Instead of using a liner, I use "sacks" for every thing. In my experience, I find that most water proof bags are water proof but after compressed a little, They do tend to swell a little by filing back up with air after being compressed. So how I manage this is to pack my backpack a "little" loosely. Then after a few minutes the air starts to fill the water proof bags and in the end the backpack usually ends up packed pretty tightly. It lets every thing stay right where you put it and you never have any movement back and forth as you walk and it makes for a pretty solid packed backpack. I can remember years ago (back in the seventies) when I used an external frame pack, I used to hate after getting the pack packed, every thing seemed to want to move around inside the pack. Not any more with the philosophy that I now use. Seems to work great and the best thing about it is that it works every time...sabre11004...