How Wool Stays Warm When Wet: Heat of Adsorption Explained

Does wool keep you warm when wet? Mostly yes, and for a reason you can put numbers on. Wool fibre takes water vapour into its structure and releases heat as it does so — roughly 960 kJ for every kilogram of water it absorbs — while keeping its loft. That is why a damp wool sweater feels warm and a damp fleece feels cold, and why a wool sleeping bag holds its rating on a humid night when a down bag does not. This article explains the mechanism, works through what it means over one night in a tent, and is honest about where it stops.

Absorbent or not: is wool absorbent, and does it matter

Two different things happen when wool meets water

Textile scientists distinguish absorption (liquid soaking into the spaces between fibres, the way a cotton towel works) from adsorption or sorption (water molecules bonding to sites inside the fibre itself). Wool does the second very well and the first rather badly, and that combination is the whole story.

The outside of a wool fibre is a layer of overlapping scales coated in a thin waxy film. Liquid water beads on it and runs off, which is why rain sits on a wool coat rather than soaking straight in. The inside of the fibre is a protein cortex full of chemical sites that attract water vapour. Vapour passes between the scales, bonds to those sites, and the fibre swells slightly to accommodate it.

How much water wool holds

Published moisture-regain figures, measured at 65 % relative humidity as a percentage of the fibre’s dry weight: wool 13–16 %, silk 9–11 %, cotton 7–8 %, nylon 4–4.5 %, acrylic 1–2 %, polyester 0.4 %. Wool can go on taking up vapour well beyond that, to around a third of its weight, before the fibre surface has any free water on it and it starts to feel wet to the touch.

That is a large reservoir. Two pounds of dry wool batting can take up on the order of a quarter of a litre of water as vapour without feeling damp; polyester fill of the same weight would hold a few grams inside the fibre and leave the rest sitting between fibres and on your skin.

Heat of adsorption: where the warmth comes from

Bonding a water molecule to a site in the fibre releases energy. Wool’s heat of sorption is about 960 kJ per kilogram of water absorbed — close to half the heat you would get by condensing the same water from steam. The effect was first recorded in 1858 by the French scientist Coulier, who noticed that dry wool moved into a humid room warmed up. You can reproduce it: leave a wool sweater by a stove to dry thoroughly, take it into the damp outdoors, and it feels warm against the skin for the first hour in a way a dry fleece does not.

Now the overnight arithmetic. An adult gives off roughly 0.5 L of water over a night as breath and insensible perspiration. Not all of it goes into the bag, but suppose a third to a half is taken up by the wool liner and batting. That is 150–250 g of water, and at 960 kJ/kg it releases 150–240 kJ inside the bag, spread over eight hours — a steady 5–8 W. A resting adult produces about 80–100 W, so the sorption heat is a top-up of a few percent, not a heater. What makes it matter is timing: it arrives as the humidity around you rises, which is exactly when a down or synthetic bag is starting to feel clammy and lose ground.

The larger effect is not the heat released but the heat not lost. Because the water is inside the fibre rather than on it, wool batting keeps its loft and its trapped air, so its insulation value barely changes as it takes moisture up. Down collapses; polyester keeps loft but has water between the fibres conducting heat away. Wool’s rating on a damp night and its rating on a dry one are closer together than either alternative’s.

Wool, down and synthetic when wet

	Wool batting	Down	Polyester fill
Moisture regain (fibre)	13–16 %	Not usually quoted; clusters wet readily	0.4 %
Heat released on taking up vapour	About 960 kJ/kg of water	Negligible in practice	Negligible
Loft when damp	Kept	Lost; clumps	Mostly kept
Insulation when damp	Slightly reduced	Badly reduced	Reduced; water between fibres conducts
Feel against skin when damp	Dry until near saturation	Clammy shell	Clammy; water sits at the surface
Dries from body heat overnight	Yes, gradually	No	Slowly
Insulation when soaked	Poor; heavy	None	Some; wrings out
Drying time after soaking	Slow	Very slow without heat	Fast

What it feels like at 3 a.m.

You wake in a tent at 3 a.m. because the temperature has dropped and the humidity has climbed. In a down bag, the shell against your cheek is cold and slightly slick; you can feel the damp patch where your breath has been landing, and the loft over your chest has gone thin. In a synthetic bag the loft is still there but the shell feels the same, and your base layer is clinging. In a wool bag the liner against your face is dry to the touch, the bag feels the same thickness it did at bedtime, and if you pull the top over your head and breathe into it for a few minutes the air inside warms and the wool takes the moisture without going clammy. That last trick works only in wool; in a nylon-shelled bag it puts liquid water straight into the insulation.

One of our customers put the alternative memorably: “wet plastic clothes inside a wet plastic bag inside a tiny wet plastic tent, in 25 degree weather”, after a night in the Sierras that ended with a 19-mile walk out in the dark. It is not that synthetics failed a lab test. It is that everything in that system held the water at the surface.

The limits: wool delays wet, it does not defy it

Three things wool will not do.

• It will not stay warm once saturated. Past roughly a third of its weight in water, free liquid fills the spaces between fibres and the bag becomes a wet blanket: heavy, conductive and slow to dry. Dunked in a creek, a wool bag is a worse outcome than a synthetic one. Keep it packed under the canvas lid, not strapped on the outside, when rain is likely.
• It cannot take up vapour forever. Sorption heat is released as the wool goes from dry to damp. On a five-night trip in constant fog the batting may already be holding a good deal of moisture by night three, and both the reservoir and the heat release are smaller. Airing the bag each morning resets it; drying it fully by a fire or in the sun resets it completely.
• It does not replace a shell. Wool sheds drizzle and dew but is not waterproof. The Rewilder’s beeswax-treated organic cotton shell handles condensation and tent drip; it does not handle a leaking fly. Fix the fly.

Care and drying

• Every morning: open the bag flat over the tent, a branch or a rock for twenty minutes. Wind matters more than sun.
• After a wet trip: hang it open in a dry room or outdoors in shade for a day or two. Do not tumble dry. Avoid direct hot sun on the beeswax shell for long periods; it can soften the wax.
• Soaked: press (do not wring) the water out, then hang. Expect it to take a day or more depending on humidity. Near a fire is fine at a distance; wool is difficult to ignite and self-extinguishes, but the cotton shell will scorch.
• Washing: the merino liner is machine washable. The bag is spot-cleaned and aired; the user’s manual covers hand-washing when it is really needed.
• Storage: loose, dry and out of the straps. Wool recovers loft well but only if it is given the chance.

Where this leaves you

Wool does keep you warm when wet in the sense that matters on most nights: damp from dew, fog, condensation and your own breath, not submerged. It holds its loft, takes the moisture inside the fibre away from your skin, releases a little heat as it does so, and dries from your body overnight. The Rewilder (4 lb, 20°F field rating, US wool batting in a beeswax cotton shell) is built around that behaviour, and the merino liner brings a good part of it to any bag you already own. For the wider comparison, read Wool vs down sleeping bags and Why damp cold feels colder.

Reviewed August 2026. Moisture-regain and heat-of-sorption values are from published textile data; the overnight calculation is illustrative arithmetic, not a measurement; Lucky Sheep ratings are field ratings, not ISO 23537-1 results.