duct sealing

duct aerosol sealant

HVAC ducts with aerosol sealant. Image: LLBL.

Ducts that run through cellars, attics, and crawlspaces are an integral part of a forced-air heating or cooling system and their purpose is to circulate air to evenly heat and cool your home. Unfortunately, ducts are often leaky – wasting a fifth or more of the heating and cooling energy used by your home. And the answer is: NOT DUCT TAPE!


How to tell

if ducts need repairs


  • Rooms are too warm or too cold
  • High summer and winter utility bills
  • Little or no air flow from registers in some rooms
  • Air filter gets dirty quickly – needs to be changed more than once a month – indicating leaks in return ducts.
  • Streaks of dust at registers or duct connections
  • No insulation on visible ducts (those in an attic or crawl space)
  • Flexible ducts are tangled or kinked

    How to seal duct leaks


    Is duct sealing something you can do yourself?

    If your ducts are easily accessible in your attic, basement or crawlspace you may be able to make some simple improvements. An easy first step is to check if ducts have become disconnected and reconnect them. Next, check the duct connections for leaks by turning on your heating and cooling system fan and feeling for leaks - seal the joints with mastic or foil tape (household cloth duct tape should not be used). Seal gaps around ducts with spray foam where they penetrate the floor or ceiling. In addition, ducts in an attic or crawlspace should be insulated. If your ducts are uninsulated or poorly insulated (i.e. you see gaps or torn insulation) seal them first, then add insulation to keep the air in your ducts at its desired temperature as it moves through the system. Use duct insulation material rated at least R-6. In many homes the ducts may not be accessible and you will need to hire a contractor who can repair your ducts.


    Who can repair your ducts?

    Heating and cooling contractors, that specialize in duct repairs use special diagnostic tools to determine duct pressures, air flow and leakage. A good duct sealing evaluation should include:


  • Identifying duct leaks with diagnostic equipment.
  • Duct sealing with mastic, metal-backed tape, or aerosol sealant. Duct tape should not be used because it can not withstand high temperatures.
  • Airflow testing after ducts are sealed.
  • Insulating ducts in an attic or crawlspace. The contractor should use duct insulation with an R-value of 6 or better to insulate ducts located in unconditioned spaces such as an attic or crawlspace.
  • Testing combustion appliances (i.e. gas/oil furnace, water heater) to make sure they are venting properly after ducts are sealed.
  • Adding returns or "jumper ducts" to help balance room pressures if needed.

    What materials are effective for sealing duct connections?


  • Mastic is a gooey, but highly effective adhesive for sealing duct connections and dries to a soft solid.
  • Foil tape has metal foil backing and an acrylic adhesive. (Mastics and tapes should be UL 181 approved.)
  • Aerosol sealant is a sticky vinyl polymer that is applied to leaks from the inside of the duct, by pumping aerosolized sealant into a pressurized duct system. Aerosol sealant can be effective for sealing ducts, but needs to be professionally installed.

  • How NOT to seal ducts: the duct tape myth

    During World War II, before it was called duct tape, the U.S. military bought quantities of the cloth-backed, rubber-adhesive tape for making emergency repairs on the battlefield. In the movie business it's called "gaffer's tape" and is used for everything from bundling cables to holding sets together. The Apollo 13 astronauts used it to repair some equipment to get home to Earth safely from around the Moon.


    Some time after WWII, heating and air conditioner contractors begin using the tape to seal the joints in heating, ventilation, and air conditioning (HVAC) ducts. This tape was manufactured in the same way, though to match the ducting it was colored silver rather than the green of the Army version. Because of this use, it became known informally as "duct tape."


    The problem is duct tape doesn't adequately seal the joints and has a short lifespan. Over a three-month period in 1998, Lawrence Berkeley National Laboratory (LBNL) researchers tested duct tape and 31 other sealants under accelerated laboratory conditions that mimicked long-term use in the home. They heated air to nearly 170°F and chilled it to below 55° before blasting it through ducts. They baked ductwork at temperatures up to 187° to simulate the oven-like conditions of a closed attic under a hot summer sun. Of all the things they tested, only duct tape failed – and they reported it failed reliably and often quite catastrophically.


    Benefits of duct sealing


  • Health. Air may leak into your duct system if it is not sealed properly. This can be harmful to your health because air leaks in the return duct may contain fumes from household and garden chemicals, insulation particles, and dust. These items can aggravate existing asthma and allergy problems. Why take the risk? By simply upgrading the energy efficiency of your duct system, you can avoid potential health problems.

  • Safety. Safety in your home should be your number one priority. Duct leaks can cause equipment to backdraft (i.e., when combustion gases flow back into your home, instead of out the vents). If fireplaces, wood stoves, water heaters, furnaces, clothes dryers or other combustion devices are in these depressurized areas, invisible gases, such as carbon monoxide (CO), can backdraft into your home instead of going up the chimney. Make the smart choice and choose to seal your ducts.

  • Comfort. Duct leaks and improper duct sizing can affect the comfort level in your home by keeping some places cooler or warmer than others. The source of discomfort in your home may be leaky ducts or improper sizing. Sealing your ducts can help you breathe easier by improving your indoor air quality, reducing the risk of pollutants entering and circulating through your home. They also save energy and money.

  • Save money. 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.

  • Protect the environment. Energy generation and use is the single largest contributor to air pollution. If you're wasting up to 20% of your heating and cooling systems' performance through leaky ducts, your home is using more energy to get the same job done. By sealing your ducts and reducing the amount of energy necessary to comfortably heat or cool your home, you will reduce the amount of air pollution generated.