Tip 9: Keep your drywall . . . dry. A good thing to have on your exterior wall is vapor barrier. This will keep moisture from penetrating the back of the wallboard, which can possibly cause mold. If you don't do the vapor barrier and you have a large exterior wall, you can get a mildew buildup behind the drywall, and then sometimes you'll get that little browning or blacking effect in the areas.
