The inventor of duct tape was a genius..
When I was a kid, they used asbestos strips to seal heating ducts. When that got banned, they started using a heavy duty tape that was already around. The silver cloth tape most people use does a poor job on heating ducts. The adhesive drys up and it falls off.
http://www.lbl.gov/Science-Articles/Archive/duct-tape-HVAC.html
I hate duct tape.