As stated, it is important to wear something under a nomex suit to create a thermal barrier between your skin and the nomex. I wear Icebreaker under-clothes under my flight suit. They are made from natual merino wool which keeps you warm in winter, and they are quite cool in summer.
They have the added benefit of going days without washing, and not starting to stink! There are different layers available and I use several when the temp really drops down in winter.
www.icebreaker.com