How to Seal Ducts with Mastic

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How to seal your ductwork in your HVAC system with mastic to improve comfort and save money.

If your ducts are easily accessible in your attic, basement or crawlspace you may be able to make seal most of your ductwork to improve comfort and system efficiency. Check the duct connections for leaks by turning on your heating and cooling system fan and feeling for leaks. You can seal the joints with foil tape, but applying a layer of mastic is what professional HVAC technicians use to obtain a tight seal that will last for years. Never use duct tape to seal ductwork, it is only a temporary fix and testing has shown that in most cases duct tape lasts less than a year under normal conditions.

Benefits to Sealing Ductwork

Health - Contaminants may enter your duct system if it is not sealed properly. Gaps in the return duct may allow fumes from household and garden chemicals, insulation particles, and dust to be distributed throughout the home. These substances can aggravate existing asthma and allergy problems.

Safety - Duct leaks can cause combustion equipment and gas appliances to backdraft. If fireplaces, wood stoves, water heaters, furnaces, clothes dryers or other combustion devices are located in areas that come under negative pressure, combustion byproducts such as carbon monoxide (CO) can enter your home instead of going up the chimney or flue.

Comfort - Duct leaks and improper duct sizing can affect the comfort level in your home by overcooling or overheating different parts of your home. While sealing your ductwork cannot completely correct improper duct sizing, sealing will improve airflow to the various zones and reduce the runtime of the HVAC equipment. Once the ductwork is sealed properly, the airflow to individual rooms can be adjusted to satisfy heating and cooling requirements.

Efficiency - Leaking ducts can decrease the overall efficiency of your heating and cooling system by as much as 20%. Duct sealing increases efficiency and lowers your energy bills. The typical family could save up to $150 annually.

Tools and Materials

2 in. or 3 in. Brush

Denatured Alcohol


Rubber Gloves

Mesh Tape

Utility Knife

Inspection Mirror

Tin Snips

Self-Tapping Sheet Metal Screws


The mastic is not as sticky as glue, but it can be messy. It is best to wear rubber gloves when working with the material. If your ductwork is insulation with a fiberglass duct wrap you will need to remove enough of it to access the duct joints. Most joints occur every 3 to 5 feet. Before starting to seal your ductwork, make sure that all support straps and hangers are in place so that the ductwork is not sagging. Moving the ductwork after the mastic has dried can cause it to crack and defeat the purpose.

You should also repair any open gaps in metal ductwork with sheet metal screws and a cordless drill and driver.

Check the connection between the return duct and the air handling unit or furnace and return ducts composed of sheet metal and floor joists. These joints are notorious for improper seals.

Return ductwork

1. Use a cloth to wipe dust from the surface of the duct. If oily film or grease covers the duct, wipe clean with a cloth soaked in denatured alcohol.

2. Apply mastic to joints smaller than 1/4 in. Load your brush with mastic. Coat the entire joint with a continuous strip of mastic. Use end of the brush to work mastic into joint. Spread mastic at least one inch on each side of the joint. Mastic should be thick enough to hide the metal surface of the duct or about 1/16 inch thick. Use an inspection mirror to make sure that the joint is sealed behind the ductwork and in tight spaces. You may also need to use your hand to apply the mastic if the brush cannot fit in tight spots.

3. For larger joints apply mesh tape first. If the crack in the duct connection is larger than 1/4 inch, use mesh reinforcing tape in addition to mastic. If the tape is sticky on one side, cut enough tape to cover the joint, press the tape into place and then cover with mastic as in step 2. Apply enough mastic to completely cover the mesh tape. If the mesh tape does not have a sticky side, first apply a thin layer of mastic, press the tape into the mastic, then apply the finish layer of mastic.

4. If your ductwork insulation was removed you can reapply the insulation after the mastic has dried. Most mastics are dry to touch in 1 to 2 hours. You can install the insulation over the wet mastic but the mastic seal could be damaged.

5. For transitions between rectangular and round ducts, called takeoffs, use mesh tape and mastic to seal the joints. Cut the mesh tape in 2 or 3 inch strips and apply the pieces around the takeoff. Apply the mastic over the mesh to seal.

1 comment

Roberta Baxter
Posted on Nov 18, 2011