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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 Insul-Plug is constructed of a ceramic based fireproof insulating material. The black outside plate cover up to a 10" diameter hole. Simply insert it in the old thimble opening and tighten the expansion screw on the face of the Insul-Plug. The sides expand and create a tight fit inside the thimble opening. The 6" adjusts from 5 3/4" to 6 1/8". The 8" adjusts from 7 3/4" to 8 1/8". It can be removed by reversing the process.