Leaky Ductwork: How to Find and Fix Duct Problems

Read the signs

High bills in summer and winter. ENERGY STAR says leaky ducts can cut the efficiency of the heating and cooling by as much as 20 percent.

Rooms that are hard to heat or cool. Or stuffy rooms that never feel comfortable, whatever the thermostat says: one room hotter or colder than the rest.

Ducts in an attic, a crawlspace, an unfinished basement or the garage. ENERGY STAR counts that as a sign on its own.

Tangled or kinked flexible duct. Another of ENERGY STAR’s signs.

A dusty house. ENERGY STAR says fumes from household chemicals, insulation particles and dust can get into the ducts and circulate through the home.

Look in the four usual places

Joints and seams. ENERGY STAR’s first common problem is leaky, torn and disconnected ducts. Look where sections join and where the ducts meet the furnace. A room with no air at all may have a duct off completely: nothing from one room.

Registers and grilles. ENERGY STAR says the connections at vents and registers, where they meet the floors, walls and ceiling, are common places to find leaks and disconnected ductwork. The Department of Energy says holes for duct boots are often cut too large.

The furnace and the filter slot. ENERGY STAR lists leaks at the furnace and the filter slot. The Department of Energy’s job aid has a cover fitted where one is missing, one that seals the opening and still comes off easily to change the filter.

Flexible duct. Kinks and crushed runs restrict the airflow. ENERGY STAR’s answer is to straighten them.

Mastic and mesh, by the size of the gap

What to use. ENERGY STAR says mastic sealant or metal tape. Its list for contractors adds an aerosol based sealant.

Clean first. The Department of Energy’s checklist says the duct surfaces that take the sealant must be clean.

Gaps under 1/4 inch. Fiberglass mesh tape over the gap, seam or joint, then mastic over the tape.

Gaps of 1/4 to 3/4 inch. Tape as a backer to hold the mastic, mastic over it, mesh pressed into the mastic, then more mastic at least 1 inch past the edges of the tape.

Holes over 3/4 inch. A patch cut to cover the hole and held with mechanical fasteners, then mastic over its edges and the fasteners, with mesh pressed in.

Round a register. Clean round the boot, bridge the gap from the inside of the boot to the floor or ceiling with mesh tape, keeping it within what the register will cover, and cover it with mastic. Let it dry completely before the register goes back.

Seal first, then insulate

Which ducts. ENERGY STAR says to insulate all the ducts you can access, such as those in attics, crawlspaces, unfinished basements and garages.

How much. Its contractor list asks for duct insulation with an R-value of 6 or higher in unconditioned areas.

The order. ENERGY STAR starts with sealing the leaks, then insulating.

It also stops sweating. Insulation with an unbroken vapor barrier keeps damp air off cold metal: why ducts drip in summer.

Know what to ask for

The whole system. ENERGY STAR says a contractor should inspect all of it, including the attic, crawlspace, garage and basement.

Supply and return balance. ENERGY STAR says many systems have return ducts that are too small.

The repairs. Damaged and disconnected ducts repaired, tangled or crushed flexible ducts straightened, and every leak and connection sealed with mastic, metal tape or an aerosol sealant.

After the work. A new filter, the airflow checked again, and a combustion safety test for gas or oil appliances.

Replacing the equipment anyway. ENERGY STAR says a well sealed duct system may let you choose a smaller, less costly system.