In my experience it depends on the weather. I've only wore something under my helmet one time(I was riding in a storm). But that's because I have a pretty basic helmet (Giro Encore) and the vents don't close or anything. So that was more to keep my head dry. For the most part it keeps your head warm because it's trapping the heat in. Especially if you have a helmet where you can close the vents.
But to answer your question, helmets are solely made for protection, but one of the side effects is keeping your head warm.
Last edited by Frankyis4; 04-28-2012 at 02:03 PM.
Reason: Forgot something