ORIGINAL: nosualc

The problem with rubber boots is that they are impervious to water. While not letting water in is good, but not letting water vapor out is bad. All that water vapor from your sweaty feet will end up in your socks and the liners. That makes your feet cold.

With rubber boots, there's nothing you can do about it except avoid sweating, and have liners and socks that stay warm when damp. Things like wool, fleece (and other synthetics) still retain heat when wet. Things like cotton actually wick heat away from you when wet (which is why cotton shirts are great in the summer).

-nosualc

ps - As MeanV2 sez, Smart Wool is a great choice.