You may have large rectangular ductwork in your attic as way of delivering hot or cold air as part of your HVAC system. Sometimes the air that gets forced through these ducts can leak out through loose or broken connections. Check to make sure no air is leaking. If it is, you are wasting money because the hot or cold air is not being delivered to the living areas of your home and your system is not very efficient.
Usually in your crawl space under the home, or in the attic above the home, you have supply ducts that deliver hot or cold air to rooms in your home, and return ducts that suck air back to your HVAC unit to heat or cool the air. These ducts have joints where they were originally put together.
Using a duct mastic, you can apply this product to these joints to eliminate any air leakage. We found a great video from The Tom and Larry Show that explains how this works but also shows you how to do it.