Fireplaces are not the most efficient way to heat your house, and burning wood is a smell some people do not find pleasing. Others do not like to drag wood and all the debris that comes with it through the house to feed the fire. For these reasons, some people opt not to use their fireplaces in the winter. When the fireplace is not in use, keeping the flue closed is your first defence against cold drafts followed by covering the hole in your fireplace opening.
You could block the fireplace opening permanently with brick or wood, but it's better to do something temporary so that you have access to build an occasional fire want some heat during a power outage. Also, should you ever sell your house, a fireplace is usually an asset.
Air-tight fireplace doors will keep the house closed off from drafts that may come down the chimney. They need to be fitted just right in the opening to do their job correctly. If you live in an extremely cold climate, the doors, which are usually made of glass, will not provide much insulation. The cold may penetrate the glass and bring in cold air.
Another idea for covering up the hole is a sheet of aluminium. An article in "Mother Earth News" suggests having a sheet of aluminium cut big enough to cover the hole. This piece can be put over the fireplace opening when it is not in use, especially during the night when everyone is sleeping.
Because it is fireproof, you can put the aluminium up before going to bed even if you have had a fire and there are still embers burning. This is not an aesthetically pleasing cover and it is not insulated, but it stops the flow of air if it is fitted correctly. This idea works best for people who use the fireplace often but want to keep the opening covered at night.
Styrofoam, Foam and Fabric
If you use the fireplace occasionally and want to keep access to it, cover the opening with a sheet of styrofoam insulation. This will take care of both the cold air and the draft. Cut the styrofoam to the exact size of the opening and then cut a piece of foam used for cushioning the same size and glue it onto the styrofoam. Cover this in a fabric that will complement the decor in the room.
The styrofoam is pliable so you can really wedge it in to the opening good and tight. This is flammable, so it cannot be used after a fire has been burning. This idea gives you a decorative element along with keeping your fireplace covered to prevent the coldness and drafts.