The envelope contains the lighter-than-air helium which gives the airship its buoyancy. Inside historical Zeppelins, the gas cells were separate entities from the hull; however, on the Zeppelin NT, the envelope serves both as the aircraft's hull and as the gas cell. It is made of a three-layered laminate: one gas-tight layer of Tedlar (PVF), one polyester fabric layer to provide stability and one polyurethane layer suitable for plastic welding that acts to connect the separate laminate panels. To preserve its outer form, a slight overpressure of about is maintained within the hull. As in blimps, the interior pressure is kept constant at all flight altitudes by using ballonets. The ballonets have a total volume of .