February 2010
Leaky Ducts Can Blow Your Money

Hundreds of dollars you spend for heating may be vanishing into thin air. Leaky ductwork is a serious - and common - problem.

Ducts are the piping that delivers heated air into living spaces from the furnace.

The ductwork in practically every home can be improved, some more than others. If you live in a new home, there's still room for worry. Many times, ducts in newer homes are much more poorly installed than those in older homes.

Why are leaky ducts so common? There are dozens, maybe hundreds, of connections in a typical duct system. Every connection has the potential to leak.

There are good reasons for a tightly sealed duct system:

Save money and energy

If 20 percent of the conditioned air escapes from the ducts of your HVAC system, its efficiency decreases by 50 percent. That means you're spending twice as much as necessary to heat your home.

It also means wasted energy, which hurts the environment.

Increase comfort

Most comfort issues can be traced to leaky or poor ductwork. Leaks are a major cause of hot and cold spots in your home.

Improve air quality

The most important reason to have properly sealed ductwork is your health. Leaky ducts can suck all sorts of dangerous substances into the living area, including mold, mildew, radon, chemicals and fumes.

Insulating the ductwork is not enough - it still needs to be sealed. Insulation helps stop heat loss, but does not stop air loss.

There are two ways to get the job done. Call a licensed HVAC contractor or if you're fairly handy, you can probably do it yourself.

If you decide to tackle the job, make sure you're agile enough to maneuver safely through the areas where ducts are typically located - crawlspaces and attics. Also be aware of sharp edges that may exist on some metal duct components.

The two main materials for sealing ductwork are mastic and tape.


Mastic is a fibrous, putty type material that is spread or painted on ductwork joints. Choose mastic that is non-toxic, water based and meets UL 181 standards.

Spread the mastic on and two inches either side of every ductwork joint. It can be painted on with a brush or applied with a gloved hand. If the gap is larger than a quarter inch, use a mesh tape first to span the gap and provide support for the mastic.

For joints in access or filter doors that will be opened regularly, do not use mastic.


Choose a pressure sensitive aluminum tape that meets UL 181 standards. Do not use cloth-backed tape commonly known as "duct" tape - it's adhesive will shortly dry out and the tape will fall off.

You'll find mastic and tape at home improvement stores and on the Internet.

See our comprehensive video on sealing ductwork:

Ductwork Sealing List

Here are the priorities in sealing your duct system:

High Priority

  • Joints that have become loose or disconnected
  • Connections between the air handler and plenums
  • Seams in the air handler, plenum and rectangular ductwork
  • Return takeoffs, boots and other connections

Medium Priority

  • Joints between sections of branch ducts

Low Priority

  • Lengthwise seams in round metal ducts