The metal frame must be the key. I have an Osprey backpack without it, and the mesh alone does almost nothing to help with sweat, even though it was sold to me

The metal frame must be the key. I have an Osprey backpack without it, and the mesh alone does almost nothing to help with sweat, even though it was sold to me as being better. A friend in high school found an old Vietnam-era? US army rucksack with a metal frame that was less comfortable in the short term but not bad in the long, sweaty term.