If you are still able to find one of these shirts…
Fabric: Midweight capilene variable knit.
If you are still able to find one of these shirts (they were discontinued in 2006, I believe), I *highly* recommend buying it. It's the finest cold-weather workout shirt I've owned. It's not terrifically warm by itself, but it does provide some limited insulation--its stellar qualities are the fact that it provides some limited wind resistance combined with excellent moisture management.
With a normal midweight Capilene top under a vest, I'm very conscious of the wind on my arms when I'm cross-country skiing in sub-20 degree weather. With the variable-knit top, I hardly notice it (I skied for an hour on a 5 degree day just this morning, for instance). If it's in the high 20s, I can ski with just this and feel fine.
Your mileage may vary--I tend to be pretty warm-blooded, so I'm usually more worried about overheating than being cold; if you're like me, this might be a very good choice. (If you need to layer up massively to endure the cold even when you're highly active, then you'll probably be more comfortable with a normal wicking top and some kind of light shell.)
One final note: I wear a medium in other Patagonia Capilene crew-tops, so I bought a medium. I was surprised to find that it fit a lot differently--it's much more form-fitting, which was no surprise, but the arms are enormously longer, so much so that I have to roll up the cuffs. The fact that I wear this shirt constantly despite that irritation should provide some sense of the regard in which I hold it.