I made a change recently in my footwear and it’s really made an improvement in keeping my feet warm.

To begin, I wear a very thin synthetic sock as the base layer.

Next is a Patagonia Insulator sock, which is made from 2mm neoprene with sealed seams.  These socks are made for fly fishing, but they work well for cycling because they are waterproof (a vapor barrier) and fit relatively snug.  Best of all they can be rinsed clean rather than washed.  Hopefully this translates into a very long life.

The outer sock is a thick synthetic Patagonia sock, though I don’t recall the name.  Most anything would work here, including wool, so long as it can stretch over the neoprene.

Of course I can’t wear my normal shoe size with this much insulation on my feet, so I wear size 13.5 Lake winter cycling shoes rather than my normal 9.5s.

I could also throw in a chemical heater outside of the neoprene, but it hasn’t been necessary.  I recently rode 15 miles in -20F windchill and my toes only felt a slight but sustainable level chilliness.

I had been using SealSkinz socks, which work well.  The problem is they have a soft lining material that absorbs odor and requires washing.  After using them for a year or so, the socks are no longer waterproof like they once were.

Link: More information on keeping your feet warm while biking