Placing Vapor Barriers

Insulation has been around for centuries, but it’s gotten more sophisticated with the energy-efficient homes of the past 50 years. We quantify R-values and use vapor barriers to prevent moisture migration. The purpose of insulation is to retard the movement of air, an excellent insulator, within a space. The purpose of a vapor barrier—whether it’s foil, paper, or plastic sheeting—is to prevent water vapor from moving into a colder space and condensing. In cold northern climates the barrier is most often positioned on the interior face of the insulation. In warm climates the vapor barrier is positioned toward the exterior side. In the moderate middle … it gets complicated. Here are rules for attic and basement insulation.