I think it depends on your sweat tolerance, where you pack, how you load your pack, and how you wear your pack.

Two packs that I have used had pretty good back ventilation: Gregory Z and Moonbow Gearskin.

I keep going back to my Gossamer Gear Mariposa, though, because it is so light. The backside on it is pretty flat. I've had pools of water from sweat accumulate on my sleeping pad, which was also the pack's back pad. If one is a menopausal female hiking in Georgia, NC, VA, etc., in July and August, she should expect to sweat. Period. One thing that makes it worse is stuffing a full water tank (lie a Platypus) into the center back water bladder pocket. Yes, that is what the pocket is made for, but it encourages the pack to round out into the hollow of the user's back. What helped was moving the water baldder to a side pocket and loosening the shoulder straps so the pack hung a bit away from my torso.

Using the pack in winter or in New England in fall and spring, my back did not get as sweaty. So, I'm thinking that in drier and cooler areas, another hiker might not be as concerned about how much ventilation a pack allows. As long as the pack carries its load comfortably, other concerns start to fade.

YMMV!

CamperMom