I call this thermal underwear outfit my "Space Suit". Man is it warm!  I was stationed in Alaska several times when I was in the Air Force and was issued old fashioned 100% cotton thermal underwear. I thought those could not be improved upon. And maybe they can't actually but these are certainly at least as good!

Two minor things to note:

1) They are a bit bulky. You'll probably need to wear pants one size bigger to be comfortable in them.

2)  While the old 100% cotton ones can be thrown in the washer with little further "management" these things can only be washed in cold or luke warm water and NO BLEACH.  Also, the tag says not to use very much heat while drying which means they take longer and you run a slight risk of  shrinking. Odd for something made from totally synthetic material.