Does merino wool keep you warm when wet?

Merino does release heat when it gets wet, but far less than the claim implies, and it gives all of it back as it dries. Wool gives off about 100 to 115 joules per gram as it takes up water, so a 200 g base layer soaked in rain releases about 20 kilojoules: four minutes of a resting adult's heat output. Drying that top later costs about 18 times as much. Thickness and a dry outer layer decide whether you stay warm. Fibre comes a distant third.

Does merino wool keep you warm when wet, or is that a myth?#

The claim that wool "generates heat when wet" is physics, not marketing. Wool is hygroscopic: its protein chains carry polar and charged groups that bind water molecules, and binding releases energy. Zhou and colleagues, in a 2025 review of wool properties, put that heat of sorption at 100 to 115 joules per gram of dry fibre, varying with wool type. Polyester, with a moisture regain of 0.4 to 0.8 percent against 13 to 18 for wool (Kadolph's figures), takes almost no water into the fibre and has almost nothing of this kind to release.

The claim that it is a myth is also right, in the sense that matters on a hill. The heat is finite, it arrives once, it is small next to what you produce, and drying the garment takes it back with interest. In the two controlled human trials we could read, wet wool did not keep people measurably warmer than the other fibres.

So merino warms you a little while it absorbs water, then costs you more than it gave. What keeps you warm in wet clothing is what keeps you warm in dry clothing: trapped air, protected from wind.

How much heat does wet wool actually release?#

Put the published figure on a real garment. We use a 200 g long-sleeved merino top as the example; weigh your own and scale the figures. The arithmetic is ours.

The sorption heat repays about 5 to 6 percent of the evaporation bill. That is the whole case in two numbers: about 21 kJ in, about 389 kJ out.

There is a second route by which wool takes up water: as vapour, from sweat that has already evaporated from your skin or from humid air. Here the energy released is larger, because the vapour condenses into the fibre and gives up its latent heat as well. Take the same top from a regain of about 15 percent to the "one-third of its weight" Zhou gives as wool's upper limit. That is 36 g of water, and condensing it releases at least 36 x 2.257 = 81 kJ. Spread over half an hour, that is about 45 watts, which you can feel.

But look at where the vapour came from. If it was your sweat, your skin spent the same 81 kJ evaporating it a moment earlier. Wool hands that heat back to you near the skin instead of letting it escape, which is a real buffer when you stop walking. It is not a heat source, and when the top dries the 36 g leave and take the 81 kJ with them.

The amounts in practice are also smaller than that ceiling. Cernych and colleagues report that in an earlier trial (MacRae 2014) merino and polyester tops took up only 3 to 4 g of moisture during exercise. At 2.257 kJ per gram, 4 g is about 9 kJ: under two minutes of resting output.

What the human trials found#

Two studies put people in wet or sweaty wool and other fibres under controlled conditions, and neither found the advantage the label promises.

Bakkevig and Nielsen (1994). Eight men rested for 60 minutes at 10 degrees C and 85 percent humidity in a two-layer outfit. Before each test, 175 g of water was spread through the underwear. The underwear was polypropylene (dry and wet), and wet cotton, wet wool, wet polypropylene fishnet and a wet double layer of 47 percent wool and 53 percent polypropylene. The abstract reports "the significant cooling effect of wet underwear", that textile construction changed mean skin temperature significantly, and that "the thickness of the underwear has more of an influence on the thermoregulatory responses and thermal comfort, than the types of fibres tested". Wool was one of those fibres.

Cernych and colleagues (2017). Ten men and eleven women each cycled for an hour at 8 degrees C and then sat for an hour of recovery, once in merino long underwear and once in polyester, both rib knits from the same maker. The merino fabric had twice the thermal resistance of the polyester (0.053 against 0.027 square metre kelvin per watt by ISO 11092) and absorbed more water. Despite that head start, "both the PES and MER garment sets performed similarly" on most measures, and the differences found were mostly between men and women, not fabrics. Merino was linked to greater sweating in both sexes. In women, it was linked to a faster cooling rate and stronger shivering during recovery. The authors suggest the absorbed sweat behaved like a humid environment against the skin.

The same paper reports one trial that did favour merino (Laing 2008, running at 8 degrees C with an airflow of about 11 km/h) and a follow-up in which that benefit "disappeared" when an airflow of about 7.2 km/h was applied (MacRae 2014). Same fibres, different air movement, and the advantage was gone.

Why wet clothing gets cold, whatever the fibre#

Insulation is trapped still air. Water conducts heat about 23 times better than still air (0.6 against 0.026 watts per metre kelvin), so every gram that displaces air in a fabric short-circuits it. Fudge's review puts the size of the effect plainly: at 5 degrees C, wet clothes "may double the heat loss seen in dry conditions".

Then there is evaporation: water leaving your clothing takes 2,257 kilojoules per kilogram with it, mostly from you. So the amount a garment holds matters more than how it feels. At Cernych's absorption figures, two 200 g tops hold 172 g (merino) against 107 g (polyester): about 389 kJ against 241 kJ to dry, or 65 against 40 minutes of resting output.

Merino's real wet advantage is comfort, not heat. Cernych and colleagues note that wool "can easily absorb up to 30% of its weight in moisture without feeling damp or clammy", so a damp merino top feels less wet against the skin than a damp synthetic one. Feeling less wet is not the same as losing less heat, and the trials above did not find that it was.

Merino or synthetic for wet weather: a decision by situation#

Fibre is a small term in this sum. If you like merino for other reasons, wear it; just do not count on it once you are wet.

What actually keeps you warm in wet clothing#

In the order to do them:

1. Stop the wind. A shell over a wet layer restores the still air film outside it and stops air being driven through the wet fabric. What makes a shell keep working once it is soaked is covered in rain jackets and breathability.
2. Add thickness, not fibre. Bakkevig found thickness mattered more than fibre. For a stop, put an insulated jacket on over the damp layers rather than taking them off in the wind. Synthetic fill tolerates this better than down, for the reasons in down vs synthetic insulation.
3. Wring it out. Every gram removed by hand is 2.26 kJ you do not have to supply later. If wringing takes a soaked top from 172 g of water down to 86 g, that saves about 194 kJ, over half an hour of resting heat output. The test below tells you what your own wring achieves.
4. Keep moving until you can change. At walking output you can afford the evaporation bill. Sitting still in wet layers is when it becomes dangerous.
5. Keep one layer completely dry for camp and sleep, sealed in a bag: the same dry line that camping in the rain holds inside the tent. Swapping into it ends the bill entirely.

Test it yourself: weigh what your base layer holds#

You need a kitchen scale and a sink.

1. Weigh the garment dry, to the gram, after a day indoors.
2. Soak it fully in cold water for five minutes, lift it out and let it drip for one minute without wringing.
3. Weigh it again. The difference is the water it holds when soaked.
4. Wring it hard by hand and weigh a third time. That is what it holds after a field wring.
5. Convert to heat. Multiply grams of water by 2.257 to get kilojoules, then divide by 6 to get minutes of resting heat output at 100 watts (100 W is 6 kJ per minute).

Do the same with a synthetic top of similar thickness. This measures what actually separates them in wet weather: how much water each makes you evaporate. Expect different figures from the standard absorption test above, which uses its own method.

Common mistakes#

Treating "warm when wet" as a safety rating. The sorption heat for a 200 g top is about 21 kJ, roughly four minutes of resting output. It is not a reason to skip the shell or the dry spare.

Choosing a thin merino layer for cold wet stops. A thin merino top is still thin when it is wet. Thickness and wind protection did more in the trials than fibre.

Sleeping in the wet base layer because it "stays warm when wet". It will dry overnight on heat from you, inside your bag. Change into dry clothes, as the layering system recommends for camp.

Comparing garments by fibre alone. The merino fabric in Cernych's trial had twice the thermal resistance of the polyester and still performed similarly in most measures. Knit, thickness and fit change the result as much as the label does.

Where this answer stops applying#

The heat-of-sorption figure is for dry fibre and varies with wool type. The 86.2 percent absorption figure comes from one fabric and one test, so your top will hold a different amount: weigh it. The two trials used small groups (8 men; 10 men and 11 women), controlled rooms at 8 to 10 degrees C, and garments from a limited set of fabrics. For the 1994 study we read only the abstract, so we report its conclusions and not its numbers. Neither tested hikers in rain, wool blends with nylon, or wool mid layers. We found no study measuring wet merino against wet polyester insulation on a person walking in the rain, and nothing here should be read as one.

Frequently asked questions#

Is wool warm when wet?#

Wool releases about 100 to 115 joules per gram as it absorbs water, so it does give off a little heat while getting wet. For a 200 g top that is around 20 kJ, about four minutes of resting heat output. Drying it later costs far more. In controlled trials, wet wool did not keep people measurably warmer than the other fibres tested.

Does merino wool insulate when wet better than synthetic?#

Not measurably, in the trials available. Resting at 10 degrees C in wet underwear, thickness had more influence than whether it was wool, cotton or polypropylene. Cycling and resting at 8 degrees C, merino and polyester long underwear performed similarly on most measures, and merino was linked to more sweating. Choose by thickness and wind protection first.

Why does wool feel warm when it is damp?#

Wool can absorb up to about 30 percent of its weight in moisture without feeling damp, and absorbing it releases heat near your skin. That makes damp wool feel less clammy than damp synthetic fabric. The heat is mostly your own, returned: your skin spent it evaporating the sweat that the wool then took up. It leaves again as the garment dries.

How long does a wet merino base layer take to dry while wearing it?#

It depends on your output, the air and what you wear over it, so there is no single time. The energy is fixed, though. A 200 g merino top holding 172 g of water needs about 389 kJ to dry, which is around 65 minutes of a resting adult's entire heat output. Every gram you wring out first cuts that by 2.26 kJ.

Should I wear merino or synthetic hiking in the rain?#

Either works while you are moving under a shell, because your heat output dominates. For hard climbs followed by long cold stops, a thinner, less absorbent base layer with a dry layer for the stop holds less water for you to evaporate. If you are soaked through, neither fibre helps; changing into dry clothing is the only real fix.

Does wet clothing really double heat loss?#

Research cited in a 2016 Sports Health review found that at 5 degrees C, heat loss with wet skin and clothing can be twice that with dry clothing. That applies to wool as much as any other fabric. Water conducts heat about 23 times better than still air and has to be evaporated eventually, taking 2,257 kJ per kilogram with it.