2 Easy Ways to Seal Your Leaky Ductwork

Updated on May 14, 2018
Cre8tor profile image

Dan has been in the HVAC industry for 23 years with experience ranging from installation and service to sales and distribution.

A gap in your duct work as small as this can still leak large amounts of air.
A gap in your duct work as small as this can still leak large amounts of air.

How Do You Fix Leaky Ducts?

Homeowners are looking for any way possible to shave money off their energy bills as well as not waste energy to minimize their ecological footprint. Most owners of newly built homes have the luxury of knowing that their home is "up to snuff" since codes and regulations in many municipalities have made it nearly impossible to build a home that is not energy efficient, but those who own older homes have improvements to make if they want to cut down costs and waste.

While some of these improvements are expensive and typically require a skilled tradesman to perform, there are some that are quite simple, cost very little, and just about anyone can do. One of them is sealing up leaks in their ductwork.

There are 2 very cheap, very easy ways to fix your leaky ductwork yourself: HVAC tape and duct sealant.

What is HVAC Tape?

HVAC tape is basically any tape rated for use on HVAC systems. While there are certain tapes for certain jobs, they are all considered HVAC tape. For the purpose of sealing air distributing ducts I've provided some images and descriptions of different types of tape that are commonly used on both residential and commercial applications.

Various HVAC Tapes

Click thumbnail to view full-size
This is your basic foil tape (Polyken 337) and what I use for a typical, exposed duct type system.Skrim Tape (Polyken 338) is another type of foil tape that is reinforced with threading making it very durable and must be cut with scissors or a knife rather than torn by hand. Great for ductboard and flexible ducts.Polyken 339 is the upgraded version of the 337. It's a bit more sticky and durable and some inspectors like to see that red UL stamp on the outside if you're going through an inspection process.FlexFil is again a foil tape with it's rating stamped visibly that holds up well to smoke, mold, humidity, and higher temperatures. Also good for flexible ducting this might be the best choice for a crawl space. Mastic tape (Polyken 367-17) is a foil tape too but has mastic putty on the backing. While it has it's uses, it's expensive and I don't recommend it over the others for typical home application. If you do though, best to heat up the duct first.Lineset tape (Polyken 444) is one I'm just throwing in here so you know it exists. Using this on your A/C lines where the insulation is torn helps maintain proper temperatures inside the lines for efficient operation.
This is your basic foil tape (Polyken 337) and what I use for a typical, exposed duct type system.
This is your basic foil tape (Polyken 337) and what I use for a typical, exposed duct type system.
Skrim Tape (Polyken 338) is another type of foil tape that is reinforced with threading making it very durable and must be cut with scissors or a knife rather than torn by hand. Great for ductboard and flexible ducts.
Skrim Tape (Polyken 338) is another type of foil tape that is reinforced with threading making it very durable and must be cut with scissors or a knife rather than torn by hand. Great for ductboard and flexible ducts.
Polyken 339 is the upgraded version of the 337. It's a bit more sticky and durable and some inspectors like to see that red UL stamp on the outside if you're going through an inspection process.
Polyken 339 is the upgraded version of the 337. It's a bit more sticky and durable and some inspectors like to see that red UL stamp on the outside if you're going through an inspection process.
FlexFil is again a foil tape with it's rating stamped visibly that holds up well to smoke, mold, humidity, and higher temperatures. Also good for flexible ducting this might be the best choice for a crawl space.
FlexFil is again a foil tape with it's rating stamped visibly that holds up well to smoke, mold, humidity, and higher temperatures. Also good for flexible ducting this might be the best choice for a crawl space.
Mastic tape (Polyken 367-17) is a foil tape too but has mastic putty on the backing. While it has it's uses, it's expensive and I don't recommend it over the others for typical home application. If you do though, best to heat up the duct first.
Mastic tape (Polyken 367-17) is a foil tape too but has mastic putty on the backing. While it has it's uses, it's expensive and I don't recommend it over the others for typical home application. If you do though, best to heat up the duct first.
Lineset tape (Polyken 444) is one I'm just throwing in here so you know it exists. Using this on your A/C lines where the insulation is torn helps maintain proper temperatures inside the lines for efficient operation.
Lineset tape (Polyken 444) is one I'm just throwing in here so you know it exists. Using this on your A/C lines where the insulation is torn helps maintain proper temperatures inside the lines for efficient operation.

You probably noticed that good old fashion duct tape isn't mentioned and that is because while duct tape had been the most commonly used for decades, it has begun to fall by the wayside in the HVAC world. While duct tape is strong, it simply begins to dry out and fall away leaving leaks over time where the newer foil tapes do not. Don't worry, there are still a million other ways to use our beloved duct tape...it just won't be to seal your ducts.

Now let us take a look at these images of common leak points and the basic foil tape I applied to seal them. I chose basic tape because it seals great, tears easy, and I can cut it if I ever need to take the duct apart again.

Small Duct Leaks Often Go Un-Noticed

Click thumbnail to view full-size
"Dove-tailed" connections often leak.Leaks near the furnace are under more force so they can leak large amounts of air.Where duct fittings join together are common leakage points in every home.The plenum connection is a good place to start sealing your duct.A piece of foil tape around that joint should do the trick. You could even tape the seams that rotate the elbow if you want to get every bit you can.
"Dove-tailed" connections often leak.
"Dove-tailed" connections often leak.
Leaks near the furnace are under more force so they can leak large amounts of air.
Leaks near the furnace are under more force so they can leak large amounts of air.
Where duct fittings join together are common leakage points in every home.
Where duct fittings join together are common leakage points in every home.
The plenum connection is a good place to start sealing your duct.
The plenum connection is a good place to start sealing your duct.
A piece of foil tape around that joint should do the trick. You could even tape the seams that rotate the elbow if you want to get every bit you can.
A piece of foil tape around that joint should do the trick. You could even tape the seams that rotate the elbow if you want to get every bit you can.

What is Duct Seal For?

It's important we don't confuse duct seal, which is a type of putty used to seal around electrical wires and conduit where it might penetrate your home, with the duct sealant that you use to seal your ductwork.

What is Mastic Sealer?

Mastic is another product that is commonly used to seal up duct leaks. This pasty, thick, paint-like substance can be applied to cracks and gaps in your ductwork with a caulking gun and spread with a paintbrush or if using a gallon pail, just scooped and applied directly with the paintbrush.

I recommend using a water based sealant as it holds up well and cleans up easy also. Trust me when I say easy clean up is important. I didn't pay much attention to this once and ended up shaving off my arm hair as a result. It was not my proudest moment. Moving on...

Mastic sealer is probably the longest lasting option for sealing your duct work but I would consider whether or not you'll ever need to access the area again before using it. In spots that you may want to open up again, like near the evaporator coil that you may want to have cleaned or an end cap where you may want to clean your ducts, I would use one of the tape options.

Duct Sealant (not duct seal)

Pick your poison. Seal your duct by the gallon or by the tube. Whichever way suits you best.
Pick your poison. Seal your duct by the gallon or by the tube. Whichever way suits you best.

What is Aeroseal Duct Sealing?

I know I said there were 2 ways to seal duct but the key words there were "easiest" and "your own". Aeroseal is a very nice system developed to seal ducts of all kinds but is not a process that you as a homeowner can do yourself and is typically far more expensive than what most are willing to spend. That said, it may be the only answer for those who have underground ducts or ducts that they cannot access to significantly seal by hand. It is also becoming more and more popular in new home construction as then the cost is built into the purchase price and not coming directly out of your pocket all at once. Check out this video to see what Aeroseal is all about and how they do it.

(Note: Aeroseal is a brand name and there are other companies that use the same type of technology. I am not recommending a brand or company here, only the technology.)


Different Strokes for Different Folks

Which of these methods do you think you'll be trying?

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Do It Yourself Duct Sealing

There is not a whole lot that I can tell you about duct sealing technique. It really is a pretty self explanatory process. Apply the tape or sealant over the holes and seams...pretty simple right? There are however a few things that might help your job go smoother, be more effective, and last longer.

  • Use a water based mastic.
  • Wear rubber gloves and old long sleeve shirt if using mastic. (Maybe a hat too)
  • Whether you use tape or mastic, make sure the surface your sealing is clean and dry.
  • Tapes will stick much better if they are warm. Make sure your tape is at least room temperature and maybe even apply the tape while the heat is running.
  • Every little bit counts. Cover all the holes and seams you can.

Sealing Duct Images

Click thumbnail to view full-size
Peel the tape backing off a little at a time as you start tapingContinue to pull the backing and smoothing out the tape as you go.Finish peeling the backing off the tape and smooth over the top of the pipe as far as you can with your hand.Begin applying mastic up as high as you can get your caulking gun.Continue to run the bead around the area to be sealed.Use your paint brush to spread the mastic. Make sure you push and pull the brush so the mastic works into the seams.
Peel the tape backing off a little at a time as you start taping
Peel the tape backing off a little at a time as you start taping
Continue to pull the backing and smoothing out the tape as you go.
Continue to pull the backing and smoothing out the tape as you go.
Finish peeling the backing off the tape and smooth over the top of the pipe as far as you can with your hand.
Finish peeling the backing off the tape and smooth over the top of the pipe as far as you can with your hand.
Begin applying mastic up as high as you can get your caulking gun.
Begin applying mastic up as high as you can get your caulking gun.
Continue to run the bead around the area to be sealed.
Continue to run the bead around the area to be sealed.
Use your paint brush to spread the mastic.
Use your paint brush to spread the mastic.
Make sure you push and pull the brush so the mastic works into the seams.
Make sure you push and pull the brush so the mastic works into the seams.

The Benefits of Sealing Your Ductwork

Many will argue that air escaping your ducts is still in the house so the energy is not really lost and others that little gaps, holes, and seams don't amount to much loss anyway. I beg to differ.

If you add up the square inches of leakage in an older home's ducting you'd find that a large majority of homes would be losing roughly the same amount of air as cutting a baseball sized hole in the duct and typically it is leaked into an unconditioned area like a basement or attic where it is not needed. So while you wonder why that one room is so cold or hot all the time, consider that these little leaks can add up to roughly the same amount of air needed to heat and cool a room.

The other consideration here is that we're not only sealing the air in but the dust out. Seal both the supply and the return air ducts. Return air duct sucks air in so sealing them keeps smelly basement air and/or attic dust from creeping into our air supply also. Sure, our filter will get most of that but then you'll be changing your filter and cleaning your ductwork more often.

This all leads back to the benefits of energy cost savings, a smaller environmental impact, and better indoor air quality. Even if only one of these benefits appeals to you then it would be worth the time and little cost to seal up your ducts.

~ We're all in this together ~

Questions & Answers

    © 2018 Dan Robbins

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      • Cre8tor profile imageAUTHOR

        Dan Robbins 

        6 months ago from Ohio

        Thank you Robie! One thing you might try to balance out temperatures in your house is to leave the fan running all the time. I run mine about 320 days a year and it evens rooms out quite nicely while costing next to nothing. You may even find the A/C or furnace run just a bit less on milder days. I know it sounds weird to run the fan all the time, especially in the winter, but it works.

      • Robie Benve profile image

        Robie Benve 

        6 months ago from Ohio

        Great information Dan! Now I need to go look at my ducts, trying to figure out why two of my rooms are always the wrong temperature. Thanks!

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