Seal ducts with mastic or metal foil tape, never duct tape

Seal the duct joints you can reach with duct sealant (mastic) or metal-backed foil tape, never with duct tape. ENERGY STAR's duct sealing page says that in a typical house about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts, and that duct tape should never be used because it is not long-lasting. The one condition comes from DOE's 2022 Energy Saver guide: all duct sealing should be tested by a qualified professional to ensure combustion safety and proper air flow, so the job ends with an HVAC contractor's test. It is an owner's bigger job; a renter reports the symptoms to the landlord.

Uneven rooms and attic ducts are the signs to look for

ENERGY STAR's duct sealing page lists five signs of poorly performing ducts: high summer and winter utility bills, rooms that are difficult to heat and cool, stuffy rooms that never seem comfortable, ducts in an attic, crawlspace or garage, and tangled or kinked flexible ducts.

Free checks come first. DOE's Energy Saver guide (2022) says discomfort from heating and cooling can often be fixed by clearing blocked registers, freeing stuck dampers, fixing disconnected or damaged ducts, or replacing register grilles that don't direct the air adequately. Pull the rug or sofa off a floor register and open any closed ones. That costs nothing, and renters can do it too.

Do this, and seal the big air leaks around the house, before shopping for equipment. ENERGY STAR's heating and cooling page says to address the big air leaks in the house and the duct system before investing in a new HVAC system, because sometimes they are the real source of the problem rather than the equipment.

What as much as 20 percent means for a Texas air conditioner

DOE's 2022 guide says ducts that leak air into an attic or crawl space can add hundreds of dollars a year to heating and cooling bills, and that insulating and sealing ducts in unconditioned spaces is usually very cost effective.

Example, with published figures and one assumption. EIA's RECS 2020 Table CE5.3a puts air conditioning at 3,873 kWh a year for the average household that uses it in the West South Central census division, which includes Texas. Texas's average residential price was 15.88 cents per kWh in July 2026, the latest month in EIA's Electric Power Monthly, Table 5.6.A (preliminary). At that price the air conditioning costs 3,873 × 15.88 cents = $615.03 a year. ENERGY STAR's benefits page says leaky ducts can reduce heating and cooling system efficiency by as much as 20 percent; this example assumes that such a loss wastes the same share of the kWh the air conditioner draws. As much as 20% of 3,873 kWh is 774.6 kWh, and 774.6 × 15.88 cents = $123.01 a year. That is cooling alone. The arithmetic is the one used to price any appliance.

Seal the joints you can reach with mastic or foil tape

ENERGY STAR's heating and cooling page sets the order. Seal the ducts that run through the attic, crawlspace, unheated basement or garage first, with duct sealant (mastic) or metal-backed (foil) tape on the seams and connections. Then wrap those ducts in insulation. Next, seal any other ducts you can reach in the heated or cooled part of the house. The same page says sealing and insulating ducts can improve the efficiency of a heating and cooling system by as much as 20 percent, and sometimes much more.

Don't skip the registers. The ENERGY STAR duct sealing page says the connections at vents and registers, where they meet floors, walls and ceilings, are common places to find leaks and disconnected ductwork. Mastic and foil tape are a small buy. Of the four materials ENERGY STAR names, two are yours:

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An HVAC contractor tests the work and fixes what you cannot reach

DOE's Energy Saver guide (2022) sets the limit: hire a qualified professional to perform changes and repairs to a duct system, and have all duct sealing tested by a qualified professional to ensure combustion safety and proper air flow. ENERGY STAR's benefits page explains the safety half. Gas appliances such as water heaters, clothes dryers and furnaces release combustion gases, carbon monoxide among them, through their venting, and leaky ductwork may cause backdrafting, which draws those gases back into the living space. Sealing leaks can minimize that risk. In a home with gas appliances, carbon-monoxide alarms and a yearly inspection come first, and the flue and gas line stay with an HVAC contractor or a licensed gas fitter.

Ask the HVAC contractor for the work on ENERGY STAR's contractor list:

1. Inspect the whole duct system, including the attic and crawlspaces.
2. Evaluate the supply and return air balance; many systems have return ducts that are too small.
3. Repair damaged and disconnected ducts, and straighten tangled or crushed flexible ducts.
4. Seal all leaks and connections with mastic, metal tape or an aerosol-based sealant, and registers tightly to the ducts.
5. Insulate ducts in unconditioned areas with duct insulation that carries an R-value of 6 or higher.
6. Fit a new filter, and evaluate airflow after the repairs.
7. Check that gas or oil-burning appliances do not backdraft, and run a combustion safety test after the ducts are sealed.

The ENERGY STAR duct sealing fact sheet adds one hiring tip: look for NATE or BPI certified contractors.