Duvet fillings and tog ratings explained

What the tog rating tells you

The space under a duvet becomes a small climate of its own by the early hours. How warm it stays depends on how much still air the filling traps and how well it lets body moisture escape.

In the UK, duvet warmth is described by the tog scale, a measure of thermal resistance. A higher number holds in more heat. Light summer duvets sit at the bottom of the range, medium weights suit spring and autumn, and the highest ratings are meant for cold bedrooms in winter. The exact numbers used vary a little between makers, so compare like with like.

Tog says nothing directly about weight. Warmth comes from trapped air rather than from the mass of the fibre, so a lofty down duvet can match a much heavier synthetic one. Some people love a heavy duvet for its settled feel; others prefer to barely notice it. Both can be equally warm.

The room matters as much as the season. A well insulated, centrally heated flat may only ever need a light or medium tog. An old house with draughty windows may call for the warmest rating in midwinter. People who run warm, or who share a bed with someone who does, often choose one step lower than they first expect.

Animal fibres

Natural animal fibres have been used in bedding for centuries because of how they trap air and handle moisture. Each behaves differently.

Down and feather

Down is the soft undercoat of ducks and geese. Its clusters have no quill; fine filaments spread out from a central point and hold a great deal of air for very little weight. Goose down clusters tend to be larger than duck, which is why goose down is often associated with higher loft.

Feathers are flatter and have a quill. They add weight and body but less warmth for their weight. A duvet with a higher share of feather feels heavier and settles more closely; a higher share of down feels lighter and puffier. The proportions are usually printed on the label.

A tightly woven, downproof casing keeps the clusters inside. Daily shaking and occasional airing keep the filling evenly spread and help it recover its loft.

Wool

Wool fibres are naturally crimped, which gives a wool filling springiness and plenty of small air spaces. Its strength is moisture. Wool can take up water vapour from the body without feeling damp and release it again as the air dries, which makes it a steady choice for people who swing between too warm and too cold.

A wool duvet lies flatter than down and drapes closely around the shoulders, which cuts out draughts when you turn over. Some wool duvets are washable on a wool programme; others are not, so the care label decides.

Silk

Silk duvets are made from long silk fibres laid in layers to form a thin, even batt. The result drapes closely, is light, and feels cool when you first get into bed before settling to skin temperature. Because the fibres are layered rather than blown into pockets, the filling stays evenly spread. Most silk duvets are not designed for machine washing; airing and the cleaning method on the label keep them in good condition.

Plant-based and synthetic fillings

These fillings are often chosen for easy washing and for people who prefer to avoid animal products.

Bamboo and other cellulose fibres

Duvets sold as bamboo usually contain a fibre regenerated from bamboo pulp, such as viscose or lyocell. These cellulose fibres take up moisture readily and feel cool and smooth, so they suit warm sleepers and summer weights. Many are machine washable, but the label sets the limits.

Hollowfibre and microfibre

Synthetic fillings are made from polyester. Hollowfibre has a hollow core that holds air and gives the duvet bounce. Microfibre uses very fine fibres to imitate the softness of down.

Their great advantage is practical: most can be washed at home, often at higher temperatures than natural fillings tolerate, and they dry quickly. Their weakness is moisture. Polyester does not absorb water vapour the way wool or silk does, so a synthetic duvet that is too warm for the room can feel clammy. Choosing a lower tog than you would in down often solves this.

Allergies and sensitivities

Night-time allergy symptoms are often linked to house dust mites, which live in bedding and feed on shed skin, rather than to the filling itself. Mites thrive in warm, humid conditions, so a bed that is aired daily and kept dry helps. Washable covers, regular washing of the duvet cover and a well ventilated bedroom all play a part.

When down seems to cause a reaction, poorly cleaned filling is one possible reason, and a tightly woven casing reduces the dust that can escape. "Hypoallergenic" on a label is a marketing term, not a medical guarantee. People with a diagnosed allergy should take their doctor's advice; a duvet that can be washed often is a common recommendation.

One duvet or two

A single winter-weight duvet used all year leads to restless summer nights. An all-season set solves this with two duvets, one light and one medium, joined by press studs or ties. The light one does summer, the medium one does spring and autumn, and together they cover the coldest months.

The alternative is two separate duvets and a cupboard to keep one in. Either way, matching the warmth to the season does more for comfortable sleep than any single filling.

Frequently asked questions

What is a tog rating?

Tog is a UK measure of thermal resistance, meaning how well a duvet holds in warmth. Lower ratings are made for summer and higher ones for winter. A higher tog means warmer, not necessarily heavier.

Which duvet filling suits people who sleep hot?

Wool and silk handle body moisture well and are often chosen by people who wake up too warm. A lower tog in any filling also helps. Choosing the right warmth for the room matters as much as the fibre.

How does an all-season duvet work?

It is two duvets of different weights that fasten together. The lighter one is used in summer, the heavier one in spring and autumn, and the two together in winter.

Can people with dust mite allergies use a down duvet?

Some can. Well-cleaned down in a tightly woven casing, plus regular airing and washable covers, helps keep mites down. Anyone with a diagnosed allergy should follow medical advice; washable synthetic duvets are a common alternative.