Drafts are the most common callback builders experience with fireplaces. They make the room's air and floors feel uncomfortable.
Drafts make up almost 90% of all fireplace complaints. In addition, drafts are usually the most complex issue to solve after the home is built. With this in mind, it's important to prevent the possibility of drafts at the start with proper insulating, draftstopping, and air sealing techniques, as well as with the proper combustion vent system. For homeowners, there's little that's more annoying than relaxing before a cheery fire with cold drafts moving through the space, causing both the room's air and the floors to feel uncomfortable.
Cause 1:
Improperly insulating, draftstopping, and air sealing
The major cause of drafts is improperly insulating, draftstopping, and air sealing a gas or electric fireplace installed in an exterior wall. With a B-vent gas fireplace, cold air can infiltrate the space surrounding the fireplace unit and move into the living space by traveling through the firebox. With a direct vent gas fireplace or an electric fireplace, cold air can infiltrate the space surrounding the fireplace unit and move into the living space by traveling through the gaps around the unit's face. Most fireplaces are poorly insulated because the installer insulated the wrong layer of framing, leaving the fireplace outside the home's thermal enclosure. Fireplaces also require draftstopping and air sealing, two construction details that are frequently forgotten, missed, or installed incorrectly.
Solution
To help prevent drafts, properly insulate, draftstop, and air seal the space around the fireplace unit. Install insulation in all wall cavities between the fireplace unit and the outdoors, so the unit is within the thermal enclosure. If the fireplace is located in a bump-out, insulation should be installed in the exterior walls, as well as at the top of the fireplace cavity and in the cantilevered floor. If the fireplace is located mid-plan in a single story home, insulation should be installed at the top of the fireplace cavity.
Draftstopping encapsulates the insulation and creates an effective air barrier. After installing the insulation, install a continuous rigid air barrier of wood sheathing, drywall, or Thermo-ply® along the exterior walls. If the fireplace is located against an exterior wall, draftstop the wall. If the fireplace is in a bump-out, draftstop the exterior walls, the top of the fireplace cavity, and the underside of the cantilevered floor. If the fireplace is located mid-plan, draftstop the top of the fireplace cavity. After installing the draftstopping, thoroughly seal all corners, edges, and seams with foam. Use fire-rated sealant to seal the area where the heat shield meets the exterior wall.
Cause 2:
Cold air infiltrating through the combustion air duct
A combustion air duct on a B-vent gas fireplace can create drafts in the living space. A combustion air duct brings air from the outdoors to the firebox. Once the cold, outdoor air travels into the firebox, it can move out into the living space, creating a draft. Fireplaces with operable doors are more likely to create drafts, because doors leave the firebox more open to the living space.
Solution
Make sure the combustion air duct inlet is level with or lower than the firebox. If it’s installed level with the firebox, install a small dip in the duct, known as a cold air dip, to prevent the flow of cold air. Make sure the combustion air duct is properly connected to the unit and sealed where it penetrates the exterior wall. Check the manufacturer's instructions to see if there are other points on the fireplace unit that need to be sealed as well.
The best solution is to install a direct vent fireplace. Direct vent fireplaces have a sealed combustion vent system. With a sealed combustion vent system, combustion air is drawn from the outdoors into a sealed firebox through a flue. The flue consists of two concentric circles—outside air is drawn through the outer pipe while combustion gases are exhausted outdoors through the inner pipe; air from the home isn't used to fuel the fireplace. Direct vent fireplaces are the most efficient type of fireplaces on the market.
