Bicycle helmets are fine in the cold.
Originally Posted by
aggiegrads
...but I've found that a helmet cover and a balaclava/beanie are sufficient and much more versatile.
This.
Wear a skullcap underneath and throw a rain cover over the top if it gets really cold. A scarf or a balaclava are a good idea when the temps dip below the teens.
Oh, and check out the Winter Forum.