While many people are well aware of the energy loss that occurs around windows and doors, few recognize that electrical boxes for wall switches, outlets, phone junctions, and other devices add up to a surprisingly 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.
The reason for this is simple. 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 air to pass through. And on walls where there is no insulation or insufficient insulation, air circulating inside the wall finds a perfect pathway into living spaces through the electrical box cutouts in the drywall or plaster. In winter, this becomes a source of indoor heat loss, and in summer, a source of heat gain from the outdoors.
Fortunately, correcting the problem isn't too hard. It involves adding insulation where you can and sealing air gaps around electrical boxes. There are three common methods for insulating electrical boxes.
Equipment / Tools
Spray Foam Method
- Utility knife or awl
Rigid Foam Method
- Utility knife
- Utility knife
Spray Foam Method
- Can of spray foam insulation
Rigid Foam Method
- Rigid foam insulation
- Duct tape or caulk (if necessary)
- Fiberglass insulation (if necessary)
- Exterior caulk or sealant
- Low-expanding spray foam or fiberglass insulation
- Neoprene foam outlet gaskets
How to Insulate Electrical Boxes With Spray Foam
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 to the surrounding material. 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.
Expose the Interior
Remove the screw(s) on the electrical box cover plate, and pull off the cover plate.
Find an Entry Point
Inspect the outer edges of the box. Often, there are gaps around the edges of the box that let you insert the applicator straw of the spray can. If not, you can make a few small holes with a utility knife or an awl. Be careful not to push any tool deeper than the wall material (about 1/2 inch) so that you don't contact any electrical wires.
Fit in the Applicator
Insert the applicator straw into each gap or hole and slide it in until it reaches just beyond the back of the box.
Spray a small amount of foam behind the box.
Repeat at all four sides of the box, if possible.
How to Insulate Electrical Boxes With Rigid Foam
If you can gain access to the backside of the electrical box, you can slip a piece of rigid foam insulation between the box and the wall. This is much more effective than stuffing fiberglass insulation behind the box because compressing fiberglass insulation dramatically reduces its insulating value.
Clean the Workspace
Clear away any old insulation or debris from behind the electrical box.
Cut the Foam to Fit
Cut a piece of rigid foam insulation to fit in the space between the wall studs.
Place the insulation into the space. If necessary, hold it in place with foil duct tape, caulk, or a suitable adhesive.
Cover the Box Entirely
Insulate around the side, top, and bottom of the box with more rigid foam or fiberglass insulation notched to fit around the box. Do not compress fiberglass insulation to accommodate the box.
How to Seal Electrical Box Gaps
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.
Seal Gaps Outside
Stop airflow by sealing gaps from the outside of the house, using an exterior sealant or an exterior-grade caulk. Exterior products stand up to sunlight and weather better than standard (interior) formulas.
Search for Gaps Inside
Seal boxes from inside the house by removing the cover plates on switches and outlets and looking for air gaps around the boxes where you can feel air flowing.
Plug gaps around boxes with loose insulation or low-expanding spray foam. Let the spray foam harden, then trim any excess with a utility knife.
Install neoprene insulating foam gaskets over the boxes, behind the cover plates. These have cutouts for the receptacles or switches. When the cover plate is reinstalled, the gasket forms a tight seal around the entire opening. As an added benefit, the foam gaskets also deaden sound transmission.