My shoe covers are moderately windproof and, with a pair of thin wool socks (so as not to reduce circulation that thick socks would do), I've been toasty warm down to -16C (4F). As far as I am concerned, wind resistance is vital as an outer layer, then insulation and then blood flow (not in order of importance but from outside to inside). For blood flow, I did not tighten my shoes as much as I would during the summer but kept them somewhat loose. Also, I got a new pair of gloves this year, lobster-style overmitts, that were very tight at the cuff (and therefore, around my wrist) which probably restricted circulation and, despite wool gloves inside, my hands and fingers got very cold. The cuffs have since stretched and are less tight on my wrist and my hands are good now.
The other thing I have found, although it was with hands, is that if there is an opportunity for my arms to get cold, either because of insufficient insulation or a gap between my gloves/mitts and the sleeve of my coat (even if the cuffs go over the sleeve of the coat but so loosely that there is an opportunity for cold air to get in), then my hands get cold. If you have exposed skin around the ankle or poor insulation around the ankle or on the legs, this might also affect you: my tights have a heel strap to keep them down around my ankles.