These are two things that sometimes can do the same job. How to design and install them depends a great deal on the climate and what method of construction is chosen. No matter where you are building, water vapor condensation is a major threat to the structure of a house. In cold climates, pressure differences can drive warm, moist indoor air into exterior walls and attics. It condenses as it cools. The same can be said for very Southern climates, just in reverse. As the humid outdoor air enters the walls to find cooler wall cavities it condenses into liquid water. This is the main reason why some of the old buildings in the South that have been retrofitted with air conditioners now have mold and rotten wood problems.

Regardless of your climate, it is important to minimize water vapor migration by using a carefully designed thermal envelope and sound construction practices. Any water vapor that does manage to get into the walls or attics must be allowed to get out again. Some construction methods and climates lend themselves to allowing the vapor to flow towards the outdoors. Others are better suited to letting it flow towards the interior so that the house ventilation system (see below) can deal with it.

The Airtight Drywall Approach and the Simple CS system are other methods to control air and water vapor movement in a residential building. These systems rely on the nearly airtight installation of sheet materials such as drywall or gypsum board on the interior as the main barrier, and carefully sealed foamboard and/or plywood on the exterior.