All I can say is that I am inevitably much sweatier with a helmet than without. It's one reason I stopped wearing one... without it, I'm generally able to cycle to work in my work clothes, which is a lot less of a PITA.

If I'm rec cycling and concerned about heat, a wet bandana around the neck (carotid arteries) does far more than anything else. I believe it's a common survival tip. I learned it from Fresno roofers.