The Complete Guide to Down Insulation

Down jackets are not filled with pure down. They contain a mixture of down clusters and feathers, expressed as a ratio - for example 90/10 or 93/7. The first number is the percentage of down; the second is feathers. Higher down content means better insulation, softer feel and lighter weight. Feathers are heavier, less insulating, and can occasionally poke through the shell fabric.

Budget jackets often use 80/20 or 75/25 ratios. Premium jackets use 90/10 or above. Jöttnar jackets use a 93/7 down-to-feather ratio - among the highest available, and the highest practically achievable at scale.

Fill power is the most commonly cited number in down jacket specs, and the most commonly misunderstood. It measures the volume, in cubic inches, that one ounce of down occupies when fully lofted. A 700 fill power down expands to 700 cubic inches per ounce. An 850 fill power down expands to 850 cubic inches. Higher fill power means larger, more resilient clusters that trap more air without adding bulk or weight.

What fill power does not tell you: how warm the jacket is. That requires a second number - fill weight, the total mass of down in the jacket expressed in grams. A lightweight jacket might use 80–120g. A serious expedition jacket might use 250g or more. Fill power is the quality of the fuel; fill weight is how much fuel is in the tank.

When comparing jackets, always look at both numbers together. A 900 fill power jacket with 80g of down may be lighter and more packable than an 850 fill power jacket with 200g - but considerably less warm.

Why does nobody talk about fill weight? One reason is that manufacturers prefer the simple fill power headline. A less cynical reason is that down jackets use multiple different constructions - hybrid fills, body-mapped baffles, hood and collar volumes - which makes cross-brand fill weight comparisons less useful than they appear. Our jackets are engineered precisely to achieve defined use cases, which may not be achieved by simply loading up on fill weight alone. The jacket must be tuned. We are happy to discuss fill weight with enquirers in context - just get in touch.

Down's one significant weakness is moisture. When down clusters get wet - from rain, snow or perspiration - the interlocking filaments collapse and clump together, losing their ability to trap air. A saturated down jacket can lose up to 90% of its insulating value. This has historically been down's great compromise: unbeatable when dry, dramatically compromised when wet.

DownTek™ is a PFC-free hydrophobic treatment applied to each individual down cluster that fundamentally changes this relationship. The treatment causes moisture to bead and roll off the clusters rather than being absorbed by them. Down treated with DownTek™ absorbs 33% less moisture than untreated down, retains 66% more loft when wet, and dries significantly faster. All Jöttnar down jackets use DownTek™ treated down.

It doesn't make down waterproof - nothing does - but it extends the performance window in marginal conditions considerably, and makes down genuinely viable in the wet, unpredictable weather characteristic of British and Alpine mountain environments. Worn under a waterproof shell, a DownTek™ treated down jacket is an entirely credible choice for serious wet-weather mountain use.

For a full technical explanation of how DownTek™ works, see our separate DownTek™ guide.

Down is held in channels called baffles that prevent it from shifting and clumping
in one area while leaving cold spots in another. The construction type significantly affects thermal performance.

Through-stitch (sewn-through) baffles: The shell and lining are stitched directly together. Simple and has the lightweight advantage, but the stitching can create thin lines with no insulation - potential cold spots in sustained cold.

Box baffles: Three-dimensional baffles that maintain down loft without the cold-spot problem of through-stitching. More complex, heavier, more expensive, but better thermal performance. Used in deep cold expedition jackets.

Chevron and shaped baffles: Directional baffle patterns that prevent down migration and optimise warmth in specific body areas.

Different baffles within the same jacket may carry different amounts of down fill, depending on what performance that zone is required to deliver. This is body-mapped construction - engineering the jacket's thermal profile rather than simply filling it uniformly. Some jackets also use synthetic fill in specific zones, such as the shoulders (to resist rucksack strap compression) or at the front zip collar (where exhaled moisture is highest), with down in the primary insulation zones.

Down and synthetic insulation are not in direct competition - they serve different needs. Understanding when each is appropriate is more useful than arguing which is 'better'.

Choose down when weight and packability are priorities; conditions are predominantly dry or cold; you want maximum warmth for minimum bulk; or long-term performance matters. Down lasts decades. Synthetics degrade.

Choose synthetic when you'll be active in sustained wet conditions without a shell; you need insulation that performs in sustained damp without management; or budget is a primary constraint.

For a full comparison, see our separate Down vs Synthetic insulation guide.

The down industry has historically had welfare concerns around live-plucking and force-feeding. The Responsible Down Standard (RDS) addresses this directly - auditing the entire supply chain to confirm down is not sourced from force-fed or live-plucked birds.

All Jöttnar down is responsibly sourced, fully traceable, non live-plucked and DownTek™ treated for performance without PFCs - the chemical class historically used in hydrophobic treatments that carries environmental impact.

Storage: Store uncompressed. Long-term compression damages the clusters' ability to loft. Hang in a wardrobe or store loosely in a large breathable bag - never in
the stuff sack.

DownTek™ refresh: Tumble-drying on low heat reactivates the DownTek™ treatment. If you notice water absorption increasing, a short tumble dry will usually restore performance.

For full care instructions, see our separate Down Jacket Care guide.