Duct Sealing

Learn how proper duct sealing and insulation can optimize the performance of your HVAC system and reduce your energy bill.

Sealing leaky air ducts prevents the loss of valuable heating and cooling that accounts for about 45% of your energy usage. Your duct system distributes conditioned air throughout your home. Unfortunately, any potential savings from even a high efficiency HVAC unit can literally escape into your attic and crawlspace through leaks in your ductwork. That loss—in money and comfort—may add up to 20-40% percent of the air that moves through your ductwork.

Proper duct sealing and insulation optimizes the performance of your HVAC system and reduces your energy bill (especially during the summer and winter). In addition, you’ll benefit from:

  • Improved indoor air quality.
  • Longer lasting and less noisy HVAC units.
  • Less dust.

How Duct Sealing Works

When going about duct sealing, you should target (in order of priority):

  • The largest leaks, including disconnected ducts, missing end caps, etc.
  • The areas of highest pressure, including the air handler and air-mixing plenums.
  • The entire return duct system, except for hard pipe seams.
  • Any panned joists, wood plenums, wall cavities and/or other building cavities used as part of the duct system. Do not add these cavities to a new or existing duct system.
  • Any supply take offs, including starting collars.

Before sealing the ducts, it helps to have the right materials and understand the following guidelines.

  • Verify that your duct system is correctly sized.
  • Mechanically fasten sheet metal and flexible ducts. Also mechanically fasten and seal new ducts with mastic paste or mastic tape.
  • Seal ducts with UL 181 approved mastic or mastic tape before applying insulation. Do not use regular duct tape.
  • Apply mastic directly on the duct (and not on the insulation or sleeve).
  • Install the mastic on a clean, dry surface.
  • Seal gaps larger than ¼” with fiberglass mesh tape and mastic.
  • Monitor for carbon monoxide if your home has any gas appliances or an attached garage.
  • Vapor seal and weather-proof new or existing ducts so that they have a minimum R-8 insulation.
  • Install a damper at each take-off for new supply branch ducts. They must also originate no less than 12 inches from the end of the trunk line.
  • Provide at least one return air grille for each level of your home.

To seal the ducts, you will generally:

  • Pull back the outside sleeve and duct insulation to expose the flexible duct.
  • Install a tie-strap and apply some fiberglass mesh tape.
  • Apply some mastic directly on the duct.

After sealing, your goal is for no more than 10% duct leakage as a percentage of your system's fan capacity. In addition:

  • Do not turn your air handler on until after the mastic cures.
  • Ensure that your Quality Contractor Network (QCN) member invoice specifies the locations of your duct sealing, the materials used, the labor hours and any diagnostic readings (if available).
  • Do not block registers with furniture or other items.
  • Keep any interior doors and registers open.

Check your duct system quarterly for any damage that needs repair or sealing.


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