How do I seal leaky air ducts?

Seal the duct joints you can reach with duct mastic or metal-backed foil tape, seal the registers where they meet the floor, wall or ceiling, and insulate any duct that runs through an attic, crawlspace or garage. Many homeowners handle the reachable joints themselves; others hire it out. Leave these to a contractor either way: ducts hidden in finished walls, return-side leaks that need testing to find, and any old duct wrap that might contain asbestos.

Why it is worth doing

In a typical house, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.^[1] Leaky ducts can reduce heating and cooling system efficiency by as much as 20 percent,^[3] and the lost air usually ends up in the attic or crawlspace you are paying not to heat or cool. Leaky return ducts can also pull air from those spaces into the house, and in a home with gas appliances, return-side leakage can help draw carbon monoxide back down a furnace or water heater flue.^[5]

"Some homeowners choose to take on duct sealing as a do-it-yourself project," starting with the ducts they can reach in attics, crawlspaces, unfinished basements and garages, while many others hire a professional contractor.^[1] Hiring is the recommended route for repairs: "It is best to have a licensed heating and air conditioning contractor repair your system's duct leaks."^[5] Supply and return ducts work together, and "sometimes a homeowner, in fixing one problem, may inadvertently create another."^[5] If you seal the reachable joints yourself, have a professional check the system afterwards, as covered below.

What you need

• Duct mastic (a brush-on paste) or metal-backed foil tape, or both
• A cheap paintbrush or putty knife for the mastic
• Duct insulation, if your ducts are bare, and zip ties to hold it
• Gloves, safety glasses, a mask and a flashlight

Finding the leaks

Turn on the system fan and feel along each joint for moving air.^[2] On a supply duct you can often feel air blowing or see nearby insulation moving. Look for holes, tears and loose joints at duct connections, around the air handler and near vents, and make sure registers are firmly attached to the subfloor or drywall.^[5] At the rooms themselves, the joints where registers meet floors, walls and ceilings are common places to find leaks and disconnected ductwork.^[1]

Flexible ducts are worth a look too. Kinks in flexible ductwork restrict airflow even when nothing is leaking, so check that flex ducts run straight with only gentle curves and are not torn, squashed or disconnected.^[3]

Mastic, foil tape, and what not to use

Duct mastic is a paste that is more durable than foil tape,^[2] and mastic alone can seal cracks less than a quarter inch wide.^[5] For a wider gap, lay foil tape across it and brush mastic over the tape. Whatever you buy should carry the UL 181 rating for your type of duct.^[5]

Gray cloth duct tape fails quickly and should not be used.^[2] Its rubber-based adhesive dries out, so if you find it, check every place it was used and reseal those joints,^[5] except where it sits on old wrap or insulation you are treating as possible asbestos (see below).

Where ducts pass through the attic floor, seal the gap around them with foam.^[2] After the joints are sealed, wrap ducts in attics, crawlspaces and garages with duct insulation rated at least R-6.^[2] Seal first, then insulate, or you will be cutting insulation back off to reach a leak later.

Working safely in an attic or crawlspace

Attics get hot fast, so start early in the morning in summer and drink plenty of water. In the attic, walk only on joists or truss chords, not on drywall or insulation, and watch for roofing nails coming through the deck overhead.^[2] Wear an OSHA-approved particulate respirator or double-strap dust mask around insulation.^[2]

Asbestos is the main reason to stop before you start in an older home. Asbestos-containing materials, including duct insulation and register boots, need specialized procedures and should be disturbed or removed only by trained and equipped contractors.^[6] You usually cannot tell whether a material contains asbestos by looking at it, and sampling it yourself is not recommended.^[8] EPA's rule of thumb: "If in doubt, treat the material as if it contains asbestos and leave it alone."^[8] In an older home, that means any old wrap, tape, insulation or register boot on the ducts, whatever its color or texture, stays in place until a trained and accredited professional has tested it.

After you seal

Tightening up the ducts changes how air moves through the house. ENERGY STAR cautions that a tighter house can let carbon monoxide build up if gas-burning appliances are not venting properly, and says to have your heating and cooling technician check the furnace, water heater and dryer for proper venting.^[2] Even in an all-electric home, have a professional check that the system is balanced once you are done.^[3]

Stop and call a pro if

• You find old duct wrap, tape or insulation that might contain asbestos.
• The leaky ducts are inside finished walls, ceilings or floors.
• You suspect return-side leaks. Return leaks are hard to find because return ducts run at lower pressure and pull air in, and sealing only the supply side can draw even more unconditioned air into the system.^[5]
• Duct insulation is wet or moldy. Wet or moldy duct insulation cannot be cleaned effectively and should be removed and replaced.^[6]
• A duct is crushed, collapsed or disconnected badly enough that it needs a new section, rerouting or an extra return. Ask your building department whether that work needs a permit. Fairfax County, for example, requires a mechanical permit for duct installation, new or replacement, and for new, replacement, repair or conversion of HVAC systems.^[7] See whether you need a permit to replace ductwork in Virginia.
• You have a gas or oil furnace, water heater or dryer and have not had its venting checked since sealing.

For the full job, look for a contractor that inspects the whole duct system, checks the balance between supply and return, seals with mastic, metal tape or an aerosol sealant, and runs a combustion safety test after the ducts are sealed.^[4]

Related

If one room stays hot or cold after sealing, read why some rooms are hotter than others. If someone is selling you duct cleaning instead, read the warning signs of a duct cleaning scam. Browse heating and cooling services, find HVAC businesses in Blacksburg, or read more heating and cooling answers.

When the job is bigger than the ducts you can reach, describe it once and licensed local HVAC businesses quote it.