Walls typically represent the largest exterior surface area of a home. As a result, from an energy stand point, the wall system is a major component of the building envelope, particularly in extreme cold climates. When it comes to improving thermal performance, there are many ways to construct a wall, and the details of sealing, sheathing and insulating are even more numerous.

Besides the considerations of how heat travels through the walls, how moisture travels through the walls is also very important. The inside air has typically lot more moisture than the outside air, due to breathing, cooking, showering, etc. If we allow this humid indoor air to travel through the wall towards the colder outside, it can cool down and condensation inside the wall can occur. Such condensation can lead to mold, rotting material, and other problems. One common strategy to prevent these problems is the use of a vapor barrier, typically plastic sheeting. The vapor barrier is often placed right under the drywall, which means very close to the warm inside surface of the wall. That way the water vapor is prevented from getting into the colder parts of the wall, where it could condense and cause issues.