How to Insulate Electrical Boxes

Electrical box with surrounding insulation

The Spruce / Kevin Norris

Overview
  • Working Time: 1 - 4 hrs
  • Yield: 1 room
  • Skill Level: Beginner
  • Estimated Cost: $15 to $25

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 difficult. It involves adding insulation where you can and sealing air gaps around electrical boxes. There are three common methods for insulating electrical boxes.

What You'll Need

Equipment / Tools

Spray Foam Method

  • Screwdriver
  • Utility knife or awl

Rigid Foam Method

  • Screwdriver
  • Utility knife

Sealing Gaps

  • Screwdriver
  • Utility knife

Materials

Spray Foam Method

  • Can of spray foam insulation

Rigid Foam Method

  • Rigid foam insulation
  • Duct tape or caulk (if necessary)
  • Fiberglass insulation (if necessary)

Sealing Gaps

  • Exterior caulk or sealant
  • Low-expanding spray foam or fiberglass insulation
  • Neoprene foam outlet gaskets

Instructions

Materials and tools to insulate electrical boxes

 The Spruce / Hilary Allison

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.

    Electrical box cover plate unscrewed and removed

    The Spruce / Kevin Norris

  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.

    Gaps around edges of electrical box to find entry point

    The Spruce / Kevin Norris

  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.

    Applicator straw inserted into edges of electrical box

    The Spruce / Kevin Norris

  4. Apply Spray

    Spray a small amount of foam behind the box.

    Applicator straw spraying foam into edges of eletrical box

    The Spruce / Kevin Norris

  5. Repeat

    Repeat at all four sides of the box, if possible.

    Spray foam inserted on all sides of electrical box gaps

    The Spruce / Kevin Norris

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.

    Old debris cleared off electrical box

    The Spruce / Kevin Norris

  2. Cut the Foam to Fit

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

    Rigid foam insulation cut with box cutter

    The Spruce / Kevin Norris

  3. Add Insulation

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

    Foil keeping rigid insulation in place around electrical box

    The Spruce / Kevin Norris

  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.

    Rigid foam insulation covering all sides of electrical box

    The Spruce / Kevin Norris

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.

    Exterior-grade caulk ceiling gaps in house cracks

    The Spruce / Kevin Norris

  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.

    Electrical box cover plate removed to look for gaps

    The Spruce / Kevin Norris

  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.

    Spray foam inserted into sides of electrical box gaps

    The Spruce / Kevin Norris

  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. 

    Neoprene insulating foam gaskets inserted behind reinstalled cover plate of electrical box

    The Spruce / Kevin Norris