Did a 35 mi ride with my Osprey Radial 26 pack today. Brought it for a change of clothes because of iffy weather. Didn't really notice it. It has a mesh back with and air space that allows air to flow between the back and pack and it works.

I highly recommend this pack. If you do this a lot I would look for a bike-specific pack like this one. It has lots of great features that makes it a pleasure to use. like a "kick stand" so you can sit the pack upright - it basically is a stiff underside that parks the pack upright and is very stable, so you can get things in an out with ease. A pull out rain cobver, soft poacket for phone, a bottle holder, helmet holder etc, etc.