The function of an air barrier is quite specific; to stop air leakage due to differences in air pressure and temperature. It must also be continuous to be fully effective. An air barrier is not just one single product in one area of the building. It is an entire system of products that surrounds the envelope in order to reduce heating and cooling costs, prevent pollutants and debris from collecting in wall cavities, and prevent the ingress of insects and rodents. Even membrane systems installed on roofs are part of that system.