The material we have used for our test envelopes, and also for our first full-size balloon, is a type of fabric produced by Gelvenor SA of South Africa and principally used for the upper side of paragliders, due to its imperviousness. This material weighs about 50 gm/m² and consists of conventional ripstop nylon with both sides knife-over-air coated with a mixture of polyurethane and silicone. It is very slippery and repels water fiercely. You can see their data sheet here.