Common flame-retardant plastic film materials?
What have you found so far? In a situation like this honestly I would go ahead and research what hot air balloons are commonly made with already. Find somebody more experienced in the hobby and ask them if you can, or even an experienced fire fighter you can get in touch with.

From wikipedia: "For modern sport balloons the envelope is generally made from nylon fabric and the inlet of the balloon (closest to the burner flame) is made from fire resistant material such as Nomex"

The type of measurement you are asking for seems a bit more complicated than something like measuring mass or volume, it seems to be commonly represented with images, videos of tests, graphs, etc. You may feel more at ease by testing how useful materials are before and after flame testing them yourself, that is what I would do anyways. I hope this helps you.