I have to agree with Cedar on this one... When I was in the Canadian Infantry, I would use any excuse I could to put on my Mukluks (Sorels). If the ground is at all frozen, it is below 32 and you are not moving around a lot, they are the best way to go. No traditional insulated boot ever worked well enough at keeping my feet warm in those temperatures (though my wife does call me a wuss sometimes).
I have polled sleds, gone on long ruck marches with them and snowshoed with them. Are they a bit loose? Yes, but that is the compromise. I'll take warmth any day of the week unless it was a short duration outside or specific work was being done and you needed a more solid boot. Even when I go to the office, I keep the Sorels in the trunk or even wear them, and just have my dress shoes in my bag.
I also own Zamberlan boots (an Italian brand of hiking/mountaineering boots) and would highly recommend them for comfort and warm in cold/extreme cold conditions.
I also have a pair (maybe a different model?), but even still, if below 32 and I stop moving, they get cold let alone below 20 degrees like you are talking about.