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So, I used to believe that two sets of baselayers was ideal: One winter "expedition weight" set, and one mid-weight set for the rest of the time.
I have since discovered that all I want my baselayer to do is effectively wick moisture, even at the expense of insulation. My logic is that, for a baselayer to work at its best, it should be dry; the faster it dries after getting wet, the better it will be able to continue doing its thing; thinner baselayers dry faster than thicker ones; hence, the thinnest baselayer should wick the "best."
Insulation is best left to one's insulating layers, methinks. A thick baselayer, like a Patagonia R1 suit (a heavy version of Polartec's "PowerDry" fabric), while warmer than a mid-weight set like Patagonia's Capilene 2s, will not dry nearly as fast. This could leave one, after a long day of backpacking, with a semi-wet set of baselayers no matter how conscious one is of their own thermo-regulation. So then, you're getting into your bag in damp baselayers, which is really never a good thing.
I'm already sold on the concept, as the uber-light baselayer thing has worked extremely well for me ever since I started implementing it. My Smartwool NTS set weighs all of 8 ounces total. They dry in a flash, which keeps them doing their job: wicking sweat away from my skin. Their feather-light weight means that I can often bring along a more substantial midlayer, which means me being actually warmer, for the same weight.
Anyone else subscribe to this line of thought? What do you do for baselayers? I live in a fairly temperate area, so I imagine what works for me might not work for someone in Alaska or Scotland.