I've been using Hanes Cool Dri long sleeved undershirts lately as part of my cold weather layering. They are about the same weight as silk but are 100% polyester and have given me a good insulation layer (temps in the 30's) without the bulk of longjohns. If the weather warms up, they also have the advantage of not overheating you, I also used them this past summer as a sun block for my arms while wearing short sleeved shirts. No affiliation with Hanes and they are ridiculously cheap on Amazon. No affiliation with Amazon either. You could say I like them.
