How to Seal Leaky Ductwork: 7 Steps

Keep your home cozy while reducing your energy bills

A mother with her daughters laughing at home
Photo: Klaus Vedfelt / DigitalVision / Getty Images
A mother with her daughters laughing at home
Photo: Klaus Vedfelt / DigitalVision / Getty Images
Jill Harness
Written by Jill Harness
Contributing Writer
Updated Mar 4, 2025
SKILL LEVEL
Simple
COMPLETION TIME
48 hours
COST
$55–$170
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What you'll need:
TOOLS
  • Gloves
  • Safety goggles
  • Cleaning rags
  • A paintbrush or putty knife
  • Utility knife
SUPPLIES
  • A dust mask
  • Mild cleaning solution
  • Foil HVAC tape
  • Mastic
  • Fiberglass insulation blanket

Most homes lose 20% to 30% of the air running through their ductwork through leaks, holes, and bad connections. That means you’re wasting money each month on your energy bills, especially in winter. Keep your home more comfortable and reduce your energy costs by learning how to seal leaky ductwork yourself.

  1. Put on Protective Gear

    Most ductwork is surrounded by fiberglass and located in dusty, dirty areas. Before working on your ducts, protect yourself with long sleeves, pants, gloves, safety glasses, and a dust mask.

  2. Identify Duct Problems

    A man sealing ductwork with duct tape
    Photo: 1905HKN / E+ / Getty Images

    Before starting your project, identify problem areas in your ductwork. Inspect for holes, damage, disconnections, and poor sealing. Pay special attention to areas with excessive dust around the vents, as this can indicate a leak.

  3. Clean the Ductwork

    Duct sealing products will not adhere properly to dust and dirt. To prep ducts for repairs, remove the insulation, then use a rag dipped in a mild cleaning solution to clean the area around the leaks. Scrub a few inches past the gap in every direction.

  4. Fill Holes With Duct Mastic

    6 duct sealing materials, including latex, foil tape, and insulation
    Photo: photovs / iStock / Getty Images Plus / Getty Images

    Mastic is a thick substance that dries to create a strong but flexible seal over holes, seams, and joints. Use a paintbrush or putty knife to apply a thick layer of mastic over small leaks, ensuring the coverage extends a few inches over the leak in all directions to create a strong seal.

  5. Apply Foil Tape to Seams

    Use foil tape along longitudinal seams or areas where the ducts connect. Cut a piece of tape large enough to cover the entire seam. Extend it past the edges of seams or overlap the tape on joints. Push the tape onto the duct, working to ensure there are no wrinkles. Firmly smooth the tape out to remove bubbles and ensure maximum adhesion.

  6. Let Dry, Then Test

    Avoid using your heating or cooling system for 24 to 48 hours to allow the mastic to cure. Once the mastic has dried completely, run your HVAC to test it. Use a smoke pencil or incense at nearby vents to check if your seals worked properly. If the smoke escapes, apply more mastic or tape, allow it to dry, then test again.

  7. Insulate Your Ducts

    A professional wrapping duct with insulation
    Photo: MyrKu / iStock / Getty Images Plus / Getty Images

    Measure your ducts and cut fiberglass insulation to wrap around them. Secure the insulation with small pieces of foil tape before applying a long strip along the seam.

Common Duct Sealing Mistakes

Sealing ducts is pretty straightforward, even if it can be time-consuming. However, to do the job properly, avoid common pitfalls.

  • Never use duct tape. Despite its name, duct tape is not suitable for use on ducts as it deteriorates quickly and does not hold up over time. Only use foil tape designed for HVAC systems.

  • Don’t forget to check connections. The joints where ducts meet vents and other ducts are the most common areas for leaks.

  • Clean the ducts first. Nothing sticks well to dust. To ensure your seals stick around, always clean the surface before use.

  • Don’t seal ducts with major damage. When your ducts have holes over ½ inch or have been severely crushed, replace the section or hire a pro to do the repairs.

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DIY vs. Hiring a Pro

While you may be able to seal exposed ducts yourself, the majority of ductwork in most homes is difficult to access, which means hiring a pro.

An HVAC contractor or duct installer near you can inspect your whole system using pressure testers, leakage testers, and infrared cameras to identify leaks invisible to the naked eye. They can repair crushed or damaged ducts, seal and insulate the ductwork, then check the airflow to ensure their efforts were successful.

Professional duct repairs cost an average of $2,250, depending on the method and the size of your HVAC system. While you may be tempted to save money with a DIY, professional services are drastically more effective and will save you more money in the long run.

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Frequently Asked Questions

Mastic and mastic tape are two of the best materials for sealing leaks in ductwork. Professionals may also use aerosol-based sealants. Once sealed, surround ducts with fiberglass insulation blankets. Don’t use duct tape, as it will fail quickly.

Yes, spray foam insulation can be applied to your ductwork and effectively insulate your ducts. Blanket insulation is the easiest insulation product to DIY since it is easy to wrap around ducts. However, spray foam insulation can also benefit your HVAC system if applied to air ducts.

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Learn more about our contributor
Jill Harness
Written by Jill Harness
Contributing Writer
Jill is a contributing writer with over a decade of experience writing on home projects. She specializes in DIY and gardening topics and has written for Hunker and Lifehack.

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