This current cold spell has been proving just what a sound purchase my Brynje Arctic Double Zip Polo 3/4-Neck base layers have been. Riding home from work today, it was 20 miles into a solid 25mph headwind at -5.5 degrees C. On top of that, I was on the electric commuter, meaning I wasn't working as hard as I would have been on my non-assisted commuter - and when I work less hard, I usually feel a fair bit colder as I'm generating less body heat. Nevertheless, with this base layer on, I was still perfectly comfortable with just two layers on the top half - base layer and windproof thermal jacket. So much so that I reckon -7 to -8 may even be doable with just the two layers, perhaps even -10 if I'm working harder on a non-e-bike. Of course it helps that the rest of my kit is also all optimally chosen for the conditions, but the search for the ultimate warmest base layer has been a resounding success. Thoroughly recommend them.