I use a giro ski helmet with a vent system lever on the top that you can open or close. (cant remember the model) I bought it for winter riding but now I ski in it too. It's warm for riding, so the vents are always open, and it really has to be below 20F to not get too warm. I don't wear anything under it unless it's below 0F then a thin lycra balaclava. I like to wear ski goggles (Oakley A frames) when it's below 20 otherwise my eyes water way too much and freeze. Goggles really fit and work well with the ski helmet. A rode with a Giro e2 in the winter for a few years and that worked fine until I crashed on some ice and it cracked ( i was OK) That's when I decided to find a ski helmet, (off season, on sale, on line) for winter riding.

Conclusion: Ski helmets work pretty darn well if you ride in really cold temps, and if you ski it has a second use. Certainly not mandatory equipment, but based on my experience I would recommend a vented one as a good winter riding option if it's really cold where you live.