Start With the Season, Not the Price Tag

Most sleeping bag regrets begin in the shop, not in the tent. It is easy to be dazzled by a bargain or a fancy temperature figure and forget the question that actually matters: when and where will you sleep in it? A bag that keeps you snug on a frosty February bothy will cook you alive in a Cornish campsite in July, and a light summer bag will leave you shivering in a Welsh valley in October.

So before you look at anything else, picture your year. Are you a fair-weather camper who pitches between May and September? Do you sneak in a few cool spring and autumn weekends? Or are you the sort who wants to wake up to frost on the flysheet and a brew in a frozen mug? Be honest with yourself. Buying for the coldest night you will realistically meet is far wiser than buying for the warmest one and hoping for the best.

Understanding Comfort Ratings and the Sleep Limit

Sleeping bag temperatures are not a promise; they are a test result. The European standard gives three useful figures, but two of them matter most to families.

• Comfort rating — the temperature at which a typical adult woman sleeps comfortably. This is the number to trust if you feel the cold, which many of us do after a long day's walking.
• Lower limit — the temperature at which a typical adult man stays comfortable. Fine for warm sleepers, less so for everyone else.
• Extreme rating — a survival figure, not a sleeping figure. Ignore it entirely for planning.

Children need extra care here. They lose heat faster than adults, they wriggle out of covers, and they rarely complain until they are properly cold. If you are kitting out a family, add a good five degrees of margin to whatever rating you choose for the grown-ups.

Fill Power: What the Numbers Actually Mean

Fill power is a measure of how much space a given weight of down occupies — how loftily it puffs up. Higher numbers mean more warmth for less weight, and usually a higher price. A 600 fill power down bag is perfectly respectable for general camping. A 750 or 800 fill bag is lighter and more packable, which matters if you are carrying it any distance.

Down is unbeatable for warmth-to-weight, but it hates damp. In Britain's climate that is a genuine consideration. Synthetic fills are heavier and bulkier for the same warmth, yet they keep working when soggy, dry quickly, and cost less. For a family with children, damp tents and spilled hot chocolate, a synthetic bag is often the sensible choice. For a backpacker counting grams, down wins.

Shape, Width and the Art of Not Wriggling

Shape affects warmth almost as much as fill. A mummy bag tapers towards the feet, trapping warm air efficiently and saving weight. A rectangular bag gives you room to stretch but lets warmth escape around your shoulders. Most campers are happiest somewhere in the middle — a gently tapered bag that still lets you turn over.

Width matters more than many people expect. Squeeze yourself into a narrow bag and you compress the fill beneath you, creating cold spots and a miserable night. If you sleep on your side with a knee drawn up, choose a wider cut or a bag with a generous shoulder girth. Check the internal dimensions rather than the outer ones, and compare them to a bag you already find comfortable.

Details That Keep You Warm

Small features make a big difference once the temperature drops:

• A well-fitted hood with a drawcord, so you can burrow in and keep your head warm without a hat.
• A draught collar around the neck or shoulders, which stops warm air escaping every time you shift.
• A two-way zip with an insulated baffle behind it, so cold air cannot creep through the teeth.
• A decent shell fabric that blocks wind and shrugs off condensation from the tent walls.

Combine the bag with a decent sleeping mat and you will sleep far warmer. The ground steals heat relentlessly, and no bag can compensate for a mat that offers no insulation. Look for an R-value of at least 3 for three-season camping.

Storing and Caring for Your Bag

How you store a bag decides how long it lasts. Never leave it compressed in its stuff sack between trips — repeated compression crushes the loft and eventually robs the fill of its warmth.

Down bags especially should be stored loosely. Hang them in a dry cupboard by a loop, or lay them flat under a bed, or keep them in a large cotton or mesh storage sack so they can breathe and stay puffed. Synthetic bags tolerate compression a little better, but treat them the same way and they will reward you with years of service.

Wash sparingly, following the maker's instructions, and dry thoroughly — down clumps badly if left damp. Air bags in the sun before a trip to refresh them and restore loft. A bag looked after properly will keep a family warm through countless weekends, from bluebell woods in spring to frosty dawns in the Peak District.