Well I have seen bald pro-cyclists with tan circles on their foreheads from where the sun gets through their helmet vents. My hair covers enough of my head that I don't have a similar problem.

There are a few companies that sell summer helmet beanies made of thin wicking fabric. You could try something like that. TE carries them. They're low on inventory right now as they haven't moved to the new warehouse yet, but I would expect them to get new stock in soon for the spring and summer. Another brand that TE does not carry is Head Sweats.