Down or synthetic: choosing insulation by trip, not by loyalty

Down and synthetic are not rivals with a winner. They are two solutions to the same problem, trapping still air, and they fail in different ways. Down gives more warmth per gram and lasts far longer; synthetic keeps working when wet and costs less. The right question is not which is better but which failure mode your trip can absorb.

What insulation is actually doing#

Neither down nor polyester is warm. Both are structures for immobilising air, and air is the insulator: its thermal conductivity of roughly 0.026 watts per metre kelvin is lower than any common solid. The moment that air can circulate, convection carries heat across the gap much faster, which is why an empty air chamber insulates poorly and a fluffed sleeping bag insulates well.

Water is what breaks the scheme. At about 0.6 watts per metre kelvin it conducts roughly twenty-three times better than still air, so a fill that has taken on water is not merely heavy, it has been converted from an insulator into a conductor. Worse, evaporating that water takes energy, and on a wet night in a sleeping bag the only heat source available is you.

That single ratio explains most of the argument. Down builds its air cells from three-dimensional clusters of fine filaments that collapse into a wet clump when saturated. Synthetic builds them from polyester fibres with enough stiffness to hold some structure even when wet. The mechanical difference is why the two behave so differently in rain, and why nothing about the marketing changes it.

Fill power, and what it does not tell you#

Fill power is the volume in cubic inches that one ounce of down expands to under standard conditions. A sample is conditioned, dropped into a cylinder, lightly loaded and measured. Higher numbers mean each gram of down produces more loft.

• 550 to 650: budget fill. Warm, heavier, usually cheaper, often a higher proportion of feather.
• 700 to 750: good quality. The practical sweet spot for most buyers.
• 800 to 900: premium. Genuinely lighter for the same warmth, and priced accordingly.

The near-universal misreading: a bag with 850 fill power down is not automatically warmer than one with 650. Fill power is quality per gram. Warmth comes from total loft, which is fill power multiplied by fill weight. A bag with 300 g of 850 fill can easily be colder than one with 600 g of 650 fill.

Here is the arithmetic that matters. Take 400 g of 800 fill power down. That is 400 / 28.35 = 14.1 ounces, and 14.1 x 800 = 11,290 cubic inches of loft. To reach the same loft with 650 fill power you need 11,290 / 650 = 17.4 ounces, or 492 g. So the premium down saves 92 g, about 23 percent, for the same warmth. Real, worth having on a long carry, and much smaller than the price difference usually implies.

The two synthetic families#

Synthetic fills divide into two constructions with different trade-offs, and the label rarely says which you are getting.

Continuous filament is made of long, unbroken fibres laid into sheets. It resists shifting, holds loft over more compression cycles and is harder to tear apart in use. It is heavier and less compressible, and it feels stiffer.

Short staple is made of short, fine, crimped fibres, more like down in handling. It is softer, drapes better and packs smaller. Because the fibres are not anchored to each other, it migrates and loses loft faster under repeated compression.

Neither approaches down for warmth per gram. Across published specifications, a synthetic bag with the same comfort rating typically weighs noticeably more and packs to a larger volume than its down equivalent, and the gap widens as the rating gets colder because the fill weight rises. For a bag used a dozen nights a year near a car, that penalty is close to irrelevant. For a multi-week carry, it is the whole argument.

What happens when it gets wet#

The wet performance claim is often overstated in both directions. A useful way to hold it:

• Damp, not soaked. Both fills lose loft. Synthetic loses less and recovers faster. Down loses loft cumulatively over consecutive humid nights, which is a slow degradation people usually blame on the weather rather than the bag.
• Soaked. Down retains almost nothing. The clusters clump and the air cells disappear. Synthetic retains a useful fraction of its warmth, enough to be the difference between an unpleasant night and a dangerous one.
• Drying. This is the underrated half. Synthetic wrings out and dries in hours in moving air. Down holds water inside the clusters and can take a full day of sun and wind, or a tumble dryer it will not get in the field.

Hydrophobic down is a durable water repellent finish applied to the clusters. It delays wetting, buys time in humid conditions and speeds drying. It does not make down waterproof, the treatment degrades with washing and use, and it does not change what happens once the down is actually saturated. Treat it as a useful margin, not a category change. The same logic applies to insulated clothing, covered in insulated jackets.

A decision rule you can score#

Rather than argue about fills, score the trip. Each factor scores 0, 1 or 2 points, where higher points push toward synthetic.

0 to 3 points: buy down. Short trips, dry climates, weight matters, and a wetting event is recoverable. This covers most three season backpacking in continental climates and most alpine winter use, where the air is cold and dry.

7 to 10 points: buy synthetic. Long wet trips with no drying window and real consequences. Packrafting, sea kayaking, maritime winter, extended shoulder season travel in rainforest. Here the weight penalty is the price of a fill that still works after the worst has happened.

4 to 6 points: genuinely close, and the honest answer is that either works. At this range the deciding factors are outside the fill entirely: how much you can spend, whether you will actually wash and store the bag properly, how long you intend to keep it. Anyone who tells you there is a clean answer in this band is arguing from loyalty. A reasonable tiebreaker is expected lifetime use: over a hundred nights, down's durability starts to dominate; under fifty, it rarely pays back.

Two things the score deliberately ignores. Vapour barrier liners and a disciplined dry-bag system move a wet trip several points toward down, and they are cheaper than a second sleeping bag. And a hybrid layout, synthetic under the sleeper and down on top, exists precisely for scores in the middle band.

Lifespan, care and welfare#

Down is the more durable fill by a wide margin. Clusters are resilient in a way polyester fibres are not, and a bag washed correctly a few times per decade and stored uncompressed can stay serviceable for ten years or more. Synthetic loses loft measurably over a few hundred nights of use and compression, and that loss is permanent: no washing restores it. Cost per night, rather than cost, is the comparison that matters, and it is much closer than the sticker suggests.

Care is where most down bags die early. Body oils migrate into the fill and mat the clusters, compression sacks used for storage crush loft permanently, and damp storage rots baffles. The routine is simple: store uncompressed in a large cotton or mesh sack, wash with a down-specific detergent when loft visibly drops, and dry thoroughly. Details are in washing down gear and gear storage.

On sourcing, down is an animal product and welfare issues in the supply chain are real. Two schemes address them: the Responsible Down Standard, which certifies welfare through the chain including prohibitions on live plucking and force feeding, and the Global Traceable Down Standard, which adds traceability to parent farms. Neither is a guarantee of perfection, but an uncertified supply chain has been audited by nobody. Synthetic sidesteps animal welfare and raises a different issue: it is petroleum derived and sheds microfibres in washing.

Check this before you buy#

Two checks anyone can run in a shop or at home.

The loft check. Take the bag out of its stuff sack and leave it for twenty minutes, then measure its thickness at the chest with a ruler laid across it, without compressing. Loft is warmth, and it is the one property you can observe directly. Compare two bags this way rather than comparing their fill power numbers. On a bag you already own, this is also how you detect fill that has died: measure once when new and record it.

The handful test for wet behaviour. Take a small amount of the fill type in a sample or an old garment, wet it thoroughly, and squeeze the water out. Down collapses into a dense clump that does not spring back until it is dry. Synthetic springs back damp. That difference, in your hand, is the entire wet performance argument, and it is more persuasive than any specification.

Common mistakes#

Buying fill power as if it were warmth. The single most common error. Ask for fill weight in grams, or better, a comfort rating tested to a standard.

Assuming hydrophobic down solves rain. It delays wetting. Everything about the failure mode when saturated is unchanged.

Storing either fill compressed. Down recovers from compression far better than synthetic, but neither should live in a stuff sack between trips. This kills more insulation than weather does.

Choosing synthetic for a dry cold trip because it "handles moisture". In genuinely cold, dry conditions the humidity that matters is the water you emit, and the fix is venting and airing rather than a different fill. See why you sleep cold.

Comparing bags on fill weight across different fill powers. 500 g of 650 and 500 g of 850 are not the same bag. Multiply through.

Ignoring the pad. Whatever fill is on top of you, it is crushed flat underneath you and contributing nothing. Downward heat loss is the pad's job, as set out in the R-value guide.

Frequently asked questions#

Is down or synthetic better for a sleeping bag?#

Neither, in the abstract. Down gives more warmth per gram, packs smaller and lasts a decade or more with care. Synthetic keeps a useful fraction of its warmth when damp, dries far faster and costs less. Score your trip on drying opportunity, humidity, trip length and consequence of failure: dry and weight-critical points to down, wet and committing points to synthetic.

What does 800 fill power actually mean?#

That one ounce of the down lofts to 800 cubic inches under a standardised test in which a conditioned sample is placed in a cylinder under a light load. It is a measure of quality per unit mass, not of how much down is in the bag. A bag's warmth comes from fill power multiplied by fill weight, so always ask for both numbers.

Is hydrophobic down worth paying for?#

It is a modest, real benefit. The treatment delays water uptake and speeds drying, which helps most on humid multi-day trips where damp accumulates slowly. It does not make down waterproof, it wears off with washing and use, and a saturated hydrophobic down bag behaves much like a saturated untreated one. Do not let it change a trip plan.

How long does synthetic insulation last?#

Expect measurable loft loss over a few hundred nights of use, with the rate depending on how often the bag is compressed and how it is stored. Short staple fills fade faster than continuous filament. The loss is permanent, since washing restores cleanliness but not fibre structure. A down bag treated well can outlast three or four synthetic bags, which changes the cost comparison considerably.

Does a synthetic bag really keep you warm when soaked?#

It keeps a useful fraction of its warmth, not all of it. The polyester fibres hold enough structure to trap some air even when wet, and the fill wrings out and dries in hours rather than a day. Soaked down retains almost nothing, because the clusters clump and the air cells vanish. In a genuine soaking, that difference can matter a great deal.

What is the difference between continuous filament and short staple synthetic?#

Continuous filament uses long unbroken fibres in sheets: durable, resistant to shifting, heavier and less compressible. Short staple uses short crimped fibres: softer, more down-like in handling, packs smaller, but migrates and loses loft faster under repeated compression. Manufacturers rarely state which is used, so judge by weight, packed size and any durability claims.

Are down certifications like RDS meaningful?#

They are meaningful without being complete. The Responsible Down Standard certifies animal welfare through the supply chain, including bans on live plucking and force feeding, and the Global Traceable Down Standard adds traceability back to parent farms. Both involve third party audits. Neither can guarantee every bird's treatment, but uncertified down carries no audit at all.