What is the bubble-like bag of a hot air balloon called?
One can hardly believe it but this part of the balloon is known as the envelope. The structure of it is a bit more complex than that one of an envelope you imagined as it has more sections.
The bottom (skirt) of such a compound "envelope" is made of another material. This material is flame resistant and holeproof.
Finally, the top of it is made of fabric with silicone coating.
More Info:
www.eballoon.org