The Nomex will not catch fire and burn as we would expect of some fabrics. But it gets very hot and has poor heat transfer characteristics - it does not "insulate".
Go look up Brian Shul on Google, he was an old student of mine and fellow A-7D flyer in the same squadron back in the 70's. He survived a T-28D crash and escaped the burning plane easily. Sitting against a nearby tree he became extremely hot within seconds even tho the nomex flying suit was not on fire. It was very hot from the fire but had no insulation properties. So he suffered severe burns over a significnt part of his body and about a third of his face. I played soccer with the man and seeing the damage when in the locker room was a shock. There were lines where his shorts and socks were. The backs of his hands were burned, but the palms were O.K. because the gloves only had nomex on the backs. So nomex may not burn, but your best bet is wool or a coupla layers of cotton and not rayon or any nylon.