Down vs Synthetic Duvet: Which Tog Feels Warmer?

The Tog Number Doesn't Tell the Whole Story

Picking a duvet by tog number alone works right up until two shoppers buy the same 13.5 tog rating in different fills and end up with two noticeably different beds. One is light enough to barely register under the arms; the other presses down with real, settled weight. Both are rated identically for warmth, and both shoppers are right — because tog measures thermal resistance, not the two dozen other things a fill does once it's actually spread across a mattress.

One tog, under the test method set out by the British Standards Institution's BS 4745, is defined as a thermal resistance of 0.1 square metre kelvin per watt — a laboratory measurement of how well a material resists heat loss, applied identically no matter what the fill is made from. A 10.5 tog duck feather and down duvet and a 10.5 tog polyester hollowfibre duvet have, by definition, passed the same standardised test at the same number. What the test doesn't measure, because it was never designed to, is weight, breathability, moisture handling, or how a duvet settles across two people who run at different temperatures overnight. That's where "down" and "synthetic" stop being interchangeable labels for the same number and start behaving like genuinely different products.

Two Fibres, One Job

Down is a natural insulator: the soft under-plumage from ducks or geese, made up of fine, three-dimensional clusters rather than the flat quills of a feather. Those clusters trap pockets of still air between them, and still air is what actually insulates — the down itself isn't warm, it's just extremely good scaffolding for holding heat in place. Synthetic fill, most commonly hollowfibre or the finer microfibre, is manufactured polyester fibre engineered to do the same job: trap air in a springy structure that mimics what down does naturally, at a fraction of the cost and with none of the sourcing complexity.

Both approaches can be built up to any tog rating a manufacturer wants. The difference is in how much material it takes to get there. A down duvet reaches a given tog rating at considerably less weight than a hollowfibre equivalent, because down clusters trap air more efficiently per gram than a manufactured fibre can manage. That single fact is the root of almost every practical difference between the two fills — everything else in this comparison traces back to it.

Comparing Down and Synthetic Where It Actually Matters

Which? tests duvets using real sleeper panels rather than lab measurements alone, covering both natural-fill and synthetic products across the price range. Its findings are a useful corrective to the assumption that fill type settles the warmth question on its own: a well-chosen synthetic can match a natural-fill duvet on measured warmth and comfort, with the differences that actually show up falling in weight, feel and longevity rather than in how warm either one measures on test. In other words, the "which feels warmer" question isn't really a warmth question at all once the tog rating is fixed — it's a weight, breathability and construction question.

Weight and Drape

This is the difference most sleepers notice first and describe least accurately. A heavier duvet at the same tog rating doesn't sleep warmer in any measurable sense, but it can feel warmer, because more mass pressing down against the body reads to most people as more warmth. Down's lighter build at equal tog is why it's often described as feeling less oppressive; a hollowfibre duvet at the identical rating can feel like more insulation simply because there's more physical fill between the sleeper and the room, even though the standardised test says the two are thermally equivalent.

Breathability and the Overheating Question

Fill type also affects how a duvet handles the warmth it's trapping, not just how much it traps. Down's open, three-dimensional cluster structure generally allows more air movement through the fill than a denser synthetic fill permits, particularly at the lower end of the synthetic price tier. That matters because — as the same Which? testing on the UK duvet market makes clear — comfort and temperature regulation drive buyer satisfaction more than the raw tog number does. A fill that holds warmth in without letting much of it circulate can tip a correctly rated duvet from "warm enough" into "too warm," which is a more common complaint among UK buyers than the reverse.

Allergies and Care

Synthetic fill has one advantage that has nothing to do with warmth: it's hypoallergenic, which matters directly to anyone who reacts to natural down or feather, and it's generally machine washable at home in a standard domestic machine. Natural down and feather fills, by contrast, typically need a large-capacity machine or a specialist clean to avoid damaging the loft or unbalancing the fill — a real ongoing-care difference that a spec sheet rarely spells out up front.

Price Tiers Follow the Fill, Not Just the Brand

Down fill costs more to produce than synthetic fill at every tog rating, and that difference shows up consistently across UK retailers regardless of which name is on the label — a duck feather and down duvet at a given tog rating carries a real premium over a hollowfibre duvet at the same number, before brand positioning is even factored in. That premium reflects the supply chain rather than markup alone: natural fill is harvested, cleaned and graded rather than manufactured to a fixed spec, which is a slower and more variable process than extruding polyester fibre to weight. Worth knowing before comparing prices between retailers — a "premium" price on a down duvet is often close to what the fill itself costs to bring to market, not padding stacked on top of a fair synthetic price.

Construction Matters More as the Fill Gets Heavier

Whichever fill is chosen, how it's stitched into the duvet changes how it behaves over a season of use. Stitched-through construction — channels of thread running straight through both the outer casing and the fill — is the cheaper, more common method at the value end of the market, but it creates thin cold seams where there's effectively no fill at all. Baffle-box or box-wall construction, where internal fabric walls separate fill compartments without stitching all the way through, keeps the fill distributed evenly and avoids that problem. It matters more for down, which can migrate and clump inside a poorly divided casing, but a heavier synthetic duvet benefits from the same construction for the same reason: more fill means more opportunity for it to shift around unless something is holding it in place.

Which Fill Actually Suits Which Sleeper

None of this makes one fill objectively better — it makes them suited to different sleepers at the identical tog rating. A warm sleeper who already runs hot overnight is usually better served by down's lighter build and better airflow at any given tog, because the risk that matters most to them is overheating, not under-warming. A sleeper who wants low-maintenance care, or a household managing allergies, gets more practical value from synthetic fill even though it means carrying a bit more weight for the same warmth. Neither choice compromises the tog rating itself — that number is fixed by the standard either way — it's a choice about how that same warmth gets delivered.

For a deeper look at how the tog scale itself breaks down by season, the 13.5 tog vs 15 tog comparison works through where the UK's warmest ratings actually earn their keep, and the Silentnight vs Slumberdown comparison covers how two value-tier brands split their own ranges across both fill types at the tog numbers discussed here. The Dunelm duvet review goes further into how construction quality specifically varies within one retailer's synthetic and natural-fill tiers.

Where to Check Before Buying

Retailers stocking both fill types side by side make the comparison easiest to run in practice. John Lewis lists tog rating and fill type as separate filters across its full range, and Dunelm does the same across a wider spread of price points. Checking both filters together — tog first, fill second — on the exact rating a bedroom needs is the most reliable way to see the real weight and price difference between a down and a synthetic duvet at the identical warmth level, rather than guessing from marketing copy alone.

References

• Which? duvet reviews and guides
• BS 4745, thermal resistance of textiles (ANSI Webstore listing)