How to Seal Ductwork in Your Attic: A Guide

Air ducts are the home’s circulatory system, responsible for distributing the warm and cool conditioned air that keeps your residence at the desired temperature. However, as with any system, inefficiencies can arise, leading to weaker heating and inadequate cooling from air conditioning units, contributing to higher energy bills. 

Learning how to insulate ductwork in the attic is just one essential aspect of the job. Air duct sealing is another matter. Whether it’s because of the tricky size of the space or the confounding HVAC choices of a previous owner, the attic can be a prime culprit when it comes to leaky air ducts. 

In this guide, we’ll explain how to tell if your air ducts are leaking and walk through how to seal ductwork so that your HVAC system operates at maximum efficiency. 

Are Your Air Ducts Leaking? 

Before we get into how to seal HVAC ducts, let’s first run through the symptoms of a leaky duct system. If you’re experiencing any or all of these conditions, then it may be time to check the integrity of your ventilation system:

  • Higher than normal energy bills – Leaky ductwork means that your heating and cooling system has to work extra hard to maintain the home’s desired temperature. The result can be increased energy bills every month. 
  • Uneven heating and/or cooling – If some rooms in your home are warmer or cooler than others, that can suggest a duct leakage. You also might notice the phenomenon that certain areas of individual rooms are hotter or cooler than they should be. Basically, if your temperatures are uneven, that’s a sign. 
  • Excessive dust – Does your house seem perpetually dusty no matter how often you clean it? That can be caused by compromised ductwork, especially in the attic. Because the attic is typically dusty, duct leakage can begin sucking in dust and pushing it throughout the rest of the house. 
  • Poor air quality This symptom goes hand-in-hand with an excess amount of dust. If you’re noticing a musty smell throughout the house or finding that your allergies are more inflamed than normal, that’s an unfortunate indicator of leaky ductwork. 

How to Find an Air Leak

If you believe your ductwork is leaking, the next step is identifying the source. The attic is a likely location for a variety of reasons. Because of space restrictions, ductwork in attics may be constructed with many joints and bends, which are all susceptible to leaking. 

Attic ductwork can also be competing for space with other fixtures (like the assembly sides of ceiling fans from the floor below) or insulation. In the case of older homes that weren’t built with modern HVAC needs in mind, the ductwork can simply become a neglected mess. 

The simplest way to check for an air duct leak in your attic is to perform a visual inspection. When examining your ductwork, keep an eye out for sections that have:

  • Rusted
  • Been crushed
  • Developed kinks
  • Have rodent chewing

Those areas can often be the source of pesky leaks. 

Also, keep an eye out for spots that have been previously patched. Over time, sealant can wear off and this might call for an air duct replacement. As noted below, this can be particularly true when common duct tape is used as a seal.

More Ways to Measure Leakage

Most of the time, you’ll be able to feel the air escaping from the system. However, if you’re unable to spot the leak with just your eyes and hands, there are a few other tricks you might consider:

  • Use a smoke pen – Smoke pens can spot the movement of air escaping from ducts. 
  • Suds up your ducts – Soapy water can be sprayed on ducts. Leaks will then reveal themselves in the form of bubbles rising from the tubes. 

If you’re interested in more thorough results than what your bare hands and soapy water might reveal, consider having a professional conduct a duct blaster test. Duct blasting is the standard for HVAC testing amongst home inspectors and is done by sealing the vents in a household, pressurizing the ducts, and then measuring leakage. 

A duct blaster test will also give you an idea of just how much air is escaping from your HVAC system, which can be a useful comparison point after you’ve sealed your ducts. 

Benefits of Sealing Air Ducts

The benefits of properly sealed ductwork are numerous. They are pretty much the opposite of our aforementioned leaky ductwork symptoms. 

With completely sealed ductwork, you’ll enjoy:

  • Lower energy costs
  • More thorough heating and cooling
  • Better air quality

The energy benefits of sealed ductwork cannot be overstated. The Depart of Energy estimated that leaky ductwork could account for upwards of 30% of a home’s energy costs.1 That’s a lot of extra money to part with every month. 

Beyond just the benefits to the individual homeowner, sealing ductwork is also an environmentally conscious choice. In this day and age, no one wants to waste energy unnecessarily. 

Considering those positive impacts, it’s imperative that every homeowner keep their ductwork properly maintained. So, how do we go about sealing a leak?

3 Ways to Seal a Leaking Air Duct

There are three common ways to get the job done when it comes to sealing air ducts.2 We go over all three methods below. 

Keep in mind that the easier the fix, the less reliable the result. For long-term benefits, it’s best to ensure that your ductwork is sealed securely and permanently. 

Method #1: Duct Tape (But Not Literally Duct Tape)

The simplest method to seal a leaky air duct is to use adhesive tape to patch up the gap. Using tape is a quick and simple fix, but not a long-lasting one. 

Over time, the air and moisture blowing through your air ducts will wear away the tape’s adhesive. Eventually, the tape will peel away and you will need to seal your air duct again—except this time, you’ll have tape gunk on your duct that may make the job a little more complicated. 

This is especially true of duct tape. Despite its name, duct tape is not actually made for use on air ducts. While it’s good for a number of other household projects, the adhesive on duct tape is not made to withstand the temperature changes common with air ducts. 

Using tape is essentially a quick fix until you’re ready to do a more thorough sealing of your air ducts. For the best results, look for one of these kinds of tape at the hardware store:

  • Oriented polypropylene tape 
  • Aluminum foil tape

Method #2: Mastic Duct Sealant

A water-based mastic duct sealant is a much more permanent solution than tape, offering tighter seals. This thick paste can be applied directly to ductwork using either a caulk gun or a paintbrush. 

When working with mastic sealant, remember that you should:

  • Clean the area before applying the seal (especially if there’s tape residue left behind from the previous patching)
  • Wear gloves and long sleeves
  • Wear a mask, especially if working in a space without access to fresh air

Once mastic duct sealant hardens on your air ducts, removing them can be difficult and messy. For that reason, ensure that you won’t need to disassemble the ductwork in the near future. If you’re planning to clean your air ducts (always a good idea if you’re suffering from odors or poor air quality), you should do so before applying the sealant. 

Method #3: Have a Technician Aeroseal Your Duct Work

Arguably the longest-lasting solution to sealing leaky ductwork is also the least DIY. To use the Aeroseal method to seal your ductwork, you’ll need the help of a licensed technician. 

Aerosealing is done by first closing and pressurizing your ductwork, similar to how the duct blaster we described above works. Once the ducts are closed off, the Aeroseal machine pressurizes the ducts. Hot polymer glue is then sprayed into the duct and blown through the system with a fan. 

Because the air duct is sealed, the glue will be drawn to any cracks in the ductwork, where it will dry and harden. Essentially, Aeroseal patches your compromised ducts from the inside. The Aeroseal process is great for sealing hard-to-reach or hard-to-find fissures in your ducts, which can be especially common in attics. 

It’s also important to note that the glue is non-toxic and contains the same materials as pacifiers and chewing gum. 

Attic Construction Can Plug Your Leaks

By now, you hopefully understand the benefits of having a properly sealed ventilation system. From lower energy bills to improved air quality, ensuring your HVAC system is free of leaks basically pays for itself. 

If you need more help than what’s provided in this guide, Attic Construction is here to help.

We have over ten years of experience in air duct cleaning in San Diego and Phoenix areas. Our experienced contractors can help find inefficiencies in your ductwork and bring them up to standard, all with the professionalism that has made us one of the most reputable attic cleaning companies in Orange County. 

If you’ve got questions about the information provided in this guide or think you need a helping hand sealing your ductwork, get in touch with us today!



  1. ACHR News. DOE: Leaky Ducts are Top Energy Waster.
  2. Attainable Home. Beginner’s Guide  to Air Duct Sealing: Three Proven Methods.


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