Most people are well aware of the energy loss that occurs around windows and doors, but fewer people recognize that various electrical boxes for wall switches, outlets, phone junctions, and other devices can be an equally big energy drain. If you closely examine electrical boxes located on exterior walls or walls that face unheated spaces, you can often feel cold drafts coming through the box or around the edges of cover plates. And although you don't feel it, these boxes are also spots where you are losing air-conditioned air to the outdoors during the cooling season. In addition to wall boxes, a significant amount of heat loss can occur around ceiling fixtures below unheated attic spaces.
The reason for this is not hard to understand. Even if a wall or ceiling is sufficiently insulated, the insulation may have been cut away to make room for the electrical boxes, which means these spots become conduits for cold air to pass through from the outside. And on walls or ceilings where there is no insulation or insufficient insulation, cold air circulating inside the wall or attic finds a perfect pathway into living spaces through the electrical box cutouts in the wallboard or plaster.
Fortunately, correcting the problem isn't too hard. It involves adding insulation where you can and sealing air gaps around electrical boxes.
Insulate Behind the Electrical Box
Warning: Recessed ceiling canister lights are a prime area for heat loss, but be careful about packing insulation around the top of ceiling boxes. This can trap heat and increase the danger of fire. Make sure recessed canister lights are rated for close contact with insulation before insulating in this way. You can, however, still fill the air gaps around the electrical boxes for recessed light fixtures.
Where there is no insulation between the back of the electrical box and the outside wall, it may be possible to add some insulation behind it.
One solution is to use spray insulating foam, such as Great Stuff. If there is enough room around the box, you may be able to spray foam behind the box to fill the gap between the box and the exterior wall surface. Use low-expanding foam, which fills gaps nicely without applying force. One advantage of foam is that it air-seals as well as it insulates, so you solve both problems with one product. Be careful not to get the foam inside the box, though, and NEVER spray insulating foam inside an electrical box, as this can cause overheating of the wires.
The second method is to slip a piece of rigid foam insulation behind the electrical box, using a bit of panel adhesive to hold it in place. Foam insulation works well here because compression is not an issue. Although you could stuff fiberglass behind the box, doing so almost inevitably compresses the insulation, thereby reducing its R-value. Although it can be hard to get access to the space behind the electrical box, if you can manage to get a piece of rigid foam behind it, then loose fiberglass insulation or Great Stuff foam can be used to fill cavities around the sides of the boxes.
Seal Gaps Around Electrical Boxes and Pipes
You can also greatly reduce heat loss simply by sealing air gaps around electrical boxes or any cables or pipes coming through exterior walls. Sometimes wall electrical boxes are installed back to back, creating a direct pathway to the outdoors. Or, there may be TV cables or furnace vent pipes penetrating exterior walls, through which cold air can gain entry. Stop this air flow by sealing gaps from the outside with an exterior sealant or an exterior-grade caulk, which will stand up to sunlight and weather better than silicone or standard painter's caulk.
From the inside, remove the cover plates on switches and outlets and look for air gaps around the boxes where you can feel air flowing. You can plug these gaps with loose insulation or spray foam. Or, you can install neoprene insulating foam strips over the boxes, behind the cover plates. These have cutouts for the receptacles or switches, and when the cover plates are reattached, they form a tight seal around the entire opening. As an added benefit, these neoprene foam strips also deaden sound transmission.