As a member of the cold climate fraternity I have A LOT of first hand experience with full face helmets and fogging.
I'll start with the spray on chemicals out there first. While some may work for awhile I haven't found one yet to not eventually fog up. I tried the stuff you see at the events Zooks? and I proved to the people at the event it DOES fog up.
Next I tried the pin-lock system from Shoie. While it works to a degree it eventually fogs up also. Now the pin-lock system didn't fog, it was in between the two shields that fogged up. I also tried dedicated anti-fog shields. They simply took a minute or two longer before they fogged up.
Now since I also ride ATVs all winter in the snow the fogging really gets to me. One of my riding buddies purchased a Scorpion helmet. My first thought is, what is a Scorpion helmet? It is just a brand name that claims their helmets don't fog, yeah right. After a fall evening of riding my Shoie kept fogging up, his Scorpion didn't . Can't be. Winter rolled around and we were running the roads on the ATVs in one of the snow storms and sure as can be my Shoie not only fogged up it iced up. Not his Scorpion, dam. OK let me try a Scorpion... I picked one up while in Daytona so I could try it on and have it fit for my big head. I am sold, I have yet to fog it up in either the snow, cold or rain. I have a hard time accepting a $250 helmet out performs my $600 Shoie however it does. Since my buds initial purchase of his Scorpion there are now at least a dozen of them running around in town strictly for the anti fog feature. Wanna try one? I'll send you mine to try (No cooties please

).