What a duvet is actually doing
Why the insulating material is the trapped air rather than the fabric.
A duvet keeps a sleeper warm not because the fabric or the filling itself holds heat but because both exist mainly to trap a thick layer of still air against the body, and it is that motionless air, a poor conductor to begin with, that actually does the work of slowing the body's heat from escaping into the room.
Miodownik's Stuff Matters gives foam a whole chapter and glass another, treating both like characters with histories rather than as mere materials, and it was the foam chapter that made the filling inside a duvet finally make sense, since almost none of what keeps a sleeper warm turns out to be solid material at all.
Filling as scaffolding for still air
A body constantly gives off heat, and with nothing between it and the open air of a cold room, that heat would conduct and convect away quickly, since moving room air keeps sweeping fresh cool air across the skin and carrying warmth off with it. A duvet's filling, whether down feathers or a synthetic equivalent, is chosen for how effectively its structure, a tangle of fine fibres or feather barbs, traps countless small pockets of air and holds them still. Still air has a very low thermal conductivity, so once a sleeper's body has warmed the air trapped in those pockets, that warmth has nowhere convenient to go. No current is sweeping it away, and no solid path is conducting it out quickly, because the solid material at any point is only a tiny fraction of what is there.
That fraction is smaller than most people would guess. Good down fluffs up to fill a space several hundred times its own solid volume, so inside a well-lofted down duvet well over ninety-five parts in a hundred of the space are air. The fabric and filling are the scaffolding that holds a great deal of air completely still. A single loose feather or a lone synthetic fibre does almost nothing to keep anyone warm, since there is nothing around it to hold air in place, and only once millions of them are packed together into a loft does the structure have enough tiny gaps to trap the air a sleeper needs held motionless.
Layers of jumpers and pockets of air
Wearing a few thin jumpers stacked on top of each other on a cold day works the same way. Each thin layer traps its own pocket of air between it and the next, so the warmth comes as much from the still air held between the layers as from the wool itself, and a stack of thin jumpers can keep a body as warm as a far bulkier single garment. Squeeze the same jumpers flat under a tight coat and they lose much of that benefit, because the gaps that held the air have been pressed out.
A duvet is the same idea built into a single object. Its loft of fibres or feathers is many tiny separate air pockets stacked together, and the more of those small still pockets it can hold without collapsing under its own weight, the warmer it keeps a sleeper. A pocket needs to be small to stay still, since a large open cavity lets the air inside it circulate, warm air rising on one side and cool air sinking on the other, carrying heat across the gap by convection.
Loft, draughts and the summer duvet
Because the trapped, still air is doing the insulating, anything that compresses a duvet, squashes its loft, or lets a draught pass through it works against its warmth far more than the choice of filling material ever could. A duvet crushed flat under a heavy weight, or one that has lost its loft after years of washing and use, loses much of its warmth even though the same amount of fabric and filling is still present, because the air pockets have been squeezed out or collapsed. A damp duvet fails for a related reason, since water soaking into the filling takes the place of some of that air and conducts heat around twenty times more readily. Shaking a duvet out before use makes a real difference for this reason, since restoring loft restores the trapped-air structure. A tightly woven cover matters as much as the filling, since a loosely woven or damaged one lets a draught pass straight through and carry away the still air the filling worked to trap.
Swapping the filling for a different material with similar loft changes very little, because the insulating job was always being done by the still air between the fibres. A lightweight summer duvet and a heavy winter one are usually built from the same materials in the same proportions, differing mainly in how thick a layer of trapped air each is designed to hold, and that thickness is a simpler and more useful way to compare two duvets than any number printed on a label.