The only difference between the mid zeros and subzeros is the thickness of the fleece inside. There is no special wind blocking material; it's the same as the mid zeros. For me, realistically, the Firewalls would be overkill, because I don't ride when it's super windy, anyway, and I have the Ibex tights that can serve the same purpose. I wear those x country skiing and snow shoeing when it's -10 F, up to about 32. Of course, wind on your legs, when you are in the woods is less of an issue in those sports!