Temperature ratings are only a starting point

Almost every bag sold in Britain carries a European EN 13537 or its successor ISO 23537 rating, and it is worth understanding what those four numbers actually mean. The upper limit is the point at which a hot sleeper starts to sweat. The comfort figure is the temperature at which a cold-sleeping adult woman can sleep through the night without waking. The limit is the equivalent for a warm-sleeping adult man. The extreme figure is a survival statistic and nothing more — do not plan a night out around it.

In practice, buy to the comfort rating if you sleep cold, are tired, are female, or are camping above the treeline. Most people are happiest with a bag whose comfort rating sits about five degrees below the lowest temperature they expect to meet. A bag with a limit of −10°C and a comfort of −4°C is a sensible British winter bag, not an extreme one.

Down or synthetic: the damp question

Down wins on warmth per gramme, packability and lifespan. Synthetic wins on price and on performance when wet. The classic British winter is damp cold rather than dry cold, which is where the decision gets interesting.

• 550–600 fill power — a workhorse for UK three-season use and mild winter camping. Cheap, durable, bulkier than it looks.
• 650–750 fill power — the sweet spot for British winter. Warm, reasonably light, still tolerant of a bit of moisture.
• 800 fill power and above — lightest and most compressible, ideal for long Scandinavian trips where every gramme in the pack counts. Less forgiving if it gets soaked.
• Synthetic fills — around 600–800 g of decent continuous-filament insulation will get you a genuine winter bag. It will be half again as heavy and bulky as an equivalent down bag, and it will lose loft after a few hard seasons.

Hydrophobically treated down is a useful compromise: it still dislikes standing water, but it shrugs off condensation and damp air far better than untreated clusters.

Matching the bag to the trip

A lowland campsite in Devon in December is not the same proposition as a Norwegian winter. Be honest about which you are packing for.

• British winter lowland and valley camps — comfort around −2°C to −5°C, plus a decent mat and a liner for the coldest snaps.
• Scottish Highlands, Snowdonia, Pennines — comfort −7°C to −10°C. Expect wind chill, condensation and long, long nights.
• Scandinavian winter — comfort −15°C to −25°C. Dry cold, so down comes into its own, but the bag needs proper loft, a full hood and a seriously warm mat underneath it.

Scandinavian snow camping is often colder but kinder than a wet night in the Lake District. Damp air steals warmth from any insulation; dry air does not.

Shape, hood and draughts

A mummy bag is warmer for its weight because there is less dead space to heat. Rectangular bags are more comfortable for car camping but need more fill to do the same job. Look for a well-filled hood you can cinch down to your eyebrows, a neck baffle, a draught collar behind the zip, and a zip baffle that stops cold air tracking along the teeth.

Check the shoulder and hip circumference honestly. A bag that is too tight compresses its own insulation wherever your body presses against it, which is exactly where you will feel coldest. If you might share a bag, buy a left-zip and a right-zip pair from the same range.

Pack size and carrying it

Down compresses brutally well. A −10°C down bag may squash into 8–10 litres and weigh around 1.1–1.4 kg. A synthetic equivalent will want 15–20 litres and 1.8–2.5 kg. For a Scottish winter bothy trip that hardly matters; for a multi-day ski tour in Norway it decides whether you can carry enough food.

Always pack the bag inside a waterproof stuff sack or a dry bag, roughly the size of your winter rucksack's lower compartment. At home, store it loose in a large cotton or mesh bag — compression kills loft over time.

Getting the most out of what you have

The bag is only one part of the system. A mat with an R-value of 4 or more is non-negotiable for winter; underneath a cold bag, the ground will win every time. Sleep in dry baselayers, wear a hat, and consider a silk or fleece liner — it can add five to eight degrees and keeps the bag clean.

• Fill a sturdy bottle with hot water, wrap it in a sock, and put it in the bag ten minutes before bed.
• Eat something before sleep; digestion generates heat.
• Don't wear every layer you own inside the bag — it compresses the loft and makes you colder.
• Air the bag each morning to shift the moisture you breathed into it overnight.
• Vent the bag if you wake up clammy. Sweat is the enemy of warmth.

Buy once, buy warm, and treat the rating as a guide rather than a guarantee. A well-chosen bag, a warm mat and a dry base layer will turn a miserable winter night into one you will happily repeat.