When I install a plastic vapor barrier over a wall or ceiling, I ignore the electric outlet boxes until I’m finished hanging the sheet. Then, rather than cut the plastic on the outside of the box, I make the cut 1/4-in. to 1/2-in. inside the perimeter of the box. I stretch the plastic around the outside, which gives me a nice tight fit that cuts down on cold-air infiltration.
This trick works for other utilities that poke through the vapor barrier, such as ductwork or plumbing. A bead of caulk or a duct-tape collar can make the seal even tighter.
Susan Caust Farrell, Searsport, ME