"Don't confuse heat with flame. Nomex doesn't repel heat, it protects against flame. You need to wear something under the nomex to protect against heat. "

That doesn't mean nomex isn't heat resistant, only that the material is designed to be "Inherently flame-resistant, NOMEX® won't melt, drip, burn or support combustion in the air." It is heat resistant in that it's stable at high temps. http://www.dupont.com/nomex/

OTOH, if you are wearing a flight suit and aviator gloves, intense heat can burn you through the nomex even though the nomex doesn't burn. I always wore cotton (levi jeans and a cotton turtle neck) under my flightsuit. The old cotton thermal u/w formed a good thermal barrier under nomex.