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Duct, Duct, Loose: Leaks In Your Ductwork Is No Kid's Game! Learn How To Seal Ductwork And Improve Your Heater's Efficiency

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If you notice that your home isn't heating as well as it used to, you may be tempted to assume that there is something wrong with your furnace. While that is possible, there is a more likely cause. Your ductwork may be leaking hot air into unused areas of your home. According to ENERGY STAR, you may be losing 20 to 30 percent of the heat forced through your ductwork due to holes, leaks and poorly sealed joints. That means sealing your ductwork will go a long way to improving the comfort of your home.

Locating Ductwork - Locating your ductwork may seem like an obvious step, but it can be more difficult than it seems. Because the ductwork from your furnace carries the heated air to all areas of your home, you may find ductwork in nearly any part of the home. It is commonly run through basements, attics and crawl spaces where it is out of the way. Locating it and identifying possible problems before beginning repairs lets you concentrate on the work at hand, instead of crawling around looking for the ducts.

  1. Follow the ducts from your furnace as they work their way through your home.
  2. Note any loose joints, separated seams or holes in the ductwork. Examine any rusty or discolored areas, as these are likely to create problems.
  3. Test the ductwork with your hands to see if it is secure.
  4. Sketch a diagram of your ductwork, noting areas that need sealing.


Gathering  all the needed supplies before you begin will save time and effort and avoid wasted time looking for that drill.

  • Duct Tape: You are probably thinking that all you need to seal your ductwork is a trusty roll of duct tape. Surprisingly, duct tape  isn't recommended for repairing and sealing ductwork. According to researchers from the Environmental Energy Division at Lawrence Berkeley National Laboratory, duct tape dries out and becomes brittle when exposed to heat. It can also catch fire and release toxic fumes. It is prohibited for use on ductwork in most of the states in the United States. 
  • Mastic Sealant: Mastic sealant is a liquid sealant that forms an elastic substance when it dries. It is heat resistant and stands up well to the normal vibrations in ductwork. Mastic sealant is best for sealing joints in your ductwork, explains doityourself. You can find mastic sealant in the heating and plumbing aisle of your local hardware store. HGTV recommends UL 181-rated mastic and claims it adheres better and lasts longer.
  • Foil Tape: You can also use foil tape to seal your ductwork. This tape is sold in the hardware store and comes in a roll. The tape looks like shiny aluminum and has a paper backing that must be removed when you apply it.
  • Sheet Metal Screws & Drill: If the joints and seams along your ductwork are loose or have separated, you will need sheet metal screws to repair them. These can also be found in the hardware store.
  • Hammer: A hammer comes in handy to remove small dents or nudge the ductwork into place.
  • Work Clothes and Protective Gloves: Sealing your ductwork can get messy, especially is you choose to use mastic. Protective clothing and gloves save you from the hassle of ruined clothes or the pain of injured hands.
  • Cleaning Supplies: You will need a vacuum cleaner and old rags.

Sealing Your Ductwork

Now that you know what needs to be repaired and have assembled the needed supplies, you are ready to get down to the business of sealing your ductwork.

  1. Dust or vacuum the outside of your ductwork to remove any surface soil. Wipe them down with a damp cloth, if the residue resists your efforts. Allow the ductwork to dry.
  2. Reconnect any loose seams or joints and replace missing screws. 
  3. Apply a piece of mesh tape designed for use with mastic, before applying the mastic, if the crack is more than 1/4-inch wide, says HGTV.
  4. Apply mastic to seams and joints by brushing a generous layer over the area. You can use your hand for this, but wear protective rubber gloves to protect your skin.

Sealing your ductwork is likely to improve the efficiency of your furnace and make you feel more comfortable. If you still experience problems with inefficient heating, contact your local heating repair service and get a professional opinion. He can determine whether the problem is with your furnace and offer suggestions for improving its efficiency.