A picture of a hand being placed up to an open air vent in a house with gaps between the drywall and the air vent, letting hot attic hot air leaking into the house.

Unsealed ductwork is one of the biggest culprits of energy loss in HVAC systems. As we learn in BPI energy auditor training and out in the field, air sealing ductwork can have some of the biggest impact on your energy home performance.

Ductwork is the system of pipes that carry air to and from your HVAC unit. If these pipes and tubes are not properly sealed, they can leak a tremendous amount of air and, as a result, waste a lot of energy. 

This article will take you through a step-by-step approach to properly air seal HVAC duct boots.

We also built a pretty slick home air sealing calculator that uses your own utility bill (and other factors) to show a wealth of data on the benefits of air sealing. You can find that here.

What is The Best Way to Air Seal an HVAC System?

The best way to air seal your HVAC system and save energy is to air (gap) seal the ductwork at the boot where the ductwork connects to the floor, ceiling, or wall. You can do this by applying mastic sealant to cracks and leaks.

An air duct with the cover off, and caulking around the outside of it. This helps prevent hot attic air from getting into the room.

Sealing duct boots is a relatively easy and inexpensive way to reduce energy consumption and save money. It’s also an effective way to improve the indoor air quality of your building. 

How To Air Seal Duct Boots

There are several measures you should take to properly air seal your HVAC duct boots. 

Steps involved in the process include:

1.) Check if Your Ducts Are Leaking

There are many ways to inspect your duct boots for air leaks. All you need is a flashlight and a mirror. First, shine the flashlight into the duct boot. Then, look for any light coming through the boot or around the edges. If you see any light, then you have an air leak.

Other methods of checking include:

  • Using wet hands or a damp cloth to feel for any air leaks around the edge of the boot
  • Using smoke from a candle or incense burner to identify areas where the smoke is drawn into the ductwork
  • Using toilet paper to identify areas where it’s being sucked into or blown away from the ductwork

Once you’ve found the air leak, you can repair it with sealant. Then, with a little effort, you can easily prevent expensive air leaks in duct boots and keep your home’s HVAC system running efficiently.

2.) Inspect Air Duct Insulation Around the Attic and Basement

It’s essential to keep an eye on the air ducts in your home, especially if they pass through the attic or basement. Over time, the insulation around these ducts can damage or degrade, leading to drafts and energy loss.

Humidity in air duct insulation resulting in it corroding

Inspecting and replacing the insulation around your air ducts is a relatively straightforward process. In most cases, you can do it yourself with just a few tools and some basic knowledge. 

3.) Clean Surfaces Near Leaks

After identifying a leak, you must ensure that the surfaces around it are completely clean. Most leaks will cause condensation and moisture buildup, damaging furniture, carpets, and drywall. Make sure that your furnace is off before applying any sealant. 

If you have an HVAC duct boot leak, there are a few things you can do to clean the surfaces around it, including:

  1. First, you must remove any debris or dirt around the leak.
  2. Next, you can use a household cleaner to clean the area around the leak. Make sure to follow the instructions on the cleaner to avoid damaging the surfaces around the leak. 
  3. Finally, use a sealing agent to seal the leak and prevent further damage.

4.) Apply Mastic Sealant for Smaller Leaks

If you have a small leak (less than ¼ inches or 6.3 mm) in your HVAC ductwork, you can apply mastic sealant with fiberglass tape to fix it. This technique is a great way to repair your HVAC ducts without having to replace them entirely. Plus, it’s a much cheaper and easier fix than you might think.

Mastic being applied to a leaky air duct

Here’s what you’ll need:

  • Mastic sealant
  • Fiberglass tape
  • A putty knife
  1. First, you’ll want to clean the area around the leak with a putty knife to help the mastic sealant to adhere better. 
  2. Then, apply the mastic sealant to the leak, using a putty knife to spread it evenly. 
  3. Apply the fiberglass tape over the mastic sealant, making sure to smooth it out, so there are no air bubbles. 
  4. Finally, let the seal sit for a few hours to harden and close any open cracks properly. 

5.) Apply Aluminum Foil Tape for Larger Leaks

Using aluminum foil tape is a quick and easy way to patch more significant leaks (greater than ¼ inches) in your duct boots. Simply apply it to areas where the leak occurs, and you’ll be good to go. This measure is a great temporary fix until you can try a more permanent solution.

A technician applying metal-backed tape to an cylindrical duct to prevent air duct leakage

Here’s a quick step-by-step guide on how to apply aluminum foil tape to your HVAC duct boots:

  1. Clean the boot’s surface with a rag or brush to remove any dust or debris.
  2. Next, cut a piece of tape slightly larger than the area of the leak.
  3. Apply the tape to the duct boot, and smooth it out so there are no wrinkles or bubbles.
  4. Press the tape down firmly and wait for it to set.
  5. You can also apply mastic sealant to strengthen the aluminum foil tape further. 

6.) Seal Duct Boots To Dry Wall or Floor

Most people don’t realize that their HVAC ducts can leak air if they’re not correctly sealed. Duct leaks can not only lead to higher energy bills, but they can also make your HVAC system work less efficiently. 

That’s why it’s essential to seal your HVAC duct boots, especially the ones that connect to your home’s exterior walls or floors.

There are a few different ways to seal HVAC duct boots, but we recommend using mastic sealant or silicone caulk. Both materials are easy to use and create a lasting seal to prevent air leaks. Just make sure to follow the instructions on the product you choose so that you get the best results.

7.) Avoid Using Duct Tape

If you’ve ever been in the process of sealing HVAC duct boots and thought to yourself, “hey, this would be a great time to use duct tape,” then we’re here to tell you that you should avoid doing that. 

Duct tape may seem like it would be strong enough to hold the boots in place, but it’s not. The adhesive on duct tape is not designed for HVAC ducts and can come loose over time. Duct tape also leaves behind a sticky residue that’s difficult to clean.

8.) Install Insulation in the Attic and Basement

Duct boot leaks occur when the insulation around the duct boot, which is the connection point between the duct and the floor, is damaged or not correctly installed. Faulty insulation can cause the conditioned air to leak out of the ductwork and into the attic or basement, wasting energy and money.

A worker lifts a batt of rockwool insulation off a stack as he readies it to be applied to an attic ceiling

Older homes don’t have the best insulation, which can be a problem in the winter when your energy bills skyrocket, and you can’t seem to keep your home warm. Insulating your attic and basement is a great way to improve your home’s energy efficiency and reduce your heating and cooling costs.

There are a few different ways to insulate your basement and attic, so it’s essential to research and find the best option for your home. Once you’ve decided on the best insulation for your needs, installation is a reasonably straightforward process. 

9.) Install Supply/Return Air Ducts in Basement and Attic

Supply duct boots are used to connect supply ductwork to the floor joists, and return boots are used to connect return ductwork to the ceiling joists. 

There are a few different reasons why you should install HVAC duct boots in your basement and attic, including:

  • Supply/return air ducts help to seal the connection between the ductwork and the floor or ceiling joists. This bond helps to prevent air leaks, which can lead to lower energy bills. 
  • It will help improve the air quality in your home.
  • It will make your home more energy efficient by helping to regulate the temperature.
  • It can help reduce noise levels from your furnace and air conditioner.
  • It can help to prevent mold and condensation buildup. 

10.) Consider Installing a Vapor Barrier 

A vapor barrier is a material that helps to block moisture and humidity. Vapor barriers can help prevent leaks and improve the overall efficiency of your HVAC system.

A homeowner applies aluminum foil tape to a seam on a radiant barrier beneath a ceiling

Most HVAC contractors will tell you that installing a vapor barrier is a must if you want to avoid further leaks in your system. There are many benefits to installing a vapor barrier in your HVAC system. However, none are more important than the fact it will help prevent further leaks.

11.) Check Carbon Monoxide Readings 

A leaking HVAC duct boot can release deadly carbon monoxide gas into your home, so it’s vital to take action as soon as you notice a leak. If you have a carbon monoxide detector in your home, check the readings and make sure they’re normal. 

If you don’t have a carbon monoxide detector, open a window and get some fresh air into your home. If you have any pets, bring them outside as well.

Once you’ve ventilated your home, call a professional to repair the leak. In the meantime, keep an eye on your carbon monoxide readings and take preventive measures to avoid exposure to the gas.

12.) Contact an HVAC Duct Professional 

If your home needs improvement in terms of energy efficiency, it might be time to consider contacting an HVAC duct professional. Unfortunately, people often try to take on energy efficiency projects themselves, only to find that they are in over their heads. 

HVAC duct professionals have the experience and expertise to get the job done right, and they can also offer advice on other ways to improve energy efficiency in your home. So if you’re interested in improving your home’s energy efficiency, contact an HVAC duct professional today.

Why Sealing Duct Boots Matter

HVAC systems are essential for keeping your home and business comfortable year-round. But did you know that your HVAC system can also be a significant source of energy waste? In fact, studies have shown that up to 40% of the energy used by HVAC systems is lost through ductwork. 

As a homeowner, you’re always looking for ways to save energy and lower your utility bills. One of the most effective ways to do this is to seal your home. Air sealing is the process of sealing up any cracks or gaps in your home’s envelope that allow air to escape. 


The best way to save energy in your home is to ensure that the ductwork is properly sealed. Applying mastic sealant or aluminum foil tape can seal open leaks and cracks. You can also save energy by insulating the ductwork and using energy-efficient HVAC products.


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