Vinyl wallpaper works terrifically as a moisture barrier, so it might seem the perfect choice for these rooms. But if you live in a hot, humid climate and keep an air conditioner running in the summer, this kind of wallpaper on exterior walls puts the moisture barrier on the wrong side of the wall. If you have it, peel back a section. You'll almost certainly find mildew growing on the back. Paper-based wallpaper or properly applied paint won't cause the problem.