The envelope is the fabric covering that holds heated air in a hot-air balloon.
A balloon’s envelope is the large, air-filled body suspended above the basket. Modern envelopes are usually made from lightweight, heat-resistant nylon or polyester fabric, with stronger webbing and load tapes distributing the forces created during inflation and flight.
The envelope is open at its lower mouth, where the burner directs heated air upward. A parachute valve at the top allows the pilot to release hot air, while a skirt around the lower opening helps protect fabric from burner heat.
The envelope should not be confused with the basket or the rigging. The basket carries people and equipment, and the rigging connects the basket to the envelope. Together, these components form the complete balloon aircraft.