The best way to handle a fireplace is to avoid it. A standard fireplace, whether masonry or metal, generates so much air leakage that it can undermine all your other air sealing efforts. Although few people would leave a window open, many people leave fireplace dampers open for weeks at a time. The heat loss is comparable. Even when closed, dampers could have as much “leakage area” as all the windows and doors combined. Energy analysis programs often ignore these losses. So the house really isn’t as energy efficient as you might think.
More important, fireplaces pose a backdrafting threat. In a tight house, the suction created by a exhaust fan can be strong enough to cause a fireplace to backdraft. Clothes dryers and downdraft cooktops can pull a suction on any house. Even when there’s no fire, air often flows into the house through the fireplace, carrying with it emissions from the flue deposits, such as creosote. To protect the occupants from these potentially harmful gases, you must totally isolate the combustion chamber from the living space. This simply can’t be done with an open hearth fireplace.
Over the years, features have been added to open fireplaces to reduce these impacts. Dampers are installed, but seldom used. Outside air inlets to the firebox reduce the amount of heated indoor air used for combustion. Glass screens separate the firebox from the room.
Glass screens never fit tightly. In fact, manufacturers build them leaky to keep the glass cool. They purposely avoid tight screens that would force them to comply with federal regulations for “airtight” wood burning appliances. Loose fitting screens can’t stop harmful gases from leaking into the living space and heated air from leaking out. Without tight screens, the combustion air inlet is just another air leak in the house. |
