Down or synthetic: choosing the fill in your sleeping bag

Down is the clustered plumage from under the feathers of ducks and geese. Each cluster is a three-dimensional tangle that holds air still, and still air is what insulation actually is. Nothing manufactured matches it for warmth per gram or per litre of packed space, which is why it has survived every attempt to replace it.

Synthetic fill is fine polyester fibre, spun and layered to do the same job. It is heavier and bulkier for the same warmth. In exchange it keeps a usable part of its loft when damp, dries far faster, costs less, and does not care whether you can look after it.

Of the 87 bags here, 71 are down and 16 are synthetic. That ratio is a fact about this shelf rather than a verdict on the argument, and it is worth explaining before anyone reads it as one.

These are bags meant to be carried on a back for days, and in that market the selection pressure is weight and packed volume. So down is where brands put their flagship models, and where the model count multiplies: several ratings, several lengths, several fill powers of one design. Most synthetic bags sold in Europe are sold for campsites, festivals and the back seats of cars, and never enter a backpacking list at all. The shelf looks lopsided without the argument being settled.

In practice you will find more down choice at every temperature. If you want synthetic, filter for it early and expect a shorter list — the bag pool will show you how short, and it is still long enough to hold the right bag.

Fill power is the volume one ounce of down lofts to under a standard test, in cubic inches. An 850 fill power down lofts to 850 cubic inches per ounce; a 750 lofts to less. It is a measure of the quality of the down — how much air a given weight of it can hold — and nothing else. The down bags here run from 650 to 950, which covers most of what a backpacking bag is built with. Synthetic bags are recorded as 0, because the figure does not apply to them.

Higher fill power means the same warmth for fewer grams and less packed volume, and it costs sharply more per gram as the numbers climb. Between 650 and 950 the difference is genuine but smaller than the price gap implies. It is also worth knowing that the American and European test methods are not identical, so the same down can be labelled differently on either side of the Atlantic. Treat a ten or twenty point difference between two brands as noise.

What fill power does not tell you is how warm the bag is. It says nothing about fill weight — the actual grams of down inside — and a 950 bag with 300 g of it is a summer bag while a 750 bag with 900 g of it is a winter bag. It says nothing about baffles, where sewn-through seams leave cold lines and box walls do not, nor about the shell, the cut, the hood or the draught collar. The number that accounts for all of it at once is the temperature rating . Shop on that, and read fill power afterwards.

The honest case for synthetic

It is stronger than the market share suggests. Walk in a wet maritime climate — western Scotland, Ireland, western Norway, the Alps in a bad week — and you will eventually have a trip where nothing dries: condensation in the tent, rain on the pack while you pitch, a river crossing that goes wrong, a stuff sack that turns out not to be waterproof. A soaked down bag is close to no bag at all, and stays that way until you find a dry room. A soaked synthetic bag is heavy and clammy and still keeps you alive.

The second half of the case is money. At a fixed budget, synthetic buys a lower temperature rating than down does. If the choice is a synthetic bag rated for the nights you will meet or a down bag five degrees short of them, take the synthetic one. The rating decides the night; the fill only decides how pleasant the carrying was.

Most down sold today is treated at the cluster level with a durable water repellent finish. The marketing names vary by brand; the mechanism does not. Each cluster is slower to take on water, so the bag resists a spill or a damp night longer and dries faster once wet.

That is a real improvement, and it is not the same as waterproof. Treated down still wets out under sustained soaking, still collapses once it does, and still needs a warm dry place to recover. The finish wears with washing and time. Buy it as margin against condensation and bad luck, not as permission to stop protecting the bag.

The things that actually keep a bag dry are duller and more effective: a shelter that does not drip on you, a waterproof liner or dry bag inside the pack, not breathing into the bag on cold nights, and airing it over the tent for ten minutes every morning the weather allows. Those habits matter more to a down bag than any treatment on the down does.

This is where the running cost of the decision shows up. A down bag that is looked after will outlast the person who chose it. One left compressed in its stuff sack between trips will be noticeably flatter in three years, because down recovers from compression only if it is given the chance.

• Store any bag loose, in the big mesh or cotton sack it came with, hanging or under a bed. Never long-term in the compression sack.
• Use a liner and go to bed reasonably clean. Body oils mat the clusters together and cost loft long before dirt does.
• Wash rarely. When you do, use a down-specific wash in a machine with no central agitator, rinse twice, and tumble dry on low for hours with a couple of dryer balls to break up the clumps.
• Synthetic washes more easily and more often, but nothing recovers loft that has been lost to broken fibres. Compress it less than you are tempted to.
• Down can be repaired, re-lofted and in extremis re-filled, which is part of what the price buys. A tired synthetic bag is a replacement.
• More detail on all of it in looking after your kit .

1. 1

   Count the trips you made last year, not the ones you plan

   Write down the nights you actually spent out and what the weather did. Buying for the imagined expedition is the most expensive habit in outdoor kit, and it ends with the wrong bag on the trips you really take.

2. 2

   Ask how a wet bag would end

   If a soaked bag means one cold night and a car park in the morning, down is a risk you can carry. If it means three more days in the hills, or a canoe, or a week in a climate where nothing dries, synthetic fails in a way you can live with and down does not.

3. 3

   Fix the temperature rating first, then choose the fill

   Decide the comfort rating you need, and only then spend what is left on how light it is. A bag that is warm enough in the wrong fill beats a bag in the right fill that is five degrees short.

4. 4

   Weigh it against the pack and the pad

   The bag is one of the big four, and grams saved here are only worth it if the rest of the load is in proportion. Base weight and cutting weight is the honest way to work out whether 400 g of down is the cheapest 400 g you could lose.

5. 5

   Let the finder rank both together

   Tell the finder the nights, the weather and the budget, and it puts down and synthetic bags on one list with the reason each moved. If the top of that list is not the fill you expected, the answer is usually in the weather you gave it.

One last thing, and it is the part people regret skipping. A bag works with the mat under it and the shelter around it, and a euro moved between the three often buys more sleep than a euro spent on the bag alone. Settle the R-value of the mat before you spend the last hundred euro on fill power.