How to sleep warm in a tent in winter: stop being cold in your sleeping bag

It is half past two on a night in February. The tent fabric crackles with frost every time you breathe, and you are lying curled up with your knees pulled up to your chest. The sleeping bag is rated for minus ten and it is only minus seven outside. Even so, the hip and shoulder against the ground are ice-cold. The top of your body is warm, but the underside is freezing.

That is no coincidence. Most of the cold on a winter night comes from below, and most of what helps happens before you have pulled up the zip. This guide is built on a simple model with three zones: below, around and within. Below you is the sleeping pad. Around you are the sleeping bag and your clothes. Within you are food, warmth and your bladder. If all three work, you will sleep warm even in a tent without a stove.

If you want the big picture first, with where you can camp and what to pack, start with the guide to winter camping. This one is only about the night.

Zone 1: below – why the cold comes from underneath

A sleeping bag keeps you warm by trapping air, and the air is what insulates. Underneath you, your body weight compresses the filling almost completely. The thin, flat layer between you and the ground barely insulates at all, so that is where the sleeping pad does the work. The snow and the frozen ground draw heat out of you all night, and by morning you feel it as a cold strip along your back.

R-value: the sleeping pad's number

How well a sleeping pad insulates is given as an R-value. It is measured using a common method, where the test sample lies between a warm and a cold plate and you measure how much heat gets through. The higher the number, the less heat is lost downwards. An R-value of 1 or 2 is enough for a summer night. The benchmark for winter is around 4 to 6, and higher in severe cold.

The important thing is that R-values can be added together. Two pads on top of each other insulate roughly as much as the sum of the two. It is the simplest and safest way to get a winter sleeping pad out of what you may already have at home.

Two sleeping pads, in the right order

1. Closed-cell foam next to the snow. A simple foam pad that cannot be punctured. It protects the inflatable pad from sharp edges of ice and spruce needles.
2. The inflatable one on top. That is where you lie comfortably. Inflate it properly, because a half-filled pad gets compressed under your hip.

If the inflatable pad fails in the middle of the night, you still have the foam pad underneath you. It is a miserable night, but not a dangerous one.

Tip: Put your rucksack, rope and the empty stuff sack under your feet and legs if the sleeping pad is short. Your feet also need something between them and the snow.

Why the sleeping bag rating assumes a good sleeping pad

Here is a detail that hardly anyone mentions. When sleeping bags are tested to the standard, the test mannequin lies on a well-insulated pad with an R-value of around 5. So the temperature on the label applies to a sleeping bag lying on a proper winter sleeping pad. If you put the same sleeping bag on a thin summer pad on snow, it will not live up to its label, however good it is.

Zone 2: around – the sleeping bag, your clothes and moisture

What comfort, limit and extreme mean

Most sleeping bags in the shops have three temperatures on the label. They come from the EN ISO 23537 standard, which replaced the old EN 13537, and they are measured on a heated mannequin in a climate chamber.

On the label	What it means
Comfort	A standard woman sleeps comfortably in a relaxed position, without feeling cold.
Limit	A standard man gets by without feeling cold when lying curled up.
Extreme	A survival limit. The standard woman can last six hours without dying of hypothermia, but may get frostbite.

In the standard, the standard woman is 25 years old, 160 centimetres tall and weighs 60 kilos. The standard man is 25 years old, 173 centimetres and just over 70 kilos. If you are smaller, older, tired or feel the cold easily, you will get cold sooner. That is why the comfort temperature is the most reasonable figure to go by if you want to sleep well, not the limit. The extreme temperature is not a temperature to plan a night around.

The standard itself warns that the measurement becomes less reliable below about minus 20 degrees, and it does not apply to children's sleeping bags at all. What that means for children's bags is covered in the guide to winter camping with kids.

The mannequin also lies still in a room without wind or moisture. Wind blowing through the tent, hoarfrost on the fabric and a sleeping bag that has become damp over several nights make the real night colder than the test.

Not too big, not too tight

A sleeping bag that is too big has a lot of air for your body to warm up, and every time you turn over, warm air is pumped out. An adult sleeping bag for a small child will therefore be cold. If the sleeping bag is too tight, you compress the filling from the inside, and then it insulates less well.

• Fill the empty spaces. If there is spare air around your feet, stuff tomorrow's jumper or your down jacket in there.
• Pull the hood and the draft collar tight. That is where the warm air leaks out when you move.
• Shake out the sleeping bag in good time before you go to bed, so the filling has time to loft.

A fleece or wool liner gives a little extra warmth and keeps the sleeping bag cleaner. It does not replace a sleeping bag that is too cold for the night, but it can save a night that is a little colder than expected.

Moisture is the silent thief

What ruins a sleeping bag's insulation on a winter trip is usually not the cold, but moisture. Down that gets damp clumps together and stops holding air. The moisture comes from three places:

1. Your breath. Every breath out contains water vapour. If you breathe inside the sleeping bag, it gets trapped in the filling. Pull the hood around your face so your nose and mouth are outside.
2. Sweat. Your body gives off moisture all night, and more if you go to bed warm and sweaty.
3. The snow. Brush off your clothes and boots before you crawl into the tent, so the snow does not melt on your sleeping bag.

The hoarfrost that forms on the inside of the fabric melts when the tent gets warmer in the morning and drips down. Keep the vents open unless there is a storm, and pack away or cover your sleeping bag before you light the camping stove or the wood stove.

In the morning, the sleeping bag needs airing. Turn it inside out for a while, ideally in the sun or a dry wind, before you pack it away. Every night with moisture that stays in makes the next night a little colder.

The clothes you sleep in

• Dry next to the skin. Change into a dry wool base layer before you go to bed. The day's base layer is damp with sweat, even if it does not feel like it.
• Dry socks. A pair of thick wool socks used only in the sleeping bag.
• A hat. A thin hat or a neck gaiter that can be pulled up over your ears.
• Not too much. Thick, tight layers compress the sleeping bag's filling. If you sweat in the sleeping bag, you are wearing too much.

There is more about dressing for the cold and when it is too cold for the kids in the guide to how cold is too cold on the slopes.

Zone 3: within – food, warmth and your bladder

Fuel before you go to bed

The sleeping bag does not produce heat. It only holds on to the heat your body produces, and your body needs fuel to produce heat all night. Eat a proper dinner and then something fatty and filling in the last hour before bedtime: cheese, nuts, chocolate, butter in your porridge or a kolbulle over the fire (a thin Swedish pancake of flour and water fried in pork fat). It is a good idea to put something to eat in the sleeping bag's inner pocket in case you wake up cold in the middle of the night.

Drink something warm as well. It does not warm you that much in itself, but a cup of hot chocolate or stock makes it easier to get fluids down when you do not feel thirsty in the cold. You will find ideas among the best hot drinks.

Tip: Don't crawl in cold. Take a brisk walk around the tent, do a few squats or fetch more snow for water before you go to bed. Warming up a cold body in a sleeping bag takes a long time, but a warm body keeps the sleeping bag warm.

The hot water bottle

A bottle of hot water in the sleeping bag is the best winter trick there is, if it is done right.

• A bottle that can take hot water and has a lid that screws on tightly. An ordinary thin plastic water bottle can soften and leak, and a metal bottle gets hot on the outside.
• Not freshly boiled water. Let the water cool for a while after boiling before you fill the bottle.
• Check the lid. Turn the bottle upside down and squeeze it before it goes in. A leaking bottle is the worst thing that can happen to a sleeping bag in winter.
• Never directly against the skin. Put the bottle in a sock or a cover. It can cause burns if it lies against the same spot for a long time.
• Where it does the most good. By your feet or between your thighs.
• Not against frostbitten skin. White, numb toes or fingers should be warmed slowly, skin to skin. A hot object can damage the skin further.

If you bring a rubber hot water bottle like the one at home, fill it only to about two thirds, squeeze out the air before you screw in the stopper, and do not use it more than two years after the date of manufacture. Freshly boiled water breaks down the rubber so the bottle is more likely to split. Children's healthcare services in Scotland saw burns in children double in one winter and advise that children should neither fill the bottle themselves nor take it to bed. So let the bottle warm your child's sleeping bag in advance, and take it out when your child crawls in. For the adults, a bonus is that the water in the bottle has not frozen by morning.

Pee before, and when you need to

It is tempting to stay put and hold it in. But a full bladder makes it hard to get back to sleep, and the night gets long. Pee just before you crawl in, even if it feels unnecessary. If you wake up because you need to go, go outside straight away instead of lying there waiting.

If you would rather not go outside, use a pee bottle. It should have a wide neck and a lid that seals tightly, and it should be clearly marked with tape so it is never mixed up with the water bottle or the hot water bottle. For those who pee sitting down, there are funnels that make it easier. It is a good idea to practise at home first. Empty the bottle some distance from the tent and well away from the snow you melt for water.

The morning: getting warm again

The morning is often the coldest moment. Make it easier the evening before.

1. Put tomorrow's socks and liner gloves in the sleeping bag. Then they are warm and dry when it is time to get dressed.
2. Bring in your boots' insoles or inner boots into the sleeping bag or a bag by your feet, so they have not frozen.
3. Eat something before you crawl out. A piece of chocolate or a handful of nuts makes it easier to get going.
4. Get dressed in the sleeping bag as far as you can, then get out and start moving straight away.
5. Air out the sleeping bag while you boil water for breakfast.

The night in a hot tent: when the stove goes out

In a tent with a wood stove, the evening is warm, often very warm. In our hot tent test, it got almost sauna-hot once the stove was up to temperature, and you could sit in a T-shirt even though it was around one degree outside. Condensation formed during the night, which is normal when a small tent is heated in cold air.

What matters for the night is what happens next. In our tent stove test, the last logs burned out at some point during the night, and it was the sleeping bag that had to handle the rest. A tent is not insulated, and when the fire goes out the temperature drops quickly.

• Sleep as if the stove were not there. Have a sleeping bag and two sleeping pads that can handle the night's sub-zero temperatures on their own. The stove is for the evening and the morning.
• Don't crawl in sweaty. Let your body cool down and change into dry clothes before you go to bed. Otherwise the moisture from the sauna heat stays in the sleeping bag when it gets cold.
• Keep the sleeping bag open as long as it is warm, and only zip it up when you start to feel the cold.
• The stove must not burn unattended. Either you let the fire go out before you fall asleep, or the adults take turns keeping watch and adding wood. The latter works best in bigger groups, and with children in the tent, an adult stays awake for as long as the fire is burning.

How to choose a stove and run it safely, with the stove pipe, spark arrestor and clearance to the fabric, is covered in the guide to tents with a stove.

Never heat the tent with a gas heater or a barbecue

Carbon monoxide has no smell and can be fatal in a closed tent. Never use a barbecue, gas heater or any other heater made for outdoor use inside the tent, and do not bring in a barbecue that has just been used. Turn off the camping stove before you fall asleep. If you have a wood stove, the stove pipe must lead the smoke out, a vent must be open and a carbon monoxide alarm must be close to the sleeping area. Everything about alarms, symptoms and ventilation is in the guide to carbon monoxide in a tent.

Kids in the winter tent

Small children find it harder than adults to stay warm, and they do not always say that they are cold. The same three zones apply, with a few differences.

• A sleeping bag in your child's size. An adult sleeping bag has too much air for a small child. If you have to use one anyway, fill the foot end with clothes so there is less air to warm up.
• A neck check before the light goes out. Put your hand on the back of the neck and on the forehead. If it is cold there, the child is cold. A child who shivers, turns pale and prefers to lie still needs warmth immediately.
• Sleep close to an adult. Two sleeping bags next to each other keep warm better than two on opposite sides of the tent.
• Wake up once. Feel the back of their neck during the night, and have a plan for calling it off. A house or a car nearby makes the first winter night calmer.

The first night outdoors does not have to be in the snow. Read about Leo's first time camping in a tent for a gentler start, and the whole approach with age, calling it off and sleep in the guide to winter camping with kids.

Try it at home first

A tent in the garden or a night in a cold room at home quickly shows which zone is failing. If you wake up with a cold back, it is the sleeping pad. If you are cold all over, it is the sleeping bag or the food. It is also a good way to find out whether the family's sleeping bags are up to a winter week without power, as in the guide to a warm room. You will find everything else to pack in the complete tent camping packing list.