This past Monday (12/30/13) I did my 14 mile commute at about 15°F and tried a new set of foot coverings that IMO that kept my feet significantly warmer than usual. In the past I had bought a pair of neon green shoe covers made by Gore-Tex, for wet riding. During the winter, I use platform pedals with toeclips, and my usual footwear is thin and thick socks, running shoes and Totes rubber overshoes. I use plastic bags over my running shoes to put on the Totes more easily (see the sequence below).

So with the additional Gore-Tex shoe coverings at 15ºF, I did not perceive cold until about mile 10, and I did not feel cold in the sense of permeating the soft tissues of my foot until about mile 12, but it was tolerable. At about mile 12 I have a downhill run of several hundred yards that irreversibly drains the heat from my extremities. The next day at 21ºF, I rode without the Goretex, and started feeling cold at about mile 9 and finished significantly colder at my mile 14 destination than the day before.