What three-season actually means in Britain

Three-season is a loose term, and in Britain it covers more weather than most people expect. Realistically you are looking at April through October, with the strong caveat that a Highland glen in May can produce a ground frost while a sheltered Cornish campsite in August may not drop below 15°C all night. A bag that copes with a still, damp night at 300 metres in the Peak District has to be genuinely warm, not just theoretically warm.

Look at the ISO temperature ratings and read them properly. The comfort figure is the one to trust if you sleep cold, wear a baselayer, and want to be comfortable rather than merely surviving. The limit figure assumes a warm sleeper curled up in a ball. The extreme figure is a survival number and should be ignored entirely for buying decisions. For most British three-season camping, a comfort rating somewhere between 0°C and +3°C is the sweet spot. If you mostly camp lowland and coastal in high summer, you can go warmer and lighter still.

Down versus synthetic: the damp argument

Down wins on warmth-to-weight and pack size, and it lasts for decades if you treat it well. Good down bags compress to the size of a melon and weigh as little as 600g. The catch is moisture. Down loses loft when wet, clumps into useless lumps, and in a damp tent on a wet Welsh morning it will not dry out before you need it again.

Synthetic fills are heavier and bulkier for the same warmth, and they degrade faster with repeated compression. But they keep insulating when damp, dry far quicker, and cost noticeably less. For a British climate, where condensation inside a small tent is normal rather than exceptional, that resilience matters.

• Down: best for weight, pack size and longevity; suits travellers who keep gear dry and camp in reliable conditions.
• Synthetic: best for damp nights, occasional campers, and anyone who wants to stuff a bag away without ceremony.
• Hydrophobic down: treated down resists moisture better than untreated, but it is not waterproof. Do not treat it as a licence to sleep in a puddle.

If you camp three or four nights a year, often in a tent that gets packed away wet, synthetic is usually the sensible answer. If you walk long distances and count every gram, down earns its keep.

Weight and pack size: the cost of bulk

Weight is only half the story. A bulky bag forces a bigger rucksack, which shifts weight off your hips and onto your shoulders, which makes the whole walk harder. Compare both figures before you buy. A synthetic bag at 1.2kg that fills the bottom third of your pack is often more tiring than a 700g down bag that leaves room for everything else.

Remember too that warmth comes from the whole sleep system. A mat with an R-value of 3 or higher stops the ground pulling heat out of you, and it will do more for your comfort on cold ground than an extra 200g of fill ever will.

Features worth paying for

Camping shop shelves are full of gimmicks. A few design details genuinely change how a bag performs.

• A proper draft collar and hood. The heat you lose around your neck and shoulders is significant. A well-shaped hood you can cinch down beats a flat one every time.
• Box-wall construction. Stitch-through baffles create cold spots along the seams. Box walls cost more but eliminate them.
• A full-length zip baffle. Zips leak warmth badly. A decent baffle behind the zip is worth its weight.
• A realistic shell fabric. Something in the 10 to 20 denier range with a durable water repellent finish balances weight against puncture resistance.
• Left or right zip. Worth checking if you want to zip two bags together, which is a genuine comfort win for couples on a cold night.

Skip the built-in pillow pockets, the extra internal pouches and the marketing claims about "extreme" temperatures. Put the money into fill quality and hood design instead.

Look after it and it will outlast you

Down bags repay care. Synthetic bags are forgiving but not indestructible. Either way, a few habits make a big difference.

• Never store a bag compressed. Keep it loose in a cotton or mesh sack, or hang it in a dry cupboard.
• Air it thoroughly after every trip, even a dry one. Damp air and body moisture accumulate inside the fill.
• Wash sparingly and gently. Use a dedicated down wash for down, a non-biological detergent for synthetic, and a cool, slow cycle.
• Dry down properly: a tumble dryer on low with two or three dryer balls works well, and it may take several cycles before the bag lofts fully. Never put a damp down bag away.
• Synthetic bags dry faster but still need to be bone dry before storage to avoid mildew.

Matching the bag to how you camp

If you camp a handful of nights a year, on campsites and lowland trails, buy a reasonably priced synthetic bag rated to around 0°C comfort and spend the savings on a better mat. You will sleep warmer and worry less about a bit of drizzle.

If you walk distance, carry your own kit for days at a time, and keep your gear dry, a down bag in the 700 to 900 fill power range with a comfort rating near 0°C will serve you for years and save your back in the process. Whichever you choose, buy the bag you will actually enjoy carrying. The best sleeping bag is the one that gets you out on the hill again next weekend.