Seal Leaks With Duct Mastic

• 1). Inspect the ducts, and wipe down each joint or seam with a dry rag. Check for loose or missing sheet metal screws and tighten or replace them as necessary.
  
  
• 2). Apply duct mastic over each joint and seam. Duct mastic either comes in cartridges that fit into a caulking gun, or it comes in tubs and is applied with a brush or putty knife. Spread the mastic generously, extending it at least 1 inch on either side of the seam.
  
  
• 3). Wrap fiberglass duct tape over gaps larger than 1/4-inch wide, and then apply mastic over the tape.
  
  

Seal Leaks With Metal Tape

• 1). Wipe the seams and joints and tighten or replace the screws.
  
  
• 2). Tear enough metal tape to go around the seam, and then pull off the backing.
  
  
• 3). Apply the tape to each seam, pressing it down firmly with your fingers to create a tight seal.
  
  

Insulating Ducts

• 1). Cut lengths of duct insulation to fit your heat ducts, using a pair of sharp scissors or a utility knife.
  
  
• 2). Wrap the insulation around the ducts, with the fiberglass side against the metal and the foil side facing out.
  
  
• 3). Tape the insulation securely with duct tape so the insulation is wrapped tightly around the heat ducts.