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I prefer long sleeve t-shirts, since my skin and too much sun is not good. I wonder what are your experiences using merino wool (or any other material) in hot summer hiking days? I guess 18 - 30 degree Celsius or 64 - 86 F temperature range.

Update: I will hike in the mountains, so a combo of direct sun, hot rock and lots of ascents.

I am currently using cotton or very light synthetic. Cotton seems warmer, and I sweat more, but I wear cotton because it can stay wet longer and wet t-shirt cools me better in the long run.

When I use thin synthetic t-shirt I don't sweat so much, or get hot so fast, and it dries quickly but I doubt when I hike a lot, then extra sweat won't cool me in the long run, but would evaporate. I haven't tried merino wool yet.

What are your experiences?

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    In my experience, an oversized cotton shirt is best. That is, several sizes too large.
    – ab2
    May 16, 2022 at 22:33
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    "Extra sweat won't cool you in the long run, but would evaporate" - evaporation is exactly how sweat cools you. Sweat is of course at body-temperature when it comes out of you, and wearing a shirt soaked in body-temperature liquid doesn't do a thing to cool you down, since it's as warm as you are. As that liquid evaporates, though, it absorbs heat to go from a liquid to a gas, cooling you down. This is why sweating on a humid, muggy day is particularly uncomfortable, because the sweat doesn't evaporate well, meaning you don't cool as much.
    – Nuclear Hoagie
    May 17, 2022 at 19:14
  • @NuclearHoagie correct; but that is beneficial to the wearer only if the wet shirt is in the contact with the body. If it is loose, shirt itself would cool via evaporation, but skin would remain hot and get hotter (as air being heated by muscle action via skin would not be cooled by shirt enough, and wet shirt would act as an effective barrier keeping hot humid air trapped in and preventing further evaporation from skin).
    – Matija Nalis
    May 18, 2022 at 0:23
  • Did you consider short sleeves and a powerful sunscreen?
    – FluidCode
    May 19, 2022 at 11:55

6 Answers 6

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My experience is: The sooner you get rid of all the cotton when hiking, the better. Especially when it comes to longer or multi-day hikes.

Merino dries faster and doesn't smell. You can get through a two week hike with two shirts and even save a few grams over a comparable cotton shirt. It feels cooler when its hot and if it gets cooler and you supplement it with another layer, it feels warmer than a cotton shirt which is more prone to soaking with sweat. If it does get a littler cooler, the first layer I like to add is a long-sleeved cycling under-jersey beneath the merino shirt. Usually made out some magic synthetic poly material designed to transport sweat to the outer layers.

You also seem to have some misconceptions about sweating. If you sweat, it's your body telling you "I'm too hot I need to expend (valuable) water to cool down". So if you have a choice, avoid sweating more than you have to, you will have to replenish water and electrolytes.

[..] but I doubt if you hike a lot then extra sweat won't cool you in the long run, but would evaporate.

What do you mean by that? The sweat evaporating on your skin is what is cooling you. That is why merino shirts or synthetic cycling jerseys etc. feel cooler than cotton shirts. They don't hold on to water as much as cotton does, it evaporates faster and therefore you feel cooler.

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  • "What do you mean by that?" I thought that if cotton shirt absorbs a lot of water, sweat that extra water in cotton will cool me. That is like you hike in a wet shirt that cools you.
    – Alex J.
    May 17, 2022 at 14:55
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    The water has to evaporate in order to cool. A wet shirt just means that evaporation is delayed by the shirt. So you need to produce more sweat until cooling sets in, and you still get cooled down when you don't need it anymore, e.g. when you sit down on the summit.
    – user24582
    May 17, 2022 at 15:21
  • Not so sure the evaporation is delayed. There’s certainly no requirement for the entire T-shirt to be wet first before it can start to dry up in the sun. That said, I also find merino wool way more comfortable.
    – Sebastiaan van den Broek
    May 24, 2022 at 2:47
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Other answers have addressed the thermal issues well; I'll tackle something different.

I prefer long sleeve t-shirts, since my skin and too much sun is not good.

Bad news: bleached cotton is quite transparent to UV, and it gets even worse when it's wet.

You'll want to do some web-searches on Ultraviolet Protection Factor (UPF), which indicates how much UV radiation (both UVB and UVA) a fabric allows to reach your skin.

Try to choose clothing with a high UPF if you're trying to keep the effects of the sun off your skin.

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First off, 64 and 86F are VERY different temperatures. I'm ideally in completely different clothes for those temperatures.

For the upper end my go-to is a 100gsm wool blend hooded shirt made by Voormi. I like it better than any pure cotton or synthetic options I've tried. On the lower end I'm probably switching to a 120gsm wool blend short sleeve shirt (they also make long sleeve versions) made by Ortovox if the UV index is low enough (basically just not high altitude).

Compared to cotton, wool does a better job with temperature regulation on both ends of the temperature spectrum. In warm weather it breathes well creating an efficient layer of material to aid in the evaporation of sweat to cool you down. There is a saying the "cotton kills" because of it's poor moisture regulation performance. In really hot weather (~100F) it is preferred by some people because it won't wick moisture which can feel cooling. Over an extended period of time though I find even at extreme temps wool to be better since eventually the water in the shirt heats up and the "benefit" of cotton wanes.

Compared to synthetics, wool will have similar performance, but I find wool to be much more comfortable feeling against my skin. There is definitely some level of personal preference here. Anecdotally I find wool does a bit better at the "shoulder temps" compared to synthetics where the breathability doesn't seem to kick in until there is already moisture. Synthetics can be a bit more durable, but wool shirts lasts me years and when they do need to be replaced are much more environmentally friendly overall.

Compared to both it doesn't get smelly which is a huge bonus. The only real downside to wool in my opinion is cost. Some of the high end blended shirts have unbelievably high retail prices.

Another aspect to consider is fit. I prefer a slightly looser cut in warm weather.

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    Interesting thing is about bedouins in the desert. They wear a lot of clothes and they are loose. Seems that loose in that case is good, because it puts more distance between outer fabric and skin and if there is wind it creates so called "chimney" effect and heat trapped between the 2 layers is removed.
    – Alex J.
    May 17, 2022 at 19:58
  • Upvoted for the first sentence already. The question is basically two or three questions (18°C - 24°C - 30°C, each with their specific clothing demands).
    – Peter - Reinstate Monica
    May 18, 2022 at 11:54
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Generally in the outdoors, cotton's rotten.

Cotton is hydrophilic. When it gets wet it stays wet for a long time, sucking our heat and energy as it tries to dry. In short, wet cotton clothing is a fridge. Especially if there is any wind.

In a hot dry environment, this might be desirable. 18-30°C is not a hot dry environment.

For those termperatures, I'd use either merino wool (which I find is cool on hot days) or nylon/polyester. For example, I have some of these Macpac travel shirts, which I love.

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    It has always boggled my mind, I live in a warm country (30s and 40s summer right now) and cotton shirts feel better any day than a polyester.I dont understand your logic.
    – manu muraleedharan
    May 17, 2022 at 11:39
  • The shirts you link to claim to be made of "a blend of nylon and elastane", not merino wool nor polyester.
    – dotancohen
    May 17, 2022 at 12:11
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    "18-30°C is not a hot dry environment." Perhaps it is no the hottest, but I will hike in the mountains. That means that there will be little to no shadow from trees, lots of direct sun, hot rock and ascends. Ascents in that conditions are quite a strain and you get hot a lot.
    – Alex J.
    May 17, 2022 at 14:57
  • One remark here: while this is 110% true of living in drier, temperate, climates, wearing synthetics over a period of time in tropical countries can also cause skin irritation and rashes. I lived for about 8 years as a kid in the Caribbean and quickly learned to prefer cotton to synthetics, at least those available in the 70-80s.
    – Italian Philosophers 4 Monica
    Jun 5, 2022 at 20:12
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I actually like cotton in these conditions - but not T-shirt material.

Button-up shirts are made of a thinner, less stretchy and less absorbent material, and when travelling (day hikes more recently, but trekking in the past) I often have one to keep off the sun or an evening breeze. They may be pure cotton or polycotton, even linen blends occasionally. I go for a size or two bigger than I really need.

Yes, they wick less than synthetic, but they also cling less, and I find clinging fabric very unpleasant - getting a breeze under the shirt is preferable. They dry pretty quickly compared to cotton T shirts, but not as quickly as the better synthetics

My synthetic tops are hard to get non-stinky, especially without a proper wash; after a few days of just rinsing or hand-washing in a basin with shower gel they're not pleasant, but cotton tops seem to handle it better.

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As the old saying goes: 'In the cold, cotton kills'. As others have stated in this discussion, when cotton gets wet, it retains its moisture. When the temperature drops, it will freeze. Wool may be more insulating (run hotter), but it has the advantage of not retaining moisture and so will be better when (or if) the temperature drops.

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