Synthetic vs Down Sleeping Bags for UK Camping: Which Should You Buy?

Two sleeping bags can carry the same temperature rating and still feel like completely different pieces of kit. One packs down to the size of a melon and weighs less than a bag of sugar; the other fills half your rucksack and shrugs off a soaking. Choosing between down and synthetic usually comes down to how and where you actually camp — and British weather has strong opinions on the matter.

What Is Actually Inside the Bag

Down is the plumage that sits under the feathers of ducks and geese. Each cluster has a fluffy structure that traps air, and trapped air is what keeps you warm. Its performance is measured in fill power — roughly, how much space a set weight of down occupies. Higher numbers mean more loft for less weight, and bags commonly sit between 550 and 800. Be careful with the numbers: European and US fill-power scales are not measured the same way, so comparing a European figure against an American one will mislead you.

Synthetic insulation is polyester, engineered into sheets or loose fill. Continuous-filament versions hold their structure better than short-staple fills, which can migrate and break down with repeated compression. Neither is as light as good down, but both are far more forgiving.

Warmth-to-Weight: Down Wins on the Numbers

If you are carrying your kit, down is hard to beat. A good three-season down bag might weigh under a kilo and compress to the size of a large melon. A synthetic bag of comparable warmth can easily weigh several hundred grams more, sometimes close to a kilo, and take up noticeably more room.

On a weekend camp with a short walk from the car, that hardly matters. On a four-day trek through the Cairngorms, or a bikepacking trip where every litre of pack space is contested, it matters a great deal.

Damp Performance: The British Question

Here is where down shows its weakness. When down gets soaked, the clusters collapse, the loft disappears and the bag stops insulating. It also takes a long time to dry — a genuinely wet down bag can take days to recover in a damp climate without a tumble dryer.

Synthetic insulation absorbs far less water, keeps a useful amount of its loft when damp, and dries much faster. It will not keep you warm in a puddle, but it degrades gradually rather than catastrophically, which is exactly what you want when the weather turns.

The condensation problem

The enemy in Britain is rarely a soaking. It is condensation. Breath, wet clothing, a damp groundsheet and a sealed tent on a still, humid night combine to leave everything clammy by morning. Modern down bags usually have a water-repellent treatment applied to the clusters, which slows absorption and helps a bag survive a night of condensation. It helps. It does not make down waterproof, and it does nothing for a bag that goes for a swim during a river crossing.

Choose synthetic if…

• You camp in the UK year-round, in tents that collect condensation.
• You paddle, bivvy or camp in genuinely wet conditions.
• You are buying for a child or a teenager, or for festival camping, where a bag gets treated roughly.
• You often pack up damp and stuff the bag away until the next trip.
• You want a lower upfront cost and a bag you can machine wash without worry.

Choose down if…

• Weight and pack size matter — backpacking, bikepacking, long-distance routes.
• You sleep cold and want maximum warmth for the weight.
• You are happy to look after kit: dry it properly, store it loose, keep it inside a waterproof liner.
• You want a bag that will still be performing well in ten years.

Cost, Value and Longevity

Synthetic bags are cheaper to buy, which matters if you are equipping a family or testing whether you even enjoy camping. Down costs more, sometimes a lot more, but it holds its performance far better over time. A well-looked-after down bag can last a decade or more, and it can be professionally cleaned and topped up. Synthetic insulation gradually loses loft with every compression and wash cycle, so a cheap bag used heavily may need replacing sooner than you expect.

If you buy down, look for a certified responsible-sourcing standard on the label. It is a fair question to ask of any brand.

Care: What Actually Keeps a Bag Alive

Most sleeping bags die of storage, not use.

• Store it loose. Keep the bag in a large cotton or mesh storage sack, uncompressed, in a dry cupboard. Never leave it crammed in its stuff sack for months.
• Air it after every trip. Hang it over a door frame or a washing line, indoors if the weather is wet, until it is properly dry.
• Wash it rarely. Down should be washed only when it needs it, using a detergent made for down — not biological washing powder, and never fabric softener. Tumble dry on low with a couple of clean tennis balls to break up clumps, or air dry for several days, working the fill apart by hand.
• Synthetic is easier. A 30°C machine wash and a fast line dry usually does it. Skip high heat, which damages the fibres.
• Use a liner. A silk or cotton liner keeps body oils off the bag and adds a little warmth.

Ratings, Fit and the Bit Everyone Forgets

Temperature ratings follow a European standard, and bags usually show a comfort rating, a lower limit and an extreme rating. The comfort figure is the one to trust. The lower limit is where an average man stays warm but cold and uncomfortable; the extreme figure is a survival number, not a camping one. Men's bags are often labelled with the lower limit, women's with comfort, so a "minus five" bag is not always what it sounds.

Then there is the mat. Insulation underneath you comes from your sleeping mat, not your bag, because the fill is crushed flat under your body. A cheap foam mat under an expensive down bag will leave you cold. If you want to sleep warm in the UK, sort the mat first.

A Simple Way to Decide

If you walk to your campsite and can keep your kit dry, buy down: it is lighter, packs smaller and will outlast most of your other gear. If you camp in the wet, camp with children, or know you will stuff a damp bag into a rucksack at 7am, buy synthetic and stop worrying about it.

Either way, buy for the coldest night you realistically expect, add a liner, look after the bag properly, and it will look after you.

Photo: Alex Moliski / Pexels