virtually anything can be used between your base layer and your shell as insulation and since there are hundreds if not thousands of products in different weights and different materials, the possibilities are endless. try a heavy fleece sweater or down vest for super cold days. if it's still too cold, wear both and shed one when the skies open up. on warm days, just wear your shell and your base layer. for in-between days, rock a light fleece hoody under your shell. and anything an insulated jacket provides can be acheived with a shell and an insulated layer and, in fact, your shell gives you the flexibility to change it up as conditons change without having to waste money on different jackets with different insulation. my opinion though.
when you say thermal underwear, you mean a proper base layer? meaning it should be thin, snug-fitting, moisture-wicking material (brands are generally irrelevant they all do the same thing).
Last edited by hanzosteel; 01-20-2010 at 12:11 PM.