I have a pair of older Lake MXZ __? Can't remember model number, that I wear down to around 15 F. Below that it is platform pedals and BIG hiking boots. My hiking boots are probably a couple sizes larger than normal. I wear two pairs of thick wool socks w/ the hiking boots and still have lots of wiggle room. The absolute key to warm feet is not constricting them in extreme cold. That is why cycling specific shoes fail in low temps, they are cut to tight.