How Do I Cap Off an Air Duct?

Last updated on October 2nd, 2022 at 01:13 pm

If you are working with air ducts in your home, office space, or other location, then you may find yourself needing to cap off one of the ducts.

The process is not really that hard, and all that it takes is some sheet metal, tin snips, a scribe, folders, a bar fold, and an S-lock for assembling ducts. Basically, after creating the end cap all you need to do is fit the cap to the duct and lock it in place.

If you are interested in learning how you can cap off air ducts on your own for whatever purpose, go ahead and keep on reading. We will be going over how to make a cap for the air duct as well as how to install it.

Capping off the Ducts

Make an End Cap for Your Duct

Before you get to the installation of the air duct, you will first need to create an end cap that you will use to cap off the end of the duct.

First, you will need to get yourself some sheet metal. For a standard duct, you would need an end cap that is 8 inches by 12 inches. In order to leave space for your folds, later on, you will need to add one inch extra to each of the sides of the sheet.

That means that you will want to cut out a rectangle of sheet metal that is 14 inches by 10 inches. After this is finished, scribe each side by 1 inch. 

The next step is to cut out each of the corners of the sheet. Do not cut the corners at right angles, you will want to bias each corner in order to leave space for when you bend edges up.

Biasing the cuts means that you do not cut each corner in a perfect square, but instead cut it at an angle in order to ensure that the corners of the folds do not hit each other once they are folded up.

Now you will want to fold each of the edges up. Starting with the drive edges, you will want to fold over only half of an inch. When folding, do not fold it all the way and ensure that you do not close the drive as you will need it for installation later on.

Do this for both drive edges, and then begin folding over the other edges. Ensure that all edges are folded over at 90-degree angles in order to prepare them for installation later on.

After making the end cap itself, you will want to prepare two drives which will be used to lock the end cap in place to the duct. Drives are sheets of metal bent over at a half-inch on either side which is used as a locking mechanism to keep the cap in place.

After these are prepared, you will be ready to install the end cap onto the duct.

Installing Your End Cap

Installation of your newly crafted end cap is easy and fast. 

Before getting started, bend over two drive edges onto the duct itself in order to have a place to slide your drives on later on in the installation process. They should be bent over only half an inch in order to fit the drive edges you made previously for the end cap itself.

First, you will want to slide on the S-locks of the end cap right onto the air duct itself. After this, slide the end cap that you have made onto the end of the duct.

If the locks of the endcap have closed or become a bit bent up, you can use a screwdriver or hammer to open them back up.

After this, you can slide in your drives onto the drive edges of the end cap and air duct in order to lock it into place.

If the edges of the duct or end cap have become bent or folded, you can open them back up gently with a screwdriver and then guide the drive itself onto the two flaps.

Sometimes it may be a bit difficult to slide the drive into place, so you may need to use a hammer to gently tap the end of the drive so that it slides into place.

Once this is finished, bend off each end of the drive and direct it into place towards the S-lock edges of the end cap.

Repeat this process on the other drive edge of the duct and you will have fully installed your new end cap on your air duct!

You may also choose to further seal the edges of the duct with silicon or rivets, but this all depends on how secure or permanent you want the seal to be.


Capping off an air duct is a simple process that can be finished in under an hour depending on your skill level.

For newbies, the most difficult part of this may be preparing the end cap itself. There are many guides online for how to create your own end cap, and the steps are simple to follow and execute.

Hopefully, this guide has helped you to get an idea of how you can cap off your air duct in the simplest and easiest way possible.