Down insulates by holding still air in a structure that collapses when wet

All insulation is a way of holding air still

Air is a poor conductor of heat, which makes it an excellent insulator, but only when it isn’t moving. Free-moving air carries heat away by convection, so every insulating material in a garment is really a structure for trapping air in small pockets and preventing it circulating.

That single fact explains why loft matters more than thickness of material. A thin, dense layer of fibre holds less still air than a thick, open one, and the thick open one will be warmer despite containing less substance. It also explains why compressing insulation destroys its function: squash the air out and you are left with fibre, which conducts heat perfectly well.

Down is exceptionally good at this. The individual cluster is a three-dimensional filament structure radiating from a central point, without the flat central quill of a feather, and it interlocks with its neighbours into an open lattice that holds a great deal of air for very little weight.

Fill power and fill weight measure different things

Two numbers appear on down products and they are frequently confused. Fill power describes the loft of the down — the volume that a given mass of it occupies when allowed to expand freely — and higher figures indicate larger, more mature clusters that trap more air per gram.

Fill weight is simply how much down is in the garment. A jacket with a high fill power and very little down in it is light and not especially warm, while a jacket with modest fill power and a lot of it can be warmer and heavier. Warmth comes from the two together, and quoting only one is a common way of implying more than is being said.

The other thing to look at is the down-to-feather ratio, since most fills are a mixture. Feathers have quills, which add weight, poke through fabric and provide much less loft, so a fill described as containing a substantial proportion of feather is a different product from one that is nearly all cluster.

Water is the failure mode, and it is severe

Wet down clumps. The clusters lose their independent loft, mat together, and the lattice of trapped air collapses, taking most of the insulation with it. A wet down jacket isn’t a slightly less warm jacket; it is close to no jacket at all, and it takes a long time to dry because the moisture sits inside a dense mat.

Water reaches down from two directions. Rain from outside is the obvious one, and the reason down garments are almost always paired with a shell that resists it. Moisture from the wearer is the less obvious one, and on a long, cold, hard day it can saturate insulation gradually from the inside without any rain falling at all.

Treatments exist that apply a water-repellent finish to the individual clusters, and they do measurably delay wetting out and speed drying. They do not make down indifferent to water, and like all such finishes they lose effectiveness with washing and wear.

Synthetic insulation trades warmth for wet performance

Synthetic fills are made from polyester fibre arranged either as loose staple clusters that imitate down or as continuous filament batting held in a sheet. Being hydrophobic, the fibres absorb very little water themselves, so a wet synthetic layer retains a good deal of its loft and therefore a good deal of its warmth.

The cost is weight and compressibility. For the same warmth a synthetic layer is heavier and bulkier, and it does not pack down as small. The other cost is time: synthetic fibres take a permanent set as they are repeatedly compressed, so batting gradually loses loft over years of use, whereas well-cared-for down recovers its loft for a very long time.

That gives a reasonably clear rule of thumb. Cold and dry, with weight and packability mattering, favours down. Cold and wet, or hard use where the garment will be compressed and abused, favours synthetic. Plenty of garments now combine the two, putting synthetic where moisture and compression are worst and down where they are not.

Construction decides whether the fill stays where it belongs

Down is loose material and it will migrate to the lowest available point unless the garment is divided into chambers. The simplest method sews the outer and inner fabrics directly together in lines, which is cheap, effective at holding the fill and creates a cold line at every stitch where there is no insulation at all.

Box-wall construction uses vertical strips of fabric between the two shells, so each chamber has depth across its whole width and there is no sewn-through cold spot. It uses more material and more labour, and it is why two jackets with similar fill weights can perform quite differently.

Shell fabric matters as well, since down works its way through anything with an open weave. Down-proof fabrics are tightly woven and often calendered, which is also why a garment that sheds a few small feathers from its seams is normal and one that leaks steadily is not.