Stucco is very porous and soaks up water by design. The barrier paper your house is wrapped in is the protective shield that stops the moisture from entering the house. By design, water penetrates the stucco, comes in contact with the barrier paper and is directed to the bottom of the wall and released to the outside throught a special type of flashing that allows the moisture to drain. Walls that are in direct contact with a lot of sun, rain and wind can end up with the barrier paper becoming brittle and deteriorating which then allows water to penetrate causing insulation to get wet and water to get on the drywall. This is also a major contributer to the development of mold.