I use a wool Buff to keep warm in the winter, which isn't very bulky under the helmet and generally can be worn so that it won't obstruct vision as well. Plus, bonus face protection when you wear it as a balaclava. I use the lightweight one linked above because it was what they had two years ago, but they offer a midweight one now that I'll need to pick up; then I'd probably be comfortable (at least around the head) into the low 20s.

For waterproof/resistant head coverage, I got nothing. I don't have hair so I usually just roll with it.