Abstract: An evaporative cooler unit is disclosed for installation as an "add-on" unit to a pre-existing roof-mounted refrigeration air-conditioning unit. The conventional air supply and return ducts from and to the air-conditioning unit are removed and are interconnected with ducts in the evaporative cooler conversion unit. Pressure actuated dampers are provided which automatically cause air to be supplied through the supply duct into the building from whichever one of the units, the refrigeration unit or the evaporative cooler unit, is operated. Similarly, return air always flows from the building into the return duct. When the refrigeration unit is operating, the return air recirculates in the normal manner. When the evaporative cooler unit is operating, the return air is vented outside into the area surrounding the cooler housing.