The purpose of a vapor barrier in an exterior wall is to prevent what?

The correct answer highlights the primary function of a vapor barrier, which is to prevent humidity buildup within an exterior wall. Vapor barriers are designed to control moisture migration through building materials, which can occur when warm, moist air comes into contact with colder surfaces. This is especially important in preventing condensation from occurring within the wall cavity, where it can lead to mold growth, wood rot, and damage to insulation. By stopping moisture from permeating the walls, vapor barriers help to maintain the integrity of the building structure and ensure a healthier indoor environment.

While preventing heat loss, controlling sound transmission, and reducing air leaks are important aspects of building performance, they are not the primary role of a vapor barrier. Heat loss is typically managed by insulation, sound transmission by acoustic treatments, and air leaks by properly sealing the building envelope. Therefore, while these factors are related to overall energy efficiency and comfort, they do not directly pertain to the moisture-control purpose of a vapor barrier.