Well, you have to keep in mind were talking about hydration packs that are insulated, not heated. So if its cold enough, and you are out in the cold long enough, anything is going to freeze.
One suggestion is the Camelbak Menace ($80; camelback.com), which is a pack-sized hydration pack, meaning it can carry a bladder AND extra clothes, snacks, etc. It has about 1200 cubic inches of space in fact, which is pretty generous. And it has straps and buckles to accommodate skis or boards. So its a versatile pack. It even has a pocket for your iPod!
To keep your liquids liquid, its insulated, and has an insulated tube slot as well. So it will do about as well as any out there at keeping things in an un-frozen state.
Alternatively, you can use any hydration pack and then take extra steps to insulate the bladder and supply line. The Platypus 1 Holster ($17; platypushydration.com) comes with a 34 ounce bottle and an insulated cover. Add to it the Platypus Bite Valve and Drink Tube Insulated Kit ($16), and youve got a pretty good solution to cold temperatures.
And did I say not heated? In extreme conditions, it would make sense to me to add a Jon-E Handwarmer ($25 for two; cabelas.com) to your pack. Stick it in next to the bladder, and youre in business!
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