Why is duct sealing so important?

Return Duct with no seal
Return Duct with no seal

Everyday people ask me what is the single biggest thing they can do to improve efficiency and comfort in their home.

I usually ask a few basic questions like where they live, type of home etc.  Then I ask the two most important questions.  How old is the home and where are the ducts located?

When I hear “home over 25 years old” and “ductwork in the attic” big red flags go up.  Why?  Experience. I’ve helped thousands of people with home comfort issues and without fail this is the biggie.  Think of your house as one big refrigerator.  Leaky ducts are like leaving the door open.

Duct leakage is generally measured as a percentage.  For example, if you have 20% duct leakage this means that approximately 20% of the HOT or COLD air you are buying is being pushed OUT of the ducts and into the attic.  Or, duct leakage can also mean you are SUCKING hot or cold attic air INTO the duct system on the return side of the air handler. On new High-Performance homes, professional energy auditors are usually shooting for less than 2-5% duct leakage.  On older homes (especially with metal ducts) it is common to see duct leakage OVER 40%.

Older Metal Ducts with no seal
Older Metal Ducts with no seal

Why do older homes have such bad leaky ducts?  Back in the era between about 1950-1980 NOBODY cared about energy efficiency.  Energy was CHEAP, so installers usually didn’t bother to take the extra time or money to seal the ducts. For metal ducts this meant sliding two sections of duct together and using three screws to connect them.  Then, they would wrap the ducts with insulation to keep them from sweating or condensing moisture, not primarily to insulate them.

The ducts did not just start leaking; they were never sealed to begin with. If you have ever done some plumbing, this would be like connecting copper pipe together and NOT using solder on the joints.

As an example, think of an air conditioning duct as a long garden hose with a hundred holes in it.  Since water (or air in the ducts) is under pressure, the water will leak out of all hundred holes BEFORE it gets to the end.  Whatever does not leak out of the holes along the way is what ends up coming out of the end where you want it.  Air conditioning ducts are exactly the same and it’s pretty common to end up with only 50-75% of the air where we need it.

Older ducts – especially metal ducts are notorious for leaking.  Studies show that there is generally a direct correlation between the age of the duct system and the percentage of duct leakage.  Additionally, wrapping a duct with insulation does virtually NOTHING to reduce duct leakage just like wrapping a leaky pipe with a rag won’t stop a water leak.

AC Duct Leaks
AC Duct Leaks

Not only do leaky ducts cause higher energy bills, they cause most homes to go under negative pressure.  This means that if there is 20% leakage, then an equal amount of air must be “made-up”.  Make up air usually enters the home through windows, doors, can lights and any other “holes” in the home. Air quality can suffer since outside air is often dusty, dirty, pollen laden or humid.  Dust on windowsills or stains on carpet around the baseboards indicate outside air is being pulled into the home.

Some air-conditioning companies would rather sell a new air conditioner for thousands of dollars, rather than sealing/replacing ductwork for a fraction of the cost often neglects leaky ducts.

How To Fix Leaky Ducts

There are generally 3 methods to fix leaky ductwork:  1) Strip off insulation, seal all seams and then wrap with radiant barrier covered duct blanket.  2) Tear out metal ductwork and replace with flex duct.  This is usually not recommended since metal ducts are great (and expensive) if sealed correctly. 3) Strip off the duct insulation and have a spray foam company spray about 1-2 inches of closed cell foam on the ducts.  The foam will both seal and insulate the ducts in one shot.  WARNING: Check your local building codes and fire officials on this one.  Some cities do not allow this method.

Just remember to think of your home as one big refrigerator that you want to keep nice and cool inside.  Yes, you can add more insulation, or wrap it with radiant barrier, but bang-for-the buck, start by closing the door.

7 thoughts on “Why is duct sealing so important?

  1. Is there a product about 4″ wide that can be wrapped around the existing insulation (which is 42 yrs old in very good shape) that will seal it good enough? Perhaps a foil tape.

    Thanks in advance for your response

    1. Foil tape is going to be about 3″ wide, so you could wrap it around twice to get the coverage you are looking for. The tape will seal the outside nicely, but it won’t seal the actual ducts. Depending on the type of ducts you have (flex ducts or rigid metal ducts) , there are different ways to approach sealing each. For more info on duct sealing, check out this website: http://blocktheheat.com/ductsealing.htm

  2. What is the approximate cost to seal the ducts in a 10 yr old home? Home is a single story with 4,000 sq feet.

    1. Approximations are tricky because there isn’t a standard across the board. Several factors come in to play when sealing ductwork, so I recommend you contact some local professionals to gather price quotes.

  3. Hey Ed,
    I was reading your response to Bob Porter (the first comment above after the article) and I get a “page not found” error. Do you know where I can find the link to read up on how to properly seal flex ducting or rigid ducting?



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