13” x 13” top lock I chimney cap with damper - These are great for those fireplaces that cannot seem to seal. This damper is a tried and true industry performer and has been used for years through out the states. It installs at the top of the flue and has a draw string that installs onto a hook inside the fireplace fire box. just reach in and pull, the top damper opens and secures while the fire is going. close manually for a complete are tight system. size is the inside diameter of flue tiles.