Insulation
Down vs. Synthetic Insulation: Which Actually Keeps You Warmer
Every insulated jacket sold today is a bet on one of two materials, and most marketing does its best to blur the difference. Here's the honest comparison.
How insulation works
Neither down nor synthetic fibre is warm by itself — both work by trapping dead air, and dead air is the actual insulator. Your body heats the trapped air; the insulation's only job is to stop that air moving and mixing with the cold outside. The material that traps more air per gram, and keeps trapping it in real conditions, wins. Everything below — fill power, loft retention, wet performance — is just different ways of measuring "how much still air, and for how long."
This is also why compression kills warmth in either material: a backpack strap, a tight outer jacket, or sitting on your insulation squeezes the air out. The gear under a strap or under your seat is doing almost nothing, whatever it's made of.
Down: the efficiency champion
Down is the fluffy under-plumage of ducks and geese — not feathers, which have quills and insulate poorly, but three-dimensional plumules that loft into tangled clusters. Its warmth-to-weight ratio is still unmatched by any synthetic, and it lasts decades when cared for.
Fill power measures loft: how many cubic inches one ounce of down fills under standard conditions.
| Fill power | Quality | Typical use |
|---|---|---|
| 500–600 | Entry level | Casual winter jackets |
| 650–750 | Good | Most quality parkas |
| 800–850 | Excellent | Lightweight expedition gear |
| 900+ | Exceptional | Ultralight alpine pieces |
Goose down tops out higher than duck down (900+ vs. around 750–800), which is part of why it costs more — but a well-sorted 700-fill duck down performs exactly like 700-fill goose. The fill power number is the spec; the bird is trivia.
The number people miss is fill weight — how many grams of down are actually inside. A 900-fill jacket with 60 g of down is a spring layer; a 650-fill parka with 250 g is a proper winter coat. Warmth ≈ fill power × fill weight. High fill power doesn't mean "warmer" — it means the same warmth from less weight and packed volume, which you pay for. A brand advertising fill power while hiding fill weight is telling you half the equation on purpose.
Synthetic: the bad-weather workhorse
Synthetic insulation (PrimaLoft, Coreloft, Thermoball and dozens of house brands) is polyester fibre engineered to mimic down's structure. It's heavier and bulkier for the same warmth and it degrades faster with repeated compression — but it shrugs off the one thing that ruins down: water.
Wet down collapses into useless clumps and takes a day or more to dry. Wet synthetic keeps most of its loft and dries in hours, often on your body while you keep moving. That single property decides the choice for a lot of climates — and for a lot of humans. If you sweat hard, commute in sleet, or own a dog that swims in December, the "inferior" material is the one that will actually be insulating you at 5 p.m.
Not all synthetic is the same construction:
- Short-staple fills (classic PrimaLoft) are loose clumps of fine fibre — softer, more down-like, slightly warmer per gram, but they migrate and break down faster with washing and stuffing.
- Continuous-filament fills (Climashield and similar) are one long tangled strand — stiffer and bulkier, but far more durable under compression. Common in kids' gear and hard-use workwear for good reason.
- Active insulation (Polartec Alpha, PrimaLoft Gold Active) is deliberately air-permeable so heat and vapour escape while you move. It's the right answer for uphill skiing and winter running, and the wrong one for standing at a bus stop.
The decision table
| Your situation | Choose |
|---|---|
| Dry, deeply cold winters (interior/continental) | Down — nothing matches it at -20 °C (-4 °F) |
| Wet, hovering-around-freezing winters (maritime) | Synthetic, or down with a waterproof shell |
| Daily commuting, bus stops, school runs | Either — pick by budget; down lasts longer |
| High-output activity (skiing uphill, winter running) | Synthetic or active-insulation blends; you'll sweat into it |
| Kids, or anyone hard on gear | Synthetic — survives washing machines and abuse |
| Tight budget | Synthetic — a €120 synthetic jacket beats a €120 down one |
| Longest possible lifespan | Down — 10–20 years vs. 3–7 for synthetic |
The commuter case deserves a note, because it's most people. Ten minutes of walking, twenty at a stop, dry-ish climate: either material works, so decide on lifespan-per-euro. A €250 down jacket amortised over 15 years is cheaper than a €120 synthetic replaced every 5 — but only if you'll actually wash and store it properly.
Hydrophobic down and blends
Treated "hydrophobic" down resists moisture longer and dries faster than untreated, but it is not waterproof — treat it as down with a safety margin, not as synthetic. It buys you time in drizzle and against your own sweat; it does not survive a soaking.
Down/synthetic blends and body-mapped hybrids (down in the core, synthetic at cuffs, shoulders and hood where straps compress and rain hits first) are genuinely sensible middle grounds, not gimmicks. The synthetic is placed exactly where down fails.
Care: where the lifespan gap opens
Down's 10–20 year lifespan is earned, not automatic:
- Wash it once or twice a season with a down-specific wash — accumulated body oils matt the plumules and kill loft. Regular detergent strips the down's natural oils; don't use it.
- Dry completely, tumble low with dryer balls, breaking up clumps by hand until nothing feels lumpy. Storing damp down is how it rots.
- Store uncompressed. Hang it or use a large mesh sack. A stuff sack is for transport measured in days, not storage measured in months.
Synthetic asks for almost none of this — normal washing, no clumping, no drama — which is exactly why it wins for kids' coats and anyone who knows they won't do the maintenance. But every wash and every week stuffed in a bag costs it a little loft permanently, and a hard-used synthetic jacket is measurably less warm after a few seasons even when it looks fine.
Ethics note
If you buy down, look for RDS (Responsible Down Standard) or TDS (Traceable Down Standard) certification, which prohibit live-plucking and force-feeding in the supply chain. Recycled down — reclaimed from old duvets and garments — is the lowest-impact option and performs identically, because down doesn't wear out; garments do.
FAQ
Is down warmer than synthetic? Per gram, yes, clearly. Per euro at the budget end, no — cheap "down" jackets use low fill power padded out with feathers. Per real-world wet day, synthetic.
How do I know if my old jacket's insulation is dead? Hold it up to a light: thin or empty patches mean migrated or matted fill. If a wash-and-proper-dry doesn't restore the loft, the insulation is done.
Does sleeping-bag logic transfer to jackets? Mostly, with one difference: bags are compressed under you every night, which is why bag longevity favours down (it recovers from compression better than synthetic over hundreds of cycles) even though the wet-weather logic is identical.
The one-line answer
Cold and dry: down, and check the fill weight. Wet or sweaty: synthetic. Both, if you can — that's not a cop-out, it's a system.
