I have fought this problem for 40 years, race cars, motorcycles, googles etc. The only cure, that always works is the pin-lok inserts. I have had the shield be fogged so bad around the insert that water was dripping but the vision thru the insert was clear and dry.
Same principle applies to googles, the good snow/winter googles have twin lens that are sealed around the edge with a silicone bead which creates a air chamber between the two lens. This prevents either of the lens from fogging. Pin-lok same deal as the silicone is attached to the insert and it seals to the face shield.
Good product. Can also be removed at any time if you need to.