I've been experimenting with different layers under my Roadcrafter suit while riding on cold days. I wear my Widder electric vest over long sleeve thermal underwear and a t-shirt. Over my vest I've been switching between a Polartec 200 jacket and a shetland wool sweater.
The coldest spots on my body are the biceps, where the wind is hitting me the most. I'm still kind of undecided, but I think the wool feels warmer.
Comments or experiences welcomed.