room is comfortable
- Your ducts were recently tested and sealed
What you can safely do yourself
- Seal accessible metal duct joints in an unfinished basement with mastic (wear gloves, it is messy).
- Replace old cloth duct tape with mastic or UL 181 foil tape.
- Check flex ducts in the attic for obvious disconnections. Be careful where you step.
- Keep all registers open. Closing vents raises duct pressure and makes leaks worse. See does closing vents in unused rooms save energy?.
Leave these to a professional:
- Anything near the furnace flue or water heater vent. Never tape or seal a flue pipe.
- Work in tight attics or crawlspaces with insulation, wiring, or pests
- Leakage testing and aerosol sealing
- Return duct changes near gas appliances
Pairing duct work with lower bills
Sealed ducts also make the most of other savings steps. If you are on Xcel’s time-of-use plan, a tighter duct system holds pre-cooling better through the 5 to 9 p.m. peak window. See Xcel time-of-use rates: how to run your AC without paying peak prices.
Get a duct and airflow assessment
If your upstairs runs hot, some rooms never feel right, or your bills keep climbing, your ducts are one of the first places to look. Our NATE-certified technicians check ductwork, airflow, and static pressure as part of diagnosing comfort problems and planning new systems, so we can tell you whether duct work will actually solve the problem before you spend money on new equipment.
Comfy Cave Heating & Air is based in Arvada and serves Denver and the surrounding metro. Call 303-645-4889 or request an estimate. You can also explore our cooling services and heating services.
Frequently asked questions
How much air do leaky ducts lose?
According to ENERGY STAR, typical homes lose about 20 to 30 percent of the air that moves through the duct system to leaks, holes, and poorly connected ducts. Leaky ducts can reduce heating and cooling efficiency by as much as 20 percent. The real impact depends on whether the ducts run through conditioned or unconditioned spaces.
Is duct sealing worth it in Denver?
It is usually worth it if your ducts run through an attic, crawlspace, or garage, if you have hot or cold rooms, or if you are buying new equipment. It matters less if all your ducts are inside a conditioned basement and your home is already comfortable.
How much does duct sealing cost?
Professional sealing of accessible ducts often costs a few hundred dollars to around $1,500. Aerosol sealing of a whole system often costs about $1,500 to $3,000. The price depends on access, how leaky the system is, and whether testing is included.
Can I seal my ducts myself?
Yes, for accessible metal duct joints, like those in an unfinished basement. Use mastic or UL 181-rated foil tape, not regular cloth duct tape. Leave attic and crawlspace work, leakage testing, and anything near gas appliance vents to a professional.
What is the difference between duct sealing and duct cleaning?
Duct sealing closes leaks so air goes where it is supposed to. Duct cleaning removes dust and debris from inside the ducts. Sealing usually has a much bigger effect on comfort and energy bills. Cleaning helps in specific cases like after construction or pest problems.
How do I know if my ducts are leaking?
Common signs include rooms that are always too hot or cold, a hot upstairs in summer, weak airflow from some vents, extra dust, and higher bills. The only way to know how much they leak is a duct leakage test done by a professional.
Does duct tape work for sealing ducts?
Regular cloth duct tape is a poor choice for sealing ducts. It dries out and loses its grip over time. ENERGY STAR advises never to use it. Use duct mastic or foil tape rated to UL 181 instead.
Will sealing my ducts help my upstairs stay cooler?
Often, yes. Leaks along long duct runs to upstairs rooms reduce how much cool air reaches them. Sealing those leaks, along with checking airflow and return air paths, is one of the most common fixes for hot upstairs rooms in two-story Denver homes.
Find Out Where Your Cooling Is Going
Duct, airflow and static pressure check before you spend money on new equipment. We will tell you honestly if sealing solves it.