The Insul-Plug blocks old, unused thimble holes in masonry chimneys. It provides a thermal break from combustibles and prevents downdrafts from entering the room. The Insu-Plug is constructed of a ceramic based fireproof insulating material and will help stop the possibility of smoke and carbon monoxide from re entering the home. The black outside plate covers up to a 10" diameter hole. Simply insert it in the old thimble opening screw on the face of the Insul-Plug. The sides expand and create a tight fit inside the thimble opening. It can always be removed by reversing the process.