Insufficient make-up air can lead to increased building infiltration and higher energy costs, not to mention unhappy building owners and uncomfortable occupants. While there are no facts and figures to show how many buildings provide the correct amount of makeup air, it can be stated that there are probably a great many that do not have sufficient makeup air. And there's no one solution that is available for every building. Each building must be analyzed with a careful air balance calculation and evaluated based on the diversity of sources of exhaust air and infiltration. Natural ventilation depends on winds outside and convection currents inside a building. Winds raise air pressure slightly on the windward side of a building and lower it slightly on the lee side. The pressure difference promotes circulation into the building on the windward side and out of it on the lee side. Convection currents are caused by the sinking of colder and therefore heavier air, which displaces the warmer air. A building may have a roof ventilator to allow the rising warm air to escape. If there is an opening to the outside at the bottom of the building, fresh, cool air will be drawn in.