Building an emergency shelter

A shelter doesn't warm you up. It only stops the world from cooling you down. Its whole design comes down to that difference.

ACT

1. Choose the spot before the technique. Out of the wind, out of the valley floor where cold air pools at night, out of a stream bed even when it's dry, away from dead trees and hanging branches.
2. Build small. A shelter the size of your body, that you crawl into. Every extra litre of space is a litre you have to heat.
3. Insulate the ground first. Thirty centimetres (a foot) of dry, compressed material under you — leaves, ferns, grass, needles. It's half the thermal work, it's always the part that gets rushed, and it's what makes the difference between an uncomfortable night and Hypothermia.
4. Put up the frame. A ridge pole resting at one end on a support, branches as ribs on both sides, then a thick pile of foliage.
5. Make the cover thick. A hand's thickness against rain, an arm's length against cold.
6. Block the entrance with a bag of leaves or a bundle of branches once you're inside.

ADAPT

You're in a forest. The fastest shelter is still a pole leaning on a stump, branches laid along it like a spine, foliage on top and a thick bed inside. Allow one to two hours. Don't build it bigger to be comfortable: you'd be comfortable, and cold.

There's snow. Snow is an excellent insulator because it traps air. A hole dug under a fir tree whose low branches touch the ground gives you a sheltered space that's almost ready. A shelter dug into a snowdrift works as long as you leave an air hole and put the entrance lower than where you sleep: cold air is heavier, sinks, and stays below you.

You're in a city. A shelter already exists in almost every building. What you're looking for are the same properties: out of the wind, above freezing, insulated from the concrete, small and closable. A stairwell, a utility room, a garage. Cardboard is the best material around: light, insulating, free, everywhere.

You only have a tarp. Pitched as a lean-to facing the wind with a fire and a reflector in front of it — the setup is described in Making and keeping a fire —, it beats a closed structure with no fire. The same tarp collects water during a downpour: see Collecting and storing rainwater. As a closed tent, it keeps the rain off but builds up a lot of condensation.

You're somewhere hot and dry. The problem is reversed: you need shade and airflow, not heat retention. A raised shelter with a double roof, with air moving between the two layers, cuts the felt temperature sharply. Don't lie directly on ground that has been heating up all day.

There are several of you. A shared shelter is far more efficient than one per person: bodies warm each other and the space per person shrinks. It's one of the rare cases where the group is a direct physical advantage.

UNDERSTAND

The body loses heat in four ways, and a shelter acts on each one differently.

Conduction — direct contact with something colder. That's the ground, and it's the most underestimated loss. Someone lying on bare earth loses heat much faster than someone standing in the same cold. Hence the rule: the bed before the roof.

Convection — moving air that carries heat away. That's the wind. A shelter that simply blocks the wind, without insulating anything, already changes everything.

Radiation — heat given off to anything colder, including a clear night sky. That's why you feel colder under a clear sky than under clouds, and why a roof, even a thin one, helps more than its insulation value suggests.

Evaporation — sweat and moisture evaporating and taking heat with them. That's why you never build a shelter while sweating under your layers: take a layer off while you work, put it back on when you stop.

From this comes the single principle behind every shelter: what insulates isn't the material, it's the still air it traps. Down, wool, dead leaves, snow, corrugated cardboard and layered clothing all work exactly the same way. Compress them and they're worth nothing — that's why down squashed under your body no longer insulates, and why the bed has to be thick before your weight packs it down.

One last thing, and it often decides the night. A shelter is built before you need it. Building takes one to two hours of daylight and energy. Starting at nightfall, exhausted and chilled, gives you a failed shelter that you leave in the middle of the night. The decision to stop and build almost always has to be made earlier than you'd like.