how to insulate very wide fireplace?
To make the fireplace waste less cooled or warmed air there are several approaches to take.  Each has its own advantages and disadvantages:


Cap the chimney.  This is easy, inexpensive, and solves most airflow inefficiencies as well as most pest problems.  It can be done easily reversibly or more permanently.  It doesn't do anything to improve the aesthetics of the fireplace though.  Photo from here:


Install a direct vent gas fireplace insert.  One of these makes the fireplace a useful heating device which works even with an electrical outage.  To boost air circulation efficiency most have a blower which varies by manufacturer quality from horrible to awesome (of reliability, noise, effectiveness).  It requires installing a gas line and electricity preferably through the side or back of the fireplace though it can come from the front where it is harder to hide.  A fireplace insert with installation ranges from $2,500 to $7,000.  The "direct vent" feature completely prevents airflow from within the room to the outside and the small ducts installed to the chimney top prevent almost all pest problems.  I suppose it is reversible (by uninstalling it) if one wanted to throw wood, paper, champagne glasses into it, but the gas line makes this "not easy".  Photo from here:


Install a glass (or metal) door.  This costs a lot less than an insert and can significantly increase the appeal of the fireplace.  A well-made custom door almost completely prevents airflow, yet you still have the option of a cheery wood burning fire.  From here: