Two layers of a high-performance plastic film called ethylene tetrafluoroethylene, or ETFE, fill in the spaces between structural members and are inflated from within to create pillows with a rounded surface. Both the exterior and interior of the 12-foot-thick walls and 22-foot-thick roof have ETFE skins – the hollow space in between is used for ventilation. This design allows the structure to act like an enormous greenhouse. When the building needs heat, air is circulated from the warm walls into the interior spaces; when the building needs to be cooled, air is pulled up from ground level and expelled through the roof.