I got a helmet cover when the my head hurt from cold just above my forehead at around 15F. The cover takes away the stinging wind, and creates a nice dead air space, but hasn't gotten too hot. I've left it on up to 50F without sweating too much. I'll wear a skull cap from 45F down to 25F, and a similar balaclava below 25F, but just over the chin and not my face. YMMV, but the cover works for me. Mine is a
Sugoi.