The ducts in your home’s HVAC system should be sealed air-tight. This makes the system work most efficiently and it helps prevent pressure imbalances that can lead to air leakage outside of the house. Newer HVAC duct systems were hopefully sealed well when first installed, but many older systems are awfully leaky.
Some air leaks are of little consequence, and some are of big consequence. Ducts in the attic that are leaky will be terribly inefficient because the leaked air (that you’ve paid to heat or cool) is immediately lost to the outside, and the pressure differences that are created lead to more outside air being drawn into the house. Leaky ducts in the basement probably aren’t terribly important because the leaked air is still within the house’s conditioned space.
To seal up leaky ducts there are a couple of good options. Probably the best option is to use duct mastic. Mastic is a thick goo that can be applied with a brush or even by hand. The duct joint should be secured mechanically, with screws or other rigid fasteners, and then mesh tape can be placed over the joint and then the mastic. It lasts a long time, is quite forgiving, and is considered the best duct-sealing solution by a lot of HVAC professionals.
Another good option is to use “duct tape”. I put that in quotation marks because the idea of “duct tape” has been ruined by improper word usage. Generic gray utility tape, as pictured here, is NOT duct tape and you should never use it to seal the joints of your HVAC ducts.
This is NOT “duct tape”. It’s generic utility tape.
It will dry out and fall off in short order – maybe after several years, but that’s still an order of magnitude shorter than your ducts will last. It’s not clear how this ever got to be called “duct tape”, but it’s the worst thing to use on your ducts. This is a cloth tape coated with a polyethylene resin on one side and an adhesive on the other side. This is best called utility tape or cloth tape, but not “duct tape”.
The right tape to use is called joint foil tape or aluminum foil tape, and it should conform to standard UL 181. It has a shiny foil face and when you unwrap it from the roll you then have to peel off the backing. If you’re going to use tape to seal your ducts, this is the kind of tape to use.
This is joint foil tape. It’s clearly labelled as conforming to UL 181.
Once the tape is unrolled the backing needs to be peeled off.
Utility tape is great stuff, and it can come in very handy. Soldiers regularly carry it, and astronauts even took it to the moon! On the Apollo moon missions utility tape was used to secure bags of refuse to the ship’s bulkhead in the final days of the mission. But the Apollo 13 crew used it to construct the lithium hydroxide devices used to scrub carbon dioxide from their air. And the Apollo 17 crew used it, along with some maps from the flight plan, to build a new fender for the rover.
So generic utility tape is great stuff. But it’s not the right stuff to seal your ducts with.