Sorel makes some very good boots, or at least they did. The pair I have has the removable inside boot and is when they were still made in Canada. Realize that the temperature rating is for a person who is walking. A -40 boot will not keep your feet warm at -40 when you are stationary. I use electric wool socks, and they work great. Also have electric gloves. They come in handy with breezy and very cold temps while 20 feet in the air.