The present invention relates to a heat insulating flue duct assembly which inserts a sealing material into a portion connecting a flue duct and enables a heat insulating material to be attached to and installed in a surface of the flue duct. The heat insulated flue duct assembly comprises: a first internal pipe member; a second internal pipe member; a connection member to mutually connect one end of the first internal pipe member and one end of the second internal pipe member; a heat insulating member formed to be attached to an external surface of the first internal pipe member and an external surface of the second internal pipe member so as to insulate the inside of the first internal pipe member and the other inside of the second internal pipe member from the outside; and a finishing member formed to be attached to an external surface of the heat insulating member so as to finish the heat insulating member.
