How to Cap Off a Duct: 3 DIY Steps

Your home's HVAC ductwork hides behind floorboards or drywall, so you only think about it when it’s not functioning correctly. If you’re replacing old ducts with new ones or want to remove a register from an unused room, you’ll need to cap off the duct to promote efficient airflow and avoid overloading your HVAC unit. This guide will teach you how to cap off a duct in minutes.

Mistakes to Avoid

Capping off a duct is a relatively simple project, but without the proper knowledge, you could make several potential mistakes. Here are some of the biggest mistakes to avoid when capping off a duct.

• Buying the wrong size cap

• Not sealing the cap properly

• Skipping the cap and taping the duct closed

• Using the wrong screws to secure the duct cap

When to Cap Off a Duct

There are several times when you might need to cap off a duct. For example, if you’re removing a vent from a room you no longer use, you’d need to cap off the duct after removing the vent to maintain steady airflow in the ductwork system. Capping off a duct may also be necessary if the ductwork is damaged and you’re replacing it with new ductwork in a different part of the room. Capping off the old duct ensures that the air is directed to the new vent rather than leaking through the old one.

DIY vs. Hiring a Pro

Capping off an HVAC duct is an easy DIY for a handy homeowner. As long as you can find the right size cap for your duct and install it properly, being careful to seal it thoroughly, you can certainly do this job yourself.

However, if your ductwork is a non-standard size, you might need an HVAC professional to make a custom duct cap using sheet metal. The pro can then install the custom cap to seal off your duct, and they’ll ensure it’s done correctly so there are no leaks. An HVAC duct installer near you can also handle this job if you don’t feel comfortable tackling it as a DIY project—especially if your ducts are inaccessible.