Air Leak Testing & Sealing - The Top 10 Tips for Saving Money at Home - 2

From the Familyhandyman.com

Fill gaps around electrical boxes

Use caulk and foam to seal air gaps
Use foam sealant for bigger gaps.

The gaps around electrical boxes in exterior walls and ceilings are breezeways for cold air. If the gap between the electrical box and the drywall is less than 1/4 in., fill it with latex caulk. If the gap is bigger and lopsided, use foam sealant that's formulated for use around doors and window framing. The minimally expanding foam won't drip down your walls.

Turn off the power to the electrical box and use a noncontact voltage tester to ensure there’s no power. Remove the cover plate. Spray the foam around the box to seal it. After it dries, cut away any protruding foam, add a foam gasket (to reduce drafts through the box) and replace the cover plate. Do the same around register openings on the inside of exterior walls.