i've tested WAY too many bags for commuting. I recommend the ortlieb velocity to you seeing how ventilation is a concern. it has a series of foam rectangles running down a flexible plastic back lifting the bag from your back keeping you from that big sweaty I-wore-a-backpack square on your back. it's a rolltop so it's waterproof(i live in Oregon, waterproof is always an issue) and can hold quite a load. REI does in fact have this bag if that's where you're looking.