My regular helmet fits great during the summer but is near its largest adjustment. Anything more than a thin skull cap won't fit underneath. Also the air vents that are so nice at 90 F are freezing my forehead and top of my head now that we are hovering around the freezing mark. I plan to ride even colder as long as the roads are decent so I need to come up with something better.

Does anyone here use a "winter" helmet a size larger to allow for a knit hat underneath? Would the extra padding decrease the effectiveness of the helmet? Are there winter specific helmets with fewer vents? I've seen some helmet covers for wet weather that must be some kind of Gortex or similar material. Would something like this keep your head warmer by decreasing airflow through the helmet?