statjunk- i find that if it is bitter cold, 15 degrees and belowand you plan on sitting all day, i would use the polartec heavyweight thermals. They come in two-weights a heavy and a medium(i have the heavy)I find if anything 15 and above the underarmour is thin( it fits tight on your body, but that is how it works)and light enought to add additional layers that you might not normally use and not feel bulky and your nice and toasty. Hope this helps...I know its a tough decision when the underarmour does not come cheap.