How to Insulate Electrical Boxes

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 insulates, so you solve both problems with one product. 

Be careful not to get the foam inside the box. Never spray insulating foam inside an electrical box, as this can cause overheating of the wires. 

1. Expose the Interior

   Remove the screws on the electrical box cover plate, and pull off the cover plate.

2. 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.

3. 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.

4. Apply Spray

   Spray a small amount of foam behind the box.

5. Repeat

   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.

1. Clean the Workspace

   Clear away any old insulation or debris from behind the electrical box.

2. Cut the Foam to Fit

   Cut a piece of rigid foam insulation to fit in the space between the wall studs.

3. Add Insulation

   Place the insulation into the space. If necessary, hold it in place with foil duct tape, caulk, or a suitable adhesive.

4. 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 the 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.

1. Seal the 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.

2. 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. You'll know you've found one when you can feel air flowing.

3. Add Insulation

   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.

4. Install Gaskets

   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.