I am good down to about minus 25 Celsius below zero, it doesn't get any colder than that around here....Platform pedals + proper winter boots or insulated hiking boots + 1-2 pairs of wool socks + gaiters has never failed to keep my feet warm during longer rides in very cold weather. No need for chemical warmers, no need for shoe covers, no need for wrapping your feet in plastic bags or aluminium foil.