I dont feel at any time that it is heating my back for rides longer than 3hours, upto temp: 35deg C in normal humidity.
Camelbak setup seems to works for me.
I have a 6 or 8 years old Rogue that is quite narrow in shape which is good. But by default, it came with a 2L or 2,5L hydration reservoir which I found to be quite bulky, uncomfortable and bothersome to my shoulders/chest even when half filled.
So I use 1L reservoir which keeps the narrowest shape on the back and there is extra space to keep spare tube, Pump etc.. I freeze 1L Water overnight and keep nut/Chocolate bars, phone, key, cash in the bag pockets. Bars are not melted as side effect due to ice cold water. I don't even feel that the bag is there. When camelbak shoulder straps feel just like bib suspenders on shoulder, that's when I know the setup is optimal.
I also keep one 700ML water bottle on bike with isotonic tablet which lasts for around 50-55km. Then I refill the bottle.
Last Sunday did 86km loop full road ride around a lake. 3:05Hrs, ~570meters elevation, average 28km/hour. And ready access to ice cold water and unmelted bars in camelbak was so convenient.
And it was a lot easier to setup than writing it down here
