I think "base layer clothing" is an advertiser's catch phrase which implies that if you get that right all your cold weather problems will be solved. All the layers are important and the most effective insulant is free, it is the thin layer of air which is trapped between the clothing layers. In the very cold weather we had in UK before Christmas last year I wore (while riding a bare bike) a cotton tee shirt, a long sleeved pure wool under vest, a wool/cotton mixture shirt, a thin woolen pullover, a thick woolen pullover and a lined wind-proof bike jacket — I was as warm as toast in air temperatures about zero C on a hundred mile round trip. You can lose a lot of heat from the head, I wore a thin silk balaclava under my full-face helmet. Under my wind-proof bike trousers I wore normal cotton underwear and long pure wool underwear, with my feet in thick wool mixture socks inside leather motorcycle boots. My hands felt a bit chilly in normal motorcycle gloves, but I have now bought some thin silk under-gloves and I have heated grips on my Burgman. Winter? — bring it on! But you shouldn't have so much on that you sweat.

Google icebreaker merino wool one of the best you can buy,when you get one you wont use anything else.
Chris

Stay away from polyester. Use cotton or silk or wool. I keep a cotton pull over sweat shirt under the seat.