The aesthetic beauty and practical warmth of a fireplace in a home is a gem of architecture. However, keeping that space active may not always be the most efficient when energy bills heat up. Depending on your plans, there are both temporary and permanent coverings you may choose for your fireplace that keep the look of your room clean and consistent but also prevent the energy loss from drafts or escaping air.
To enclose a currently open fireplace, you can purchase and install flat, closed-door screens that secure to the wall just around the fireplace opening. These are usually metal and glass for best fire safety and become a clean face on your fireplace wall without taking up much floor space. You keep the doors closed, even perhaps inserting a foam board piece inside the doors when the fireplace is not going to be in use for a season, to prevent drafts. The glass doors slide open when you make a fire to allow the heat to warm the room.
Some do-it-yourselfers will take plywood, covered with a foam layer to prevent drafts, and cut it to fit the opening. Cut the plywood two to four inches beyond the size of the opening, while cutting the foam layer to fit the opening tightly. This is a temporary solution, even if you affix the plywood because it can be easily removed if you decide to sell your house or reopen the fireplace for use. You can decorate the plywood, or purchase a metal decorative folding screen to set up in front of it or use other floor decorations such as large potted plants to redirect the attention from the opening.
An easier cover is accomplished by purchasing a tin or bronze-plated wall/fireplace facing cover from a company such as The Victorian Fireplace Shop. These are measured to fit the opening of your fireplace and set in the opening as a decorative sealing cover for the fireplace.
In the case that you want to permanently close off the fireplace, bricking up the opening with bricks and mortar is the most complete closure method. You need to leave an air ventilation opening to keep from getting air pockets backing up from the top of the chimney. Choose a color that is similar or complementary to the existing fireplace. This can be a do-it-yourself project if you are familiar with masonry work and can acquire the tools to mix the cement and complete the bricking.
It's also possible to hire a chimney worker or fireplace specialist to seal the top of the chimney as well as the opening in the house. Unless you are particularly skilled in this area, hire a professional to complete this permanent chimney closure.