I ride my fat bike in winter both on single track trails and to and from work almost every day. I also happen to live in Canada's Yukon. This combination means that I ride in some pretty cold weather on a regular basis.
By no means am I a hardcore winter ultra endurance athlete like Derek, Neil or Christopher, all of whom have competed in 300+ km fat bike races. No, I'm a little more wimpy than them to say the least. My rides are much (much) shorter.
In order to keep my feet warm for the average ride—my commute to and from the office is about 10 km each way, and most of my bike rides are about 2-3 hours in length—I've had to go through some experimentation to find a solid footwear system.
What I've landed on, from outside to inside is:
These are all-around very skookum boots with an upper made of neoprene. The soles are stiff and have reliable traction both on ice and on steep, snowy slopes. What's key though to wearing boots made of neoprene is that you need to keep them dry in order for them to stay warm, which is where my vapour-barrier socks come in.
The sensation of wearing vapour-barrier socks is something that you have to experience to understand. You need to get used to the feeling of having your feet be wet all of the time and at the end of the ride, your socks will be damp but your boots will be dry. The concept is weird, but it works.
Inside of vapour-barrier socks go a pair of thin wool socks. I have a couple pairs of Surly socks that I rotate through.
These insoles use non-compressible aerogel insulation that's laminated inside EVA foam. They're quite thin but are shockingly effective and in warmer weather, they work so well I find it's almost like wearing a mini-oven underneath both of my feet. The technology of the Jaztronaut's makes a difference; it's not just marketing.
I have a number of thin snowboard socks, such as the Burton Ultralight wool sock that I use for both snowboarding and fat biking. These are durable, warm, long and breathable, going over top of the vapour-barrier socks.
I like thinner socks, but I believe a thicker pair of wool socks would work too.
Pants over top of boots
This last part is really important: Pant legs zip up and go over top of my boots. This traps any warm air that escapes inside and keeps the cold air outside. Small detail, but I've found it makes a huge difference.
This is not a minimal, feathery set-up but I'm not racing and so I don't really care much about weight. I'm much more concerned that all of my toes will be intact when I arrive home and towards that end, this system performs great.