Duct sealing that stops paying to heat the attic

Ducts are rarely airtight. Joints separate, seams loosen, connections at registers and the air handler were never sealed, and building cavities used as return paths leak into walls. According to ENERGY STAR, a typical house loses about 20 to 30 percent of the air moving through its duct system to leaks, holes and poorly connected ducts. When ducts run through attics, garages or crawlspaces, that air is lost to spaces you are not trying to condition.

Leaks on the return side are especially troublesome. They pull air from dusty, humid or hot spaces into the system and distribute it through the home, carrying dust, fumes and moisture. Supply leaks waste the air you paid to heat or cool. Both reduce comfort and make equipment run longer.

Duct sealing finds and closes those leaks at the source: plenum connections, branch takeoffs, register boots, panned joist returns and every joint along accessible runs.

The most durable way to seal ductwork is with water-based duct mastic, a thick paste that dries into a flexible seal and bonds to sheet metal and duct board. For gaps wider than about a quarter inch, fiberglass mesh tape is embedded in the mastic for reinforcement. UL 181 listed foil tapes are also acceptable in some applications. Common cloth duct tape is not suitable for sealing ducts, because its adhesive breaks down with temperature changes and the seal eventually fails.

The largest leaks are often found at the air handler and plenums, where large volumes of air pass through connections that may never have been sealed. Return systems that use building cavities, such as panned floor joists or wall chases, are also notorious for leakage because they were never airtight to begin with. Register boots, where branch ducts meet the ceiling or floor, often have gaps between the boot and the drywall that pull air from wall or attic cavities. Sealing these connections typically produces a larger improvement than sealing straight runs.

Duct leaks tend to occur in predictable places. Connections at the furnace or air handler, joints between duct sections, boots where ducts meet registers and seams in return plenums are common leak points. Panned returns, where a floor joist cavity is used as a return duct, often leak around their edges and pull air from wall cavities or crawlspaces.

Sealing begins with identifying the leaks. Visual inspection of accessible ducts reveals disconnected sections, open seams and gaps around boots. Pressure testing can quantify leakage and show whether sealing efforts are effective. Smoke or thermal imaging can help locate leaks that are hard to see.

Accessible leaks are sealed with mastic or approved foil tape rather than ordinary cloth duct tape, which dries out and fails. Boots are sealed to the drywall or subfloor, and connections at the equipment are sealed to prevent the blower from pulling in unconditioned air. Disconnected ducts are reattached and supported.

After sealing, airflow at registers typically improves, especially in rooms at the end of long runs. Homes often become less dusty because return leaks no longer draw air from dusty spaces. Rechecking pressure or airflow confirms the improvement and identifies any remaining leaks.