How to Seal a Heating Duct

two rolls of insulating foil tape
What You'll Need
Sheating tape or a roll of metal
Expanding spray foam insulation
Brush or trowel
Rubber gloves
Ladder (optional)

Heating and cooling costs suck up almost half of the average monthly electric bill, and if you have leaks in your ductwork, you’re wasting money and energy. In fact, even small leaks can result in 20-30% losses, so they're worth looking into. Fortunately, there are easy to use materials that will seal those ducts, helping you save resources and cut your electric bill.

Step 1 - Find the Leaks

Accessing and finding the leaks might be the most challenging part of the job, since ductwork is traditionally woven throughout some tight spaces in your home, like crawl spaces in your attic. Plus, there will be insulation surrounding many of the ducts (which is a good thing), making it harder to identify where the air is escaping.

opening an HVAC vent

You may not be certain you have a leak, but if any of the following occur, investigate further:

1. Rooms become too cold or too warm.

2. Utility bills are high throughout the year.

3. There is little or no airflow in some rooms.

4. You need to change the furnace filter more than once a month.

5. There are visible streaks near duct connections and air registers.

6. Visible ducts do not show any form of insulation.

7. There are kinks or tears in flexible ducts.

To find leaks in the ductwork you can access, turn your heat or AC on. Once blowing, hold a piece of toilet paper, ribbon, or other lightweight material around a seam. If there is a leak, the seam will either pull the material in or blow it away, depending on the type of duct it is (sucking or blowing).

insulating foil tape

Step 2 - Tape Small Leaks

Gaps less than ⅛ inch wide can be fixed with mastic or foil tape, specially made for the job and easily found at any home improvement store. Don’t assume just because a seam is wrapped in duct tape it's leak-free. Duct tape is not a good option for heating ducts because it will eventually break down, dry out, and create gaps. You can cover duct tape without removing it, however.

Whether there is an existing tape or not, make sure to clean the seam before applying the foil tape. It will not adhere to moisture or dust particles. When the surface is ready, peel the backing off the mastic tape and apply to the seam—press it firmly into place all the way around and test for any air leaks.

If your seam is at a junction, it may be more difficult to get a solid seal. Use several smaller pieces of foil tape to fill spaces, then wrap a larger piece around the smaller ones to ensure a tight seal.

Step 3 - Seal Large Gaps

For larger gaps, mastic tape likely won’t hold long term, so it’s best to use a silicone caulk instead. Load your caulk gun and run a stream of caulk along areas where the two pieces of metal ductwork meet. Lightly run your finger along the bead of caulking to press it into place. Wait for the caulk to dry according to directions before testing for airflow leaks.

Step 4 - Seal Really Large Gaps

If your gap is larger than a bead of caulk (about ⅜ inch), use spray foam insulation to fill the space and block airflow. It will expand as you apply it—don't panic, that's what it's supposed to do. Just cut off any excess as needed once it sets.

spray foam insulation

Step 5 - Inspect the Whole System

Getting started on the project of sealing ductwork requires collecting supplies, inspecting the system, and getting yourself in some tight spots. So while you’re there, make sure you check out the entire system. Move methodically from one seam to the next. Typically they are about six feet apart.

You may be dealing with wide ductwork, narrow metal tubes, or even flexible hoses, so you’ll have to adapt to each kind. You may also have to cut away insulation to access some areas and replace it when you’re done.

Your HVAC system not only provides comfort to your home through temperature control, it controls the air you breath. Making sure your ducts are sealed is an important step towards ensuring the safety of your family and improving the consistency of your airflow.