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