Winter sleeping bags – read the label, choose the fill and build in layers

It's a Saturday in November and you're standing in an outdoor shop with two sleeping bags in front of you. They look almost identical. One says minus five, minus eleven and minus twenty-eight. The other just says minus fifteen, in big numbers. The second one sounds warmer. But it may well be the colder of the two.

That's why a winter sleeping bag is hard to buy. The numbers look precise, but they mean different things depending on how they were arrived at. This guide is only about the sleeping bag itself. How your sleeping mat, clothes, food and hot water bottle fit into the night is covered in the guide to staying warm in a tent in winter.

Here's the way to remember it. Put these three questions to the sleeping bag, in this order:

1. Which number applies to me? The label, the standard and the margin.
2. What happens when it gets damp? Down or synthetic, and how many nights.
3. Does it fit snugly without squeezing? Shape, length, neck baffle and zip.

If the sleeping bag answers all three well, you have a bag that will do the job. If it answers one of them badly, you can often save the situation by building in layers, and that comes last.

Question 1: Which number applies to me?

What the three numbers mean

Most sleeping bags in the shops have three temperatures on the label. They come from the EN ISO 23537 standard, which replaced EN 13537. In short, comfort is the temperature at which a standard woman sleeps well in a relaxed position, limit is where a standard man gets through the night curled up without feeling cold, and extreme is a survival limit where frostbite can occur. The full explanation of the standard woman and the standard man is in the guide to staying warm.

All you need to remember is this: buy by the comfort rating. Limit is a night you can put up with. Extreme is a night you survive. Neither of them is a night you want with children in the tent.

How much margin you need

Choose a bag whose comfort temperature is a few degrees below the coldest night you expect. Add more if any of the following applies to you:

• You feel the cold easily. That goes for men too. Limit is calculated for a standard man, but a man who feels the cold easily should go by comfort.
• You'll be sleeping several nights in a row. The bag absorbs moisture every night and gets a little colder.
• The tent is cold and draughty. The dummy in the test lies in a room with no wind.
• Your sleeping mat is thin. In the test, the bag lies on a mat of around R 5. A poorer mat makes the bag colder than the label says.

What the dummy doesn't feel

The rating is measured with a heated dummy that lies perfectly still. That makes the test comparable between bags, but it also means that some of what makes a winter bag good doesn't show up in the number.

• The neck baffle and the hood. A person who turns over pumps warm air out at the shoulders. The dummy doesn't.
• The draught tube along the zip. The dummy never pushes it aside.
• Knees and hips. A person compresses the fill when lying on their side. The dummy doesn't.

Very narrow bags can therefore look better in the lab than they feel in the tent. So two bags with the same number can differ a lot on a winter night. The one with a good neck baffle and draught tube is the warmer of the two.

When the number doesn't come from the standard

Here's the detail most buying guides skip. The ISO 23537-1 standard covers only adult bags with a limit of minus 20 degrees or warmer. It doesn't cover sleeping bags for children or babies, nor bags for military use or for expeditions in extreme cold.

That has three consequences for you as a buyer:

1. Really cold bags. A bag with a limit well below minus 20 falls outside the standard. The number is the manufacturer's own calculation or estimate. It may be honest, but it can't be compared directly with bags that have been tested.
2. Children's sleeping bags. The rating on a children's sleeping bag is always the manufacturer's own. More on that further down.
3. Bags with no mention of the standard. If there's just a number, without any statement that it's tested to EN ISO 23537 or EN 13537, you don't know how it was arrived at. Take it with a pinch of salt. That also applies to bags sold with a single number in big print, like in the shop scene.

Question 2: What happens when it gets damp?

Moisture will come, even if you do everything right

A person leaves roughly half a litre of moisture in the sleeping bag over one night. It comes from the skin and the breath. One night, it doesn't matter much. But in winter it doesn't have time to dry out; it stays in the fill and can freeze there. On nights two and three the bag gets a little heavier and a little colder.

That's why the choice of fill matters more in winter than in summer.

Down or synthetic

	Down	Synthetic
Warmth for the weight	The most, and packs small	Less, weighs more and takes up more space
Moisture	Clumps together and loses its insulation	Insulates better when damp and dries faster
Durability	Lasts a long time if looked after	Breaks down faster
Fire	Must be kept away from the fire	Highly flammable
Best for	Dry cold and a few nights	Several nights in a row, lots of condensation, or as an outer bag

Soaking-wet down can insulate worse than no sleeping bag at all. That sounds dramatic, but it rarely happens on a weekend trip. The real problem is the moisture that builds up night after night, and you can deal with it by airing the bag every morning and building in layers.

Fill power – the number on down bags

Down bags often state a fill power, for example 600 or 800. It tells you how much volume a given amount of down fills. The higher the number, the more air the down holds and the warmer it is for its weight.

Two things make the number less precise than it looks:

• It's measured using different methods. The American method and the European one give values a few per cent apart for the same down.
• The method changed in 2014. The same down then got a higher number than before. So an older bag with a lower number can be just as good as a new one.

Fill power also only tells you how good the down is, not how warm the bag is. A bag with lots of lower fill power down can be warmer than one with a little high fill power down. That's why the temperature according to the standard is a better comparison than fill power when you're choosing between two bags.

Synthetic bags and the stove

If you have a tent with a stove, there's one more thing to think about. Synthetic bags are highly flammable, and there are synthetic bags treated to burn less readily, for people who sleep close to an open fire. Place the sleeping bags so they can never slide against the stove or the stove pipe, and never dry them hanging close to the stove. Where the stove should stand and how far it should be from the fabric is covered in the guide on tents with a stove.

In our tent stove test the wood burned out at some point during the night, and after that it was the sleeping bag that handled the rest. That's the right way to think about it. Choose your sleeping bag for the night's temperature without the stove, not for how warm it was in the tent in the evening.

Question 3: Does it fit snugly without squeezing?

The shape

A mummy bag tapers towards the feet and has a hood. It has less air to warm and less surface area losing heat, which is why most winter bags look like that. A rectangular bag is roomier and more comfortable to move around in, but has more air and more places where the heat leaks out. It works best indoors and in a cabin. As the outer bag in a layering system, the extra room can be an advantage, because the inner bag can then loft fully.

Try the bag in the shop. Lie down in it wearing thick clothes and roll from your back onto your side. You should be able to do that without it pulling tight over your hips or shoulders. If it pulls tight, you compress the fill from the inside, and then you get cold.

The length

A bag that's too long has an empty foot end that you never manage to warm. Many winter bags come in two lengths. If the shorter one is still too long, you can cinch off the foot end with a strap, or stuff tomorrow's clothes down there so there's less air to warm.

Neck baffle, hood and zip

• The neck baffle is a soft tube around the neck and shoulders. It stops the warm air being pumped out every time you move. On a winter bag it's close to essential.
• The hood should cinch around the face with a cord that's easy to find in the dark, so only your nose and mouth are out.
• The draught tube is an insulated strip along the inside of the zip. Without it, the cold seeps in through the zip.
• The zip is often shorter on winter bags, because a long zip is a long cold seam. Right-handed people usually find it easiest with the zip on the left side, and vice versa.

If you want to sleep two by two, there are bags with the zip on opposite sides that can often be zipped together into one. The idea also turns up among christmas gifts for him. But a zipped-together bag is wide, and it's the same as with the rectangular bag: more air to warm and more places where the heat can leak out. On a really cold night, a snug bag each is the safer choice.

Build in layers when one bag isn't enough

A winter sleeping bag doesn't have to be one single thick bag. It's often smarter to combine what you already have.

Two sleeping bags, one inside the other

Put a summer bag inside a three-season bag, or a three-season bag inside a winter bag. It's easier to ventilate than one very warm bag, and you can use the bags separately the rest of the year.

Two rules decide whether it works:

1. The outer one should be roomy. If it's too tight it compresses the inner bag, and then you'll be colder than with just one. Lie in both at home and check that the inner one has room to loft.
2. The outer one should ideally be synthetic. In severe cold, the point where moisture freezes into frost ends up inside the insulation. With a synthetic layer on the outside, that point moves out of the down and into the bag that copes best with moisture.

Liners and inner bags

A liner or inner bag made of fleece, wool or thin fabric absorbs sweat and dirt and adds a little extra warmth. It means the actual bag needs washing less often, and it can save a night that turns out a bit colder than you expected. It doesn't turn a summer bag into a winter bag.

Vapour barrier – for long trips in severe cold

A vapour barrier is a thin inner bag made of fabric that doesn't breathe. It stops moisture from your body reaching the sleeping bag's fill, so the down stays dry even after many nights. It's used inside the sleeping bag, next to the body or over thin base layers. It feels damp and clammy, and it's not something for a weekend night with the family. It's a tool for people spending a week out in severe cold.

The children's sleeping bags

The temperature rating standard doesn't cover sleeping bags for children and babies. That's because there's no model for how children's bodies retain heat, and you can't have children sleep in a climate chamber to develop one. So the rating on a children's sleeping bag is the manufacturer's own figure, however precise it looks.

That makes the three questions even more important:

• The number: allow more margin than for an adult and plan for a night that's milder than the rating.
• Moisture: children sweat easily if they've put on too many clothes, and then the bag gets cold later in the night.
• The fit: length matters most. There are children's sleeping bags where the foot end can be folded in or buttoned off, so the bag grows with the child for a few years. An adult bag only works if the foot end is cinched off.

If you have a thin children's bag or a fleece liner, it can go inside the warm children's bag on a cold night. It's the same principle as two bags one inside the other, and the outer one should be roomy here too. Everything about age, how to tell if your child is cold and when to go inside is in the guide to winter camping with kids. What to do if someone gets too cold anyway is in the guide to hypothermia and frostbite.

Your child's very first night outdoors doesn't need a winter bag at all. Leo's first time camping in a tent was a summer night in the forest, and he fell asleep as soon as he'd crawled into his sleeping bag. It's a good way to see how your child sleeps in a bag before it gets cold.

Care – how to make the bag last many winters

On the trip

• Air it every morning. Turn the bag inside out or hang it over the tent in the sun or a dry wind for a while.
• Pack it away last thing in the morning and get it out first thing in the evening. The longer it gets to lie loose, the more it can loft before the night.
• Dry it at home straight away. Even a bag that feels dry has absorbed moisture. Hang it up indoors the same evening you get home.

Between trips

• Store it loose. Hang it on a hanger or put it loosely in a large storage bag. The compression sack is only for the trip. If the bag stays compressed for a long time, the fill can clump together and lose its loft.
• Stuff, don't roll. When the bag goes into its stuff sack, stuff it in a little at a time instead of folding or rolling it. That way it lasts longer.

Washing

Wash the bag when it starts to smell or feel flat, not after every trip. Use a large machine, ideally at a launderette. A down bag needs down wash and no fabric softener. Tumble dry it on low heat until it's completely dry and lofty, and follow the manufacturer's washing instructions. A liner in the bag means longer between washes.

The sleeping bag at home when the power goes

A good winter bag isn't just for the tent. If the power goes during a cold week, the family's sleeping bags are one of the simplest things that keep you warm in a room that's cooling down. The same three questions apply there, even if it isn't snowing on you. How to make a room warm, and what the sleeping bags mean then, is covered in our guide on creating a warm room.

What to bring for the winter night

The sleeping bag is one part of the whole. If you want to see how winter differs from summer in general, start with the guide to winter camping. Everything that needs to go in the pack for adults and children is in the winter camping packing list. It's built on the same basis as our complete packing list for tent camping, with a warmer bag, more sleeping mats and a large storage bag at home.