Moisture in the air can reach a wall cavity two ways: diffusion and air leakage. Diffusion is a process by which water vapor passes through the pores of a wall due to a difference in vapor pressure. Air leakage occurs through cracks, gaps, and holes in the wall. Moisture generally is not a problem in a wall cavity as long as it stays in the vapor state. Problems occur when water vapor reaches its dew point and condenses into liquid. A vapor barrier prevents water vapor from migrating into the wall cavity and causing problems. Critical to a vapor barrier's success, however, is its proper placement in the wall system. VAPOR BARRIER PLACEMENT The traditional guideline for vapor barrier placement is to install it on the side of the wall that is warm in winter. This varies depending on climate. On double-wythe masonry walls with an insulated wall cavity constructed in cold climates, the vapor barrier should be installed on the cavity surface of the inner wythe. In warm, moist climates, install the vapor barrier on the cavity surface of the outer wythe. SELECTING A VAPOR BARRIER In technical terms, a vapor barrier's permeability is specified in grains per square foot per hour per inch of mercury vapor pressure difference. This is more commonly known as "perm rate." Today's energy-efficient construction calls for a perm rate of 0.1 or lower-especially in cold climates. WATCH THOSE HOLES! To be effective, vapor barriers must be tightly sealed and free of holes. Problems frequently occur when a high-quality vapor barrier is slashed to install switch and outlet boxes.