Yeah, thats an interesting question. Winter layering can be a little tricky, especially for the base layer. And youre right about Under Armourits great stuff for highly aerobic activities but is aimed at a little different market than backpacking.
These days, my preferred winter base layers are actually a mix of a couple of different things. One is Patagonias Capilene 1 (formerly known as Capilene Silkweight). This stuff really is one of the most versatile base layers aroundsuper-comfortable, fast drying, and with a lot of warmth in a very light package. I even use the tees as summer hiking wear. For winter, use either the long-sleeve tee ($38; patagonia.com) or short-sleeve ($42 due to its graphics) as a base layer.
Then, over that I like wool. For winter, the midweight wool pieces are best, such as Icebreakers Bodyfit 260 Long-sleeve Crew ($58; icebreaker.com). Wool is billed as being a better thermal regulator than synthetics, meaning its comfortable over a wider temp range. Im inclined to agree. It also has good wicking capabilities and will keep you warm even if you get pretty sweaty.
There are lots of ways to mix and match. Wool alone makes a good base layer. Or, start with a short-sleeve Cap 1 shirt and a long-sleeve wool layer over that. Or try a long-sleeve Cap 1 shirt, then toss over that a short-sleeve wool piece such as SmartWools Microweight Tee ($55; smartwool.com).
Ill say this, these days the base layers out there are so good its almost inexcusable to be uncomfortable. The other day I was skiing in mid-20 temps. I had on a light Icebreaker long-sleeve tee, a mid-weight Polartec long-sleeve tee, and Marmots new Exum Jacket made with Gore-Tex Pro Shell ($375; marmot.com). It is an incredibly lightweight setup, and I was perfectly comfortable both laboring down some deep snow up high, and sitting outside eating a sandwich at lunch.
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