Sealing a fireplace can mean different things. It could involve the reduction of air flow in and out of the fireplace chimney through the use of insulation, foam board, weatherstripping, new fireplace doors or a special inflatable chimney balloon. Or it could mean entirely sealing off the masonry opening with brick.
Prevent Mold Growth
If you're just trying to stop a draft or the loss of warm, indoor air, it is unlikely that the measures you'll take will promote mold growth. If, however, you're bricking the firebox in, be sure to install a special air brick with small perforations. This will allow a small amount of air to move in and out of the sealed-in firebox, inhibiting the growth of mold or mildew.