Put it outside of, but next to your sleeping bag and you should be fine. If it does freeze, hike with it next to your body and in 15min it will be ready. If you are hiking in subfreezing wx, make sure it is next to your back in your pack.

All I do is throw a small hand warmer into the storage bag before the temperatures drop at night and so far I always wake up with an unfrozen filter. This has worked down to 10F and I'm sure a second warmer would work if it got even colder.

fine... just dont leave it wet if the temps below 33 degrees... it as with any, will ofcourse freeze and become semi useless... dry and maintain your equipment and itll be just fine... Honestly I have the old PUR Voyager filter and everything Ive seen about this one so far is exactly the same... even use the Katadyn Hiker Pro replacement filters for mine because PUR I guess sold their patent on it or something.... Anyways, its always worked from the deserts outside of phoenix to the Apache White Mountains in the Northern Arizona areas... This thing has never let me down, as long as you take your time follow the instructions and worry more about doing it right than doing it fast, you'll be "OOOOO-TAY!" and withtout any stomache problems at all.... Great Product... Have fun up there you lucky!