Vapor barriers are essential to help keep unwanted, potentially damaging moisture from seeping into your home. When building or remodeling a house you should install a vapor barrier on the warm side of the wall. This means if you run the heat most of the year, install the vapor barrier over the wall studs but, if you run air conditioning most of the year, install the vapor barrier under the wall studs. This prevents condensation and mold from damaging the studs. However, cold air often finds its way through missed holes and cracks and an electrical box by itself offers no protection and unless ensure that it is has a vapor barrier, you run the risk of it corroding and causing future electrical problems. Use both rolled polyethylene plastic vapor barrier material as well as plastic vapor boxes to fully seal and protect your exterior-wall mounted electrical boxes
Things You'll Need
Screws or nails
Drill or hammer
Attach your metal electrical box to a vapor box -- a plastic shield which goes around the box's backside -- with the supplied screws. You do not need to use a vapor box if you install plastic electrical boxes.
Mount the vapor box or plastic electrical box to the wall stud, with screws or nails. Run all the necessary electrical wires to the box before you cover it with the vapor barrier.
Lay a sheet of polyethylene vapor barrier directly over the electrical outlet, extending from the ceiling to the floor. Secure the barrier's top and bottom edges to the horizontal boards with an acoustical sealant. Use an staple gun to drive staples through the barrier and into the wall studs. Use as few staples as possible while still firmly securing the barrier in place.
Use a utility knife to cut the polyethylene vapor barrier around the electrical box. Secure the cut edges to the vapor box or plastic electrical box with sheathing tape; leave no gaps for cold air to enter.