A professional installing a filter to prevent air duct leakage

Air ducts are tubes attached to the furnace/air conditioner in your central heating; they distribute the conditioned air to the various areas around the house. Of course, you want all the conditioned air to reach its intended destination. But sometimes, a problem arises and you don’t get that result—the air ducts leak.

Why are Air Ducts Leaky?

Air duct leakage results when the components distributing air from the furnace are not 100% sealed from end to end. These metal or plastic tubes may have poorly sealed connections or even holes through which the pressurized air inside escapes before it reaches the intended area. Older buildings may have as much as 35% duct leakage.

Older systems are likely to be leaky because building codes did not cover duct leakage, so HVAC contractors didn’t take the extra time or spend additional money to make air ducts tight. Some older duct systems from the ’70s use reinforced fiberglass, which can be very leaky and potentially unsafe. Replacement is advisable.

Even older ducting systems were sheet metal tubes covered with a thin layer of asbestos insulation. These are likely to be quite leaky without proper maintenance, although they can provide better airflow than plastic ducts. 

Asbestos tape was initially used to secure the joints, which usually fell off. The metal’s expansion and contraction loosened the joints, and careless tradespeople crawling over them often separated joints further.

Even with newer flexible ducting that was well-installed, things happen over time. For example, animals can tear holes in plastic air ducts, and joints vibrate loose.

Where Are Air Ducts Likely to Leak?

Air duct leakage occurs in two principal areas. Most of the time, leakage comes from joints in the air ducts, connections to other components in the system, or those components themselves (like the furnace plenum and supply registers). Sometimes, an animal looking for a warm place to sleep or collecting fiberglass insulation for its nest will make a hole in the air duct.

Sheet metal components in every system can leak. These include the furnace plenum, the box attached to the furnace that receives air from the heater. It has collars connecting the various air ducts and is big enough to allow air to mix appropriately for distribution into the air ducts. 

The joints in the plenum and its collars may have openings that should be sealed but are often not well enclosed. The junction between the furnace itself and the plenum is another joint that can be leaky if not properly sealed with mastic or tape.

An open air duct plenum with ducting attached
An air duct plenum with ducting attached

Other sheet metal components are the registers at the end of the air ducts that distribute the air to each room or area and the large intake grille. These also have joints that may not be completely sealed.

How Are Air Ducts Sealed Against Leakage?

Mastic, a water-based sticky substance that hardens to create and maintain a seal, is used to seal air ducts. In addition, certain products are made specifically for use in HVAC systems.

Mastic being applied to prevent air duct leakage.
Mastic is applied to seal an air duct. Courtesy of F.H. Furr

Joints in the plastic ducts should be thoroughly sealed with an appropriate mastic and secured with plastic ties around the metal collars at each end. Current standards require that the inner lining on the plastic duct be attached to the collar with a tie and separately to secure the outer plastic wrap. Mastic should be applied to seal the plastic to the collars, in addition to the ties.

In the past, handymen used “duct tape” to seal air ducts. However, in most cases, it was not intended for such use and has deteriorated and detached.

What Can You Do About Air Duct Leakage?

There are different methods of stopping air duct leaks, depending on where and how accessible the leaks are.

Use Tape to Seal Joints

Usually, the most accessible place where you have leaks is the plenum, especially where it joins the furnace. Regular duct tape was often used to seal this joint and is falling off or missing.

Use a high-quality, flat, metal-backed tape like this to seal this joint and any holes in the plenum. Use a suitable mastic for heating and air conditioning applications like this to secure the collars where the ducts attach to the plenum. Seal around the edge of the air duct, and if you can, secure the plastic wrap to the metal collar with a tie.

Some applying metal-backed tape to an cylindrical duct to prevent air duct leakage
Courtesy of Conklin Metal

But the fundamental issue in sealing the air ducts themselves is accessibility—can you reach them? Unless some of your ducting is in an attic or crawl space, most air ducts will be inside the building structure where you can’t get to them.

Patch Holes in Interior Lining

If you have ducts in the attic or crawl space, examine them for holes or separations. Patch any holes or gaps in the inner plastic lining with the metal-backed tape (holes only in the outer plastic wrap will not leak). Rejoin any separated air ducts using the tape.

Hopefully, the original asbestos insulation has been professionally removed and replaced with fiberglass insulation if you have older metal ducts. You can still find the joints, seal them with metal-backed tape, and reinstall the insulation. Assume all the joints leak unless they have been well-sealed with mastic or tape.

Apply Aeroseal

If your ducts are inaccessible, a product called Aeroseal is available. Essentially, a technician sprays a proprietary liquid into the air duct system, sealing all small leaks. 

A before and after comparison for an Aeroseal application
Courtesy of Aeroseal Canada

However, Aeroseal is expensive and doesn’t seal holes or considerable leaks, so you should consider using it only if the ducting system is mostly inaccessible. Professionals agree that hand sealing is more cost-effective and yields better results when it is possible.

How Do You Know If Your Ducts Are Leaking?

The most accurate way to know how much your air ducts are leaking is to have a technician measure the amount of leakage. If you have had any component of your heating/air conditioning system replaced since 2005, the HVAC system contractor was required to test your air ducts for leakage (and possibly evaluate other things). However, only recently has this been enforced since most HVAC contractors tried to avoid it to minimize costs.

If you notice that your heating or air conditioning bills seem higher or that some rooms have become warmer or cooler than they used to be, that could indicate that leaks have developed in the air duct system. Likewise, if the air seems dirtier, less healthy, and you see that your air filter is picking up more stuff than usual, these could also be signs of air duct leakage.

What Problems Do Leaky Ducts Cause?

If the leaky air ducts are between the furnace and the areas where the warmed or cooled air is headed, they will waste energy and increase energy bills. They also can lead to uneven heating and cooling in different areas of the building.

If the leaky air ducts are between the air intake from the building and the furnace, you could be sucking dust, dirt, and mold into your air distribution system and causing discomfort, even health problems. Also, this will clog the filter prematurely.

Leaky ducts make your system work harder and need repairs or replacement sooner.

Summary

Air ducts leak primarily from joints or from damage that creates holes. Fortunately, if the air ducts are accessible, you can seal those leaks without much effort. Stopping air duct leakage has significant benefits, from better health and comfort to lower energy and equipment repair costs. Fortunately, reducing or eliminating air duct leakage is not rocket science; anyone can do it.

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