The role that exterior walls play are more than one. Not only are they the foundation for the structure but they as well keep the interior from the exterior. They also help the interior by providing protection from things such as weather. Each layer to the wall plays an important role to the function and efficiency of a building. For example, the water/air barrier restricts the flow of water vapor into the insulation to keep it from forming into liquid. The rigid insulation is used to insulate the walls and keep the interior warm in colder conditions.