As the weather cools in the southern hemisphere and energy prices climb, many of us are trying to stay warm without cranking the heating.

The doona – or duvet, if you prefer – is central to that battle.

But with shelves stocked with everything from inexpensive polyester duvets to down doonas costing thousands, what’s actually worth your money?

Fortunately, there’s solid textile science to guide you – and it’s less complicated than the marketing suggests.

It’s mostly about thickness

Here’s what most doona advertising glosses over: all doonas of the same thickness provide roughly the same warmth, regardless of filling material.

That’s because a doona’s job is simply to trap air around your body – your own body does the heating.

So no, an expensive goose down doona is not automatically “warmer” than a cheaper polyester one from a discount retailer.

Thickness equals warmth – though in practice, determining how thick a doona actually is can be surprisingly difficult, as manufacturers often don’t provide this information.

But warmth isn’t the only thing that matters.

Down is the soft, fluffy clusters that grow next to a bird’s skin for insulation – distinct from the tougher outer feathers, which are heavier and less effective at trapping air.

Down creates enormous loft (thickness) with very little........

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