How to block cold air from a chimney
A fireplace can add both visual and decorative warmth to any room. However, all wood burning fireplaces require some air to escape up the chimney. Outside air kits can help with this but they do not totally eliminate the fact that some heated air from your home will escape up the chimney. Of course that loss of heated air increases energy cost for the home as it takes more energy to raise the room temperature to the desired setting.
A properly designed glass door can help keep the room air from escaping, keep the cool air from back flowing down the chimney, and add a bit of decor. Also, the glass doors can be closed when the fireplace is not in operation thereby eliminating the huge flow of heated air up the chimney.
The damper inside your fireplace greatly aides in keeping out cold air, birds, animals etc. when the fireplace is not in use. Dampers are often made from cast iron but can also be made of steel - but none will completely prevent the loss of heat up the chimney.. There are also chimney top dampers available. Chimney top dampers are operated by a chain inside the fireplace and can tightly close off the chimney, keeping cold air out.
Additionally, there are devices such as pillows, plugs and balloons. Such devices are inserted inside the fireplace above the firebox. Some are inflatable and expand to fit the dome area of the fireplace. Of course they must be removed prior to using the fireplace.
Styrofoam insulation - 2 inch thick - can be cut to fit the dome area of the fireplace and inserted there when the fireplace is not in use. Painting the foam with flat black paint will make it nearly invisible from the room.