The Flueblocker is a dense wool pad that seals and insulates the fireplace or wood stove chimney. It fits into the narrow part of the flue just above the firebox, and can be used with our without a damper. It can easily be installed and uninstalled many times. You MUST remove the Flueblocker before you light your fireplace. Cannot be used in functional vented gas log fireplace. Flueblocker is made with Herdwick wool in England.