Is it really necessary to put all that plastic vapor barrier over an insulated wall? In a word, yes. While it’s extra work, it’s an essential part of the insulation “system”.
Lots of things in your house fill the air with moisture. Drawing a hot bath. Cooking. Washing clothes. Mopping floors. Breathing. The warm air carries this moisture along, and tries to push it out through walls and ceilings. The problem occurs when it’s cold out. Insulation does such a good job of keeping the heat in, that whatever moisture gets beyond it hits cold air or building materials. It zaps back into liquid form, like beads of water on a cold summer drink.
Of course, water causes rot, paint peeling and any number of other problems.
So we have vapor barrier. A sheet of good durable polyethylene (6 mill) keeps the moisture-laden air from travelling beyond the insulation.
To work, this barrier must be a solid sheet, with no gaps. Acoustical caulking (or cement) is a gooey, messy compound that never dries out… and a couple parallel beads are the best bet to seal seams. If you’re worried about gaps in the barrier from drywall nails or screws, you can even put a bead along each stud to create a positive seal against any punctures. Use a thin bead to seal vapor barrier to window frames, put poly pans (or good plastic bags) around electrical outlets, and make sure ceiling barrier is sealed to vapor barrier on the walls.
Of course, a continuous barrier isn’t always possible. Even in new construction, it’s not always practical to wrap vapor barrier around the outside of headers holding floor joists, and between outside walls and partition walls. Just keep in mind that the more you can seal, the better.