Does a Silk Pillowcase Keep You Warm in Winter?

Silk has a reputation as the hot sleeper's friend. The cool cheek of the pillowcase, the fibre that never seems to hold onto the heat. Which raises a fair question, now that the nights are long and the electric blanket has come back out of the cupboard. If silk keeps you cool in summer, does it leave you shivering in a Dunedin July?

It does not. Silk keeps you warm through winter, and it does so without ever turning stuffy or damp. The trick is the quietly clever thing about the fibre. Silk does not choose a temperature and cling to it. It regulates.

In short: yes, a silk pillowcase keeps you warm in winter. Silk is a temperature-regulating protein fibre. It holds a thin layer of still, warm air against your skin while drawing off the moisture that makes other fabrics feel clammy. So it reads the room, cool when you are hot and warm when you are cold, and dry either way. A denser 22 momme silk feels cool for the first minute, then settles to your body's warmth.

One fabric, two jobs

Most fabrics do a single thing to temperature. Silk manages it in both directions. The thing you are really chasing in bed is your microclimate, the small envelope of warmth and humidity held against your skin under the covers. Get that envelope wrong and you know at once. Too warm and damp, and you wake at 3am flinging off the duvet. Too cold, and you never quite drop off.

Silk governs that envelope because of what it is. It is a protein fibre called fibroin, and each filament has a smooth, roughly triangular cross-section, the same shape that gives silk its low, shifting sheen. Woven up, those fibres hold a fine layer of still air against you. In summer the structure sheds heat. In winter that same trapped air becomes a light blanket of its own, keeping your warmth close without the dead, sweaty weight of a synthetic.

Why "warm but not clammy" is the whole trick

The other half of the story is moisture, and it matters more in winter than people expect. Textile scientists put silk's moisture regain at around eleven percent, and note that it can take up roughly a third of its own weight in water before it even begins to feel wet. In practice that means silk lifts the small amount you perspire overnight off your skin and lets it evaporate, rather than holding it against you. Pile a heavy duvet and a wool throw over most fabrics and you get the muggy, slept-in feeling by morning: warm, but clammy. Silk keeps the air next to you warm and the surface dry at the same time. Cosy, not stifled.

There is a skin dividend, too, in a New Zealand winter of heat pumps and dry indoor air. Cotton can be thirsty to the point of drawing moisture out of your skin and hair through the night. Silk sits closer to equilibrium, so you wake a little less parched. A small mercy in the season of chapped everything.

So why does a new silk pillowcase feel cold when you first lie down?

Because a good silk pillowcase briefly draws the warmth off your cheek, and your skin reads that as cold. A customer wrote to us this winter, a little puzzled. Her new pillowcase was a lovely fit, but it felt too cool against her face to sleep on, and, strangely, cooler than the thinner silk one she already owned. Nothing was wrong. She had simply met silk doing its job.

The measured thermal conductivity of silk is only a fraction of a watt per metre-kelvin, lower than cotton or linen. On first contact the fibre carries a little of your body heat away, which your skin registers as a cool touch. A moment later the surface has warmed to you and stays there. What feels like coldness is really the fabric equalising, quickly, with your own temperature.

Why does a heavier silk feel cooler than a thin one?

Here is the part that surprises people. The better, denser silk is the one that feels colder at first. It sounds back to front, but it follows from the very same physics. A heavier 22 momme pillowcase simply has more silk in contact with your face, so it moves a touch more warmth in that opening minute and takes a little longer to come up to temperature. A thin, lightweight silk has less fibre to it, warms almost at once, and so never feels as cool. That initial coolness, in other words, is a quiet sign you are lying on plenty of real silk. It is not a fault.

On a genuinely cold night there is an easy fix, and it is the one we give customers. Slip the pillow under your duvet for a few minutes before bed, or rest a hand on it while you read. You take the first chill off, the silk meets you already warm, and it holds that warmth for the rest of the night.

Getting the warmth you're paying for

The weight of the silk does the work, which is why it is worth knowing what you are buying. Momme is silk's unit of weight, and a pillowcase around 22 momme has the substance to insulate properly while keeping that signature slip (here is what the number actually means). Thinner, cheaper silk has less to work with, in warmth as in wear. It is the same fibre we explain in our guide to mulberry silk, the same property that runs from a single pillowcase up to full silk sheets, and back down to why silk is such a friend to hot sleepers in January. One fibre, quietly reading the room.

Our silk pillowcases are 22 momme, 100% mulberry silk, if you would like to test the theory against the next cold snap.

So: cool when you are hot, warm when you are cold, dry either way. Not magic, just a very old fibre doing the one thing it has always done beautifully, which is to keep company with human skin.

Common questions

Does a silk pillowcase keep you warm in winter? Yes. Silk holds a thin layer of still, warm air against your skin and keeps your body heat close, while wicking away the moisture that makes heavier fabrics feel clammy. You stay warm without overheating.

Why does my silk pillowcase feel cold when I first lie down? Silk briefly carries warmth away from your skin on contact, which reads as cool. Within a minute the surface warms to your body temperature and stays comfortable for the rest of the night.

Why does your 22 momme silk feel cooler than my thinner silk pillowcase? Because it contains more silk. A denser weave moves a little more heat on first touch and takes slightly longer to warm up, so it feels cooler at first. That initial coolness is a sign of quality, not a defect.

How do I take the chill off on a very cold night? Tuck the pillow under your duvet for a few minutes before bed, or rest your hand on it while you read. The silk warms up and then holds that warmth all night.