A polytunnel (also known as a tunnel greenhouse , hoop greenhouse or hoophouse, or high tunnel) is a tunnel made of polyethylene,usually semi-circular, square or elongated in shape. The interior heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building faster than heat can escape the structure. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. Temperature, humidity and ventilation can be controlled by equipment fixed in the polytunnel. Polytunnels greenhouses are mainly used in temperate regions in similar ways to glass greenhouses and cloches (row covers).