How Fabrics Perform By Temperature

Today is the first day of April, and it’s also the day that it already feels uncomfortably humid in New York. By 1 pm, it was in the low 80s with cloudy skies and a thunderstorm warning in effect. But I’m actually on a plane right now towards Southern California where oddly enough it’s colder. So packing has been a bit of a puzzle.

But anyways, while I have some time mid-flight on my flight from LaGuardia to Denver, I put the words some of the research that’s been underpinning Terlingua Threads. And we wanted to do it in a helpful way by framing what fabrics are best for each temperature.

And for this post, I wanted to organize it by temperature bands rather than organizing it by material. Usually, advice about clothes for being outside begins with what they’re made of – merino wool is breathable, polyester moves sweat away from your skin, and cotton is “bad news” (by the way, cotton gets far too much hate in the outdoor industry). But this advice doesn't usually start with what your body is doing at a particular temperature and how different materials help or hurt.

Less than ~ 40ºF / ~ 5ºC

When it's below 40°F, your body is completely focused on keeping the heat it makes. Choosing clothes at this temperature is about being warm (trapping air around your skin) and dealing with sweat in a way that doesn’t make you colder.

Wool, and especially merino wool, is fantastic for this. Wool is made of keratin (a protein) and inside the fibers are chemicals that connect with water vapor. When your sweat turns into vapor and enters the wool, this connection creates a little bit of warmth (about 1.1 kilojoules for every gram of moisture soaked up). This is called ‘the heat of sorption’, but basically, wool gives off a small amount of heat as you move in cold, damp weather. A kilogram of dry merino wool in very humid air makes roughly as much heat as an electric blanket left on for eight hours. And it stays warm even when wet, doesn't smell without being treated with chemicals, and you can wear it for several days in a row. It’s really hard to do better than wool in cold weather. I have many layers from Patagonia, Arcteryx, and even Uniqlo for these situations. They and many other brands are truly excellent at what they do.

Polyester fleece has a use here as well; it traps air, doesn’t lose its shape when wet, and dries quickly and can work as a middle layer under a waterproof coat. Where the saying that “Cotton kills” comes from is the fact thatit soaks up moisture easily but doesn’t provide any warmth when it is wet. Water fills the spaces between the fibers, and as water lets heat pass through it about 25 times faster than still air, wet cotton lets your body heat escape.

TL;DR - Wool is king when it’s cold. Don’t use our gear here at all, not even as a winter layer.

~40ºF / ~5ºC to ~75ºF / ~24ºC

From 40°F to 75°F is a comfortable range. Your body doesn’t have to work too hard to stay at the right temperature. You sweat a moderate amount, and the weather isn't trying to freeze or overheat you. Most materials are good enough here,and this is why “all-season” clothing is often marketed for this temperature. If you only go hiking in the Pacific Northwest during the spring and autumn, or you’re mostly outside between 50 and 65°F, almost everything you can buy in an REI has been made for you.

Wool and merino are still very good. At this temperature, the heat of sorption is either helpful or doesn't make a difference, it still manages sweat, and it’s useful when the weather is changing (cool mornings becoming warmer afternoons, dry spells turning into a little rain). This is where wool is most comfortable and where it’s gained its good reputation.

Cotton is fine for wearing casually. In fact, cotton is (in my personal opinion) the most comfortable material for this weather. A nice supima cotton shirt is about as good as it gets for a breezy, partly cloudy day in the 60ºs.

Some material scientists will say that hemp can work in this temperature, but for Terlingua Threads’s HeatFirst Design, we have felt too cold when testing in this temperature, especially when windy (too much air will pass through). So we don’t recommend using our gear for this range.

TL;DR - Most “all-season” clothing you’d find is for here, but this is not Terlingua Threads’ optimal temperature range.

~75ºF / ~24ºC to ~90ºF / ~32ºC

From 75°F to 90°F, things begin to change. Your body is now relying more and more on cooling by sweat. You sweat a lot more and keeping warm (which was helpful in colder temperatures) is now a problem. Instead of asking if your clothes can help you keep heat, you should ask if they help you get rid of it.

Wool starts to have problems in the warmer part of this range. By the mid-80s, the ‘heat of sorption’ that kept you warm in the winter now works against you. You're sweating more, the wool is absorbing that sweat (up to 35% of its weight when dry), and each gram of sweat absorbed releases a little heat back towards your skin. It's not a huge effect - you wouldn't say the wool is heating you up, but you might feel warmer than you should be for what you're wearing. It also gets heavier as it absorbs water, and you will notice the weight over a long day.

Linen is perfect for this temperature. It's been used for hundreds of years in hot places like the Mediterranean and the tropics for a reason. It lets air flow through it, feels cool against your skin, and its cellulose fiber structure doesn’t release as much heat when it absorbs moisture, unlike wool. The downside is it isn't as tough: linen wrinkles, gets damaged by rubbing from backpack straps, and doesn’t hold its shape as well as tougher plant fibers.

For wearing around town and doing less strenuous things, linen is a good option. In fact there’s a reason why it’s so prevalent in a lot of formal wear - it feels nice and styles well.

This is the temperature where Terlingua Threads can begin to feel more comfortable. Because breathability is becoming more important, and hemp's cellulose structure deals with moisture without making you warmer. Though for temperatures between 75 and 90º, both linen and hemp are strong players in this category.

Polyester, however, starts to fail in this temperature range. Polyester is a plastic fiber made from oil and doesn't absorb water. It’s designed to move sweat between the fibers (capillary action) rather than soaking it up. When it’s not terribly hot, the air around you can completely evaporate your sweat once it gets to your clothes. But as the temperature and humidity go up, the sweat gets to the outside of your shirt and then spreads around on the fabric, not evaporating fully - you're sweaty, but not cooler. You’ll find this with a shirt that should be cooling you down, yet still feels sticky.

Frankly, you’ll find even cotton will be more comfortable than polyester beginning in this temperature range.

TL;DR - Terlingua Threads and linen clothing perform equally well, with light wool and cotton layers following closely behind

Above 90ºF / 32ºC

Things really change at over treading 90°F, and this is why Terlingua Threads was created. The average temperature of your skin is around 93°F. Once the air is as warm or warmer than that, two of your body’s three main ways of losing heat (convection and radiation) pretty much stop working. In fact, they reverse, with the hot air then putting heat into you (I talked about this in my post about Death Valley). Evaporation is all that's left: sweat goes onto your skin, turns into a gas, and takes heat with it. That’s the whole process and anything your clothes do in these temperatures either helps or hinders it.

Wool is a problem when it’s over 90°F. The same property that makes it warm in January now releases heat back to your body which is already struggling to get rid of heat. It’s creating this hotter-than-expected environment your body now needs to deal with on top of the heat from the environment. Plus, wool loses strength when wet, becoming easily damaged at the very time your clothes are under the most strain.

TL;DR - There’s a reason why shepherds shear their sheep for the summer to prevent them from overheating.

And about the "cotton kills" we brought up before: in hot weather, polyester is even worse. The outdoor and sports industries have for years thought of “dry” as being the same as “cool”, and polyester is the material that embodies that idea. It moves moisture to the surface, feels dry on your skin and is lightweight and stretchy. But “dry” and “cool” aren’t the same. At 100°F your body needs sweat to stay on your skin or very close to it long enough to evaporate and take heat away. Polyester pushes the moisture outwards, where it either spreads across the fabric and stops evaporating in humid air, or drips onto the ground before it can. Either way, you've lost the cooling effect of the sweat. That’s why polyester can feel dry and yet be horribly stuffy and unbreathable no matter how thin it is; the dry feeling hides the fact it’s not working. And when you add the fact that bacteria grow on polyester, it sheds tiny plastic pieces with every wash, and the UV and antimicrobial chemicals it uses wear out, the idea of it being a “performance” fabric starts to look shaky.

Linen is good for less demanding situations: going from the office to a restaurant, walking around the city, or on a boat. But for long periods of hard work outdoors (hiking with a heavy backpack, working in the sun) it will fall apart under friction and loses its shape when wet. We made some prototypes with linen (and bamboo as well), but ultimately found it wasn’t durable enough under the abrasion of backpacks.

Terlingua Threads’s fabrics were developed to trap less heat than wool (tests show wool releases 20 to 50 percent more heat than hemp when exposed to humidity). Our fabrics absorbs water and spreads it out across the fabric, keeping it in contact with the air so more of your sweat can turn into cooling vapor. It naturally attracts water, so it works with the thin film of moisture on your skin, rather than pushing it away as polyester does. And hemp gets more durable when wet (the opposite of wool), so the fabric lasts better the harder you use it.

Being Forthright

If you’ve been seeing my posts on social media or my articles on here by now, it should be clear that we’re trying to be as forthright as possible with what Terlingua Threads will and won’t do. It’s not a good winter layer. But on the opposite end of the spectrum, wearing Terlingua Threads gear on a 105°F day in Saguaro or Zion won't feel like standing in air conditioning. Any brand saying that their clothing in temperatures like those is “ like AC for your pits” (and sometimes put more crassly) is belittling your intelligence. What our clothes do is work with your body’s cooling system, keep more of the evaporation that your sweat is meant to provide, and stay in good condition for hours of hard work in conditions that ruin other fabrics. The aim is to be a little more comfortable and much safer. In extreme heat, even a small advantage is important.