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. The material that traps more air per gram, and keeps trapping it in real conditions, wins.
Down: the efficiency champion
Down is the fluffy under-plumage of ducks and geese. 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.
| 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 |
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. 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 compression — but it shrugs off the one thing that ruins down: water.
Wet down collapses into useless clumps and takes a day to dry. Wet synthetic keeps most of its loft and dries in hours. That single property decides the choice for a lot of climates.
The decision table
| Your situation | Choose |
|---|---|
| Dry, deeply cold winters (interior/continental) | Down — nothing matches it at -20 °C |
| 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 |
| Tight budget | Synthetic — a €120 synthetic jacket beats a €120 down one |
| Longest possible lifespan | Down — 10–20 years vs. 3–7 for synthetic |
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. Down/synthetic blends and body-mapped hybrids (down in the core, synthetic at cuffs and shoulders where straps compress and rain hits) are genuinely sensible middle grounds, not gimmicks.
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 — is the lowest-impact option and performs identically.
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.