It takes about 5 miles before I start overheating to the point where I need to take off my helmet and sit on the side of the road for a while. Once I get over that, I usually don't have problems with my head overheating. All my helmets are well ventilated. But head is getting too hot overheating is something I get while hiking in the heat as well.

Wearing wool or cotton instead of wicking on the really hot days helps meto avoid being dehydrated.

I almost always wear my helmet, except for the couple times i forget and have gone a few miles before remembering and go back and get it - but I probably cycled for about 20 years without wearing a helmet, I didn't start till I picked up cycling again a few years ago. I've had a few crashes, but I've never actually hit my head.

I just wear it all the time to get in the habit of it, just like I wear my pfd to paddle in 2 feet of water despite knowing that I'm not gonna drown even though it's 100 degrees out - but if someone with me doesn't want to wear it, I don't tell them to, 'cause it's 2 feet of water and it's their body/risk. Same with helmet wearing, if someone's willing to risk it - their choice. I'm sure most of us grew up riding a bike without helmets as kids.