Insulating water bottles: against freezing and as a hot water bottle

Heat always moves from warmer to colder, in three ways: by conduction (for example when the bottle stands on cold snow), by convection (when cold air or wind flows past it) and by radiation. A cover of wool, fleece or foam contains many small, trapped pockets of air. Still air conducts heat poorly, and that is why the cover slows all three routes.

There are two reasons why ice forms at the top first. The lid and the bottle neck are usually the most exposed to the cold air and often have no cover. Also, water is densest at about 4 °C; colder water rises to the top, and that is where it starts to freeze – which is also why lakes freeze from the top.

If you turn the bottle upside down, the water surface where the ice forms is at the bottom of the bottle and the thread stays clear for longer. How much time this gains depends on temperature and wind. But insulation only ever delays freezing – in severe frost over a long time, water will freeze anyway.

Put the bottle into a thick sock, a hat, a piece of old sleeping pad or foam – and do not forget the lid, it should be covered too. Do not carry it in the outside side pocket of the backpack, where wind and cold hit it directly, but inside the pack close to your back, or a small bottle in an inside pocket of your jacket.

Before packing the bottle upside down, check that it does not leak: lid tightly closed, turn it over briefly and see whether anything runs out. Otherwise a leaking bottle soaks the contents of the pack, and wet gear is a real problem in the cold.

Where possible, start out with warm but not boiling water. Warm water takes longer to cool down and freeze. An insulated (vacuum) bottle keeps things warm far better than any improvised cover; a single-wall steel bottle can take hot water and is therefore also suitable as a hot water bottle.

At night you can fill a suitable, leak-proof bottle with warm water, wrap it and place it by your torso or thighs in the sleeping bag. It warms you as you fall asleep, and in the morning you have liquid drinking water instead of a block of ice.

This also works at home if the heating fails: a wrapped warm bottle in bed can help. The rules stay the same: only bottles built for hot liquids, close the lid tightly and check for leaks, always use a cover.

You can work with snow too: a little clean snow in a still warm, half-full bottle melts without a stove. Treat the water according to its source, because snow is not automatically clean drinking water either.

Hot water can cause serious scalds, and inside a sleeping bag you may notice a leak or too much heat only late. So only use bottles designed for hot liquids, close the lid tightly, check for leaks, wrap the bottle in a sock or cloth and do not place it directly on the skin.

Thin disposable plastic bottles are unsuitable for this: with hot water they deform and can leak. Boiling water never goes into a bottle like that.

For babies, small children and people who cannot feel heat well or cannot move away from it themselves, a hot water bottle in a sleeping bag or bed is not suitable. If a scald does happen: cool the area and for larger areas call the local emergency number (112 in the EU).

On winter hikes and winter camping trips, frozen drinking water is a common nuisance – and more than that, because if you do not drink you tire faster and get cold sooner. Mountaineering handbooks therefore recommend insulating bottles in winter and taking them into the sleeping bag at night.

During long waits in the cold, too – after a breakdown or at a bus stop, for example – a bottle in a cover close to your body makes sure you have something to drink. A warm drink from an insulated bottle is doubly welcome then.

And in a cold home when the heating has failed, a well-closed, wrapped hot water bottle can make the night more comfortable. But it does not replace warm clothing, blankets and – if things become critical – help from outside.