Also a hot water bottle in your sleeping bag
Nalgene Wide Mouth 1 L
Fill it with boiling water at dinner and it is a hot water bottle all night, then drinking water in the morning — that second job is why we recommend hard plastic over any soft flask above 4,000 m.
Skip it ifNobody on a cold trek. The hot-water-bottle trick alone justifies it.
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The rest of the shelf
Best insulatedWater that has not frozen by the pass
Hydro Flask Wide Mouth Insulated
On a −15 °C morning an insulated bottle is the difference between drinking and chipping ice; the weight penalty is real, and one of these plus a Nalgene is 700 g of bottles.
Skip it ifTreks below 4,000 m, where nothing freezes and it is dead weight.