What are NASCAR Drivers Suits Made of?

Personal Preference is the key to what the drivers wear under their suits. Some where underwear and some where shorts and t-shirt. There are some drivers who even wear Under Armour,

What a Nascar driver wears under their fire suits is very simple it is only his underwear. You figure they are wearing a jumpsuit driving a car and mostly in the warm/hot weather

The drivers vary in what they wear under their uniforms. Most of them wear just a t-shirt and their underwear. Most just think this is not a question they should be asked.