Choosing a sleeping bag by its comfort rating

A cold night in a tent is long. Most of the time it happens for one of two reasons: the bag was chosen by the wrong number on the label, or the mat underneath it was not up to the job. Both are easy to fix before you buy.

The three numbers on the label

Most sleeping bags sold in Europe carry temperatures measured with the method in the EN ISO 23537 standard. The test uses a heated mannequin in a cold chamber, dressed in a base layer and a hat and lying on an insulated mat, and it works out how warm the bag keeps a typical person. You will usually see two or three figures.

Comfort is the temperature at which a typical cooler sleeper should be comfortable lying on their back, relaxed. This is the number to plan with.

Limit is the temperature at which a typical warmer sleeper can sleep for the night curled up, holding their warmth in. It is not a comfortable night; it is the edge of what the bag can do.

Extreme, when it is shown, is a risk figure, not a sleeping temperature. It describes a night you would not want to spend. Ignore it when choosing a bag.

On our product pages we show comfort and limit, and we plan the checklist around comfort. So the Ember 0 down bag, with a comfort of 0 °C and a limit of −6 °C, is the bag we put on autumn lists, where a frost is possible. In winter the list moves to the Ember −8, with a comfort of −8 °C.

Why "typical" matters

The test describes typical sleepers, and no one is typical every night. You sleep colder when you are tired, hungry, dehydrated or have been standing about in the wind before bed. Some people simply run cold. If you know you do, add a few degrees of margin and choose a bag whose comfort figure is below the coldest night you expect, not just at it.

The mat is half the system

A sleeping bag works by trapping warm air in its fill. Underneath you, that fill is crushed flat by your weight and does very little. What stops the cold ground taking your heat is the mat. A bag rated to 0 °C on a thin mat on frozen ground will feel far colder than its label.

Mats are rated with an R-value: the higher the number, the better the mat holds back the cold from the ground. As a starting point:

• Around R 2, such as a closed-cell foam mat, suits warm summer nights on dry ground.
• Around R 3, such as the Loft air mat, suits three-season use down to about freezing for most people.
• R 5 and above, such as the Loft winter air mat, suits frozen ground and snow. In winter we also put a foam mat underneath, which adds warmth and keeps working if the air mat is punctured.

That is why our checklist changes the bag and the mat together when it moves from autumn to winter.

Down or synthetic

Both fills can be warm. The difference is in weight, bulk and how they cope with damp.

Down gives the most warmth for its weight and packs small, but it loses much of that warmth when wet, so it needs keeping dry.

Synthetic fill is heavier and bulkier for the same warmth, but it keeps more of its loft when damp and dries faster. It also costs less. The Hollow 0 covers the same range as the Ember 0 for about 430 g more, a trade that can be worth it on wet trips.

How to choose, step by step

1. Find the coldest night you are likely to meet. Think about the season, the height you will camp at and whether you camp in valleys, where cold air settles, or on open ground. Your camp may be colder than the nearest town.
2. Pick a bag whose comfort figure is at or below that night. Add margin if you sleep cold.
3. Match the mat. R 3 or more for three seasons, R 5 or more on frozen ground.
4. Check the fit. A bag that is too long leaves cold space at your feet; one that is too tight crushes the fill. Our product pages give the height each bag fits.
5. Think about the trip. For long, damp trips, synthetic is easier to live with. For a light pack and dry, cold nights, down earns its price.

Small things that make a warmer night

• Eat something before bed and drink something warm. Your body makes the heat; the bag only holds it.
• Go to bed warm. A short walk or a few minutes of moving about before you get in helps more than lying there waiting to warm up.
• Wear a dry base layer and a hat, not the damp clothes you walked in.
• Fill a hard bottle with hot water, close it tightly, put it in a sock and take it to your feet.
• Use the hood and draught collar. Pulled snug, they keep in a surprising amount of warmth.

A last word on margins

Labels let you compare bags; they do not promise how any one night will feel. Choose with margin, carry a warm layer you can sleep in, and check the forecast and your route before you go.