Gear & Craft
Down versus synthetic, decided by climate rather than preference
One is warmer for its weight and fails when wet. The other is heavier and does not. That is the whole comparison.
What follows is the working version of insulation choice: the decisions in the order you actually meet them, with the reasoning attached.
Before you start
- Down gives more warmth per gram and loses most of it when damp.
- Synthetic retains much of its warmth wet and is bulkier for the same warmth.
- Fill power measures loft quality, not the amount of insulation.
Loft is warmth
Both materials work by trapping still air, so the volume of loft determines the warmth rather than the material itself. Down achieves more loft per gram than any synthetic, which is why it dominates where weight matters.
Compressed insulation of any kind provides almost nothing, which is why sitting on a sleeping bag is pointless. The corollary is that two garments of the same measured thickness are close to equally warm whatever they are filled with, so anyone comparing by eye in a shop should be comparing loft rather than the name printed on the label.
Water is the dividing line
Wet down collapses and loses most of its loft, and dries slowly. Synthetic fill retains a usable proportion of its loft when damp and dries far faster.
In practice, in persistently wet maritime climates that difference outweighs the weight penalty for many users. Synthetic has its own failure mode, because the fibres take a permanent set under repeated compression and a synthetic bag loses loft over a few hundred nights in a way that a well-kept down bag does not.
Fill power is a quality measure
Fill power describes how much volume a given weight of down occupies, so higher numbers mean better down. It says nothing about how much down is in the garment, which is the fill weight. A high fill power jacket with little down in it is lighter and colder than the number suggests.
Two testing standards are in common use and give different figures for the same down, so a number quoted by one brand is not directly comparable with a number from another unless both state which method produced it.
Treated down is a partial answer
Hydrophobic treatments delay wetting out and improve drying, and they do not make down waterproof. They are worth having and should not change the decision in a genuinely wet climate. Treatments also degrade with washing over time.
The detail that matters: the shell does more than the treatment in any case, because a tightly woven, well-proofed face fabric keeps water off the fill to begin with, and a hydrophobic treatment inside a fabric that wets out in ten minutes is solving the wrong half of the problem.
The practical split
Most experienced walkers end up with synthetic for anything worn while moving and generating moisture, and down for the stationary layer in camp. Sleeping bags follow the same logic, with down dominant except where damp is unavoidable. Ethical sourcing standards exist for down and are worth checking on the label.
Those standards audit traceability back along the supply chain and prohibit live-plucking and force-feeding, which is meaningful and is not a guarantee, since the chain runs through several countries and the down is a by-product of a food industry the standard does not control.
How much of that applies depends on the terrain you are actually in.
Construction and storage decide how long the warmth lasts
Sewn-through baffles stitch shell to lining directly and leave a cold line along every seam, while box-wall construction adds a fabric wall so the down keeps its depth — which is why two garments with identical fill weights can perform differently. Down migrates through baffles over years, so an old jacket that has gone patchy usually needs the fill redistributing or a baffle repairing rather than replacing outright. Insulation of any kind left compressed loses loft permanently, which is what the large mesh storage sack that comes with a sleeping bag is for and why throwing it away is expensive.
On consecutive nights below freezing, moisture from the body condenses inside the insulation and accumulates, so a bag that felt warm on the first night can be noticeably colder by the fourth unless it is aired out whenever the sun appears.
The takeaway
Synthetic while moving, down while stationary, and check fill weight rather than fill power alone.
Go slowly enough to notice, and most of this stops being advice and starts being obvious.
Questions readers ask
How should I wash a down jacket?
With a down-specific cleaner, on a gentle cycle, then tumble dried low with dryer balls to break up clumps. It takes hours and restores loft substantially.
Does down have an ethical problem?
Live-plucking and force-feeding are real concerns in parts of the supply chain. Certification schemes exist and are stated on the label by most reputable brands.
