Duct tape (fabric-based tape with rubber adhesive) is good for many things. People use it as a bandage, to tape up broken plates, to repair their cars. There are thousands of uses. Some folks have even have websites and written books about duct tape. Apollo 13 astronauts used it to repair some equipment to get home to Earth safely from around the moon.
BUT YOU SHOULD NOT USE DUCT TAPE TO SEAL DUCTS!
During World War II, before it was called duct tape, the U.S. military bought quantities of the cloth-backed, rubber-adhesive tape for making emergency repairs on the battlefield. In the movie business it's called "gaffer's tape," used for everything from bundling cables to holding sets together.
Some time after WWII, heating and air conditioner contractors begin to use the tape to seal the joints in heating, ventilation, and air conditioning (HVAC) ducts. This tape was manufactured in the same way, though to match the ducting it was colored silver rather than the green of the Army version. Because of this use, it became known informally as 'duct tape'.
The problem...duct tape does not adequately seal the joints and has a short lifespan.
Over a three-month period in 1998, Lawrence Berkeley National Laboratory (LBNL) researchers tested duct tape and 31 other sealants under accelerated laboratory conditions that mimicked long-term use in the home. They heated air to nearly 170 degrees and chilled it to below 55 degrees before blasting it through ducts. They baked ductwork at temperatures up to 187 degrees to simulate the oven-like conditions of a closed attic under a hot summer sun.
Of all the things they tested, only duct tape failed - and they reported it failed reliably and often quite catastrophically.
Instead of using duct tape, the researchers recommended sealing ducts with mastics, gooey sealants that are painted on and allowed to harden. Metal ducts should be held together with sheet metal screws; flexible duct connections should be secured with metal or plastic bands.
Beginning October 1, 2005, Title 24 of California's Building Energy Efficiency Standards requires that ducts be tested for leaks when a central air conditioner or furnace is installed or replaced. Ducts that leak 15 percent or more must be repaired. More information can be found at: www.energy.ca.gov/title24/changeout/