I was riding all day on Sunday in temperatures that peaked a few degrees above freezing, but dry apart from the odd snow or hail shower.
It was cold enough to need layers. As I tried not to hang around outdoors, my top half was fine with just a short-sleeve jersey under bib tights and a long-sleeve jersey over, plus a buff round my neck, double gloves and home made wrist/forearm warmers. My legs and feet were fine too - 2 thin pairs of socks and pedalling effort. I didn't even need my shoe covers on.
But in between was an issue. I didn't want anything under the good chamois of the bib tights, I didn't want two layers of gel padding, but I wanted more insulation than just the bib tights. So I went for foam-padded shorts (the foam compresses quite quickly) over the tights. From an insulation point of view they were ideal - when they were in place, which was rarely. Every time I got going they caught on the saddle, and even just pedalling along seated they crept down. I think the problem is partly that cheap shorts don't grip, and expensive ones are normally gel. Loose-fitting stuff tends to get caught up and form lumps that rub, and it's nice to avoid the drag.
What are some tried and tested ways of layering for all-day rides in these temperatures? We've got a few questions on layers for cold commuting, but 12 hours door-to-door/200km is a bit different, and I expect the solutions will be, partly because of the need for something that works against the saddle. I regard minimising sweating as important; even decent wicking clothing gets cold fast if you're in the wind and damp with sweat.