Animal Fibers vs Plant Fibers - When Do You Use Each?

I love geeking out on research. I hate relying on “vibes” or whatever to justify why something outperforms something else. If we say that ABC is better than XYZ, I want to know why. Anyways, by now we know that synthetic fibers are absolute bull for the hot weather. So then we’re left with natural fabrics. But they operate in completely different ways, and understanding how they operate differently starts to make it clear how they excel in their respective use cases.

So, quick primer - natural textile fibers all fit into one of two main types depending on what they’re made of at a tiny, molecular level. Animal fibers (wool, merino, cashmere, alpaca) are constructed from keratin, a protein that’s also what your fingernails and hair are made of. Inside each animal fiber are lots of ‘sticky’ parts - carboxyl, amino and hydroxyl groups - which have a slight electric charge and really want to connect with water.

Cellulose fibers (hemp, linen, cotton, and lyocell/Tencel which is made from plant material) are made of long chains of sugar units (polysaccharides). These also have hydroxyl groups that attract water, but there are fewer of them for each amount of fiber, and they don’t bond as strongly.

But I’m not writing this just to bore you with details in cellular structure of fibers… I’m here to bore you with the difference in how they react to your body and the amount of heat they give off in the process.

Animal Fibers - Keratin

So once again, we’re talking about merino wool & alpaca wool (super underrated, btw). When you sweat, the moisture leaves your skin as vapor. With wool, this vapor goes through the waxy outer layer (which actually repels liquid water) and into the keratin inside. The water then sticks to the ‘sticky’ parts inside the fiber, and this sticking is a chemical reaction that releases heat.

It’s not a tiny amount of heat either; wool gives out roughly 1.1 kilojoules of heat for every gram of water vapor it absorbs. The fiber can hold 30%-ish of its weight in water before it feels wet because the water is held within the fiber, not just sitting on the surface. (Fun fact - a kilogram of dry wool exposed to very humid air gives off approximately the same amount of heat as an electric blanket on for eight hours). Material scientists call this ‘the heat of sorption’’ (yes, sounds like “heat absorption” when said out loud). This is why merino and alpaca wool is FANTASTIC in the winter.

Picture that you’re on a hill in November, walking in a light rain at 38°F. Your body is making heat from exercise, but losing it to the cold, wet air. Wool catches the vapor from your sweat, keeps the moisture inside where it doesn’t feel wet, and releases a little burst of warmth between the fabric and your skin. The fiber is actively warming you as the weather tries to cool you down. Also, wool’s natural curliness creates air pockets in the fabric, and it’s still air that actually insulates clothing; wool keeps those air pockets even when it’s damp, unlike cotton which flattens.

This is why wool has been the best fiber for cold weather for thousands of years. We wholeheartedly endorse wool for the winter. And keep in mind – we don’t sell anything with wool.

Plant Fibers - Cellulose

Hemp and linen are bast fibers (from the stem of plants) that contain cellulose. When moisture meets their hydroxyl groups, they bond with less energy than takes place with animal fibers. There aren't as many bonding spots on a molecular level as in keratin, and so less heat is released when the moisture is absorbed.

Cellulose fibers also deal with absorbed moisture in a different way. Instead of trapping the water inside the fiber like keratin does, cellulose spreads it across the fiber's surface and along its length. This means the moisture is available for air to take it away, which is important when considering what your body is trying to do when it's hot.

For the record, all of the fabrics at Terlingua Threads are cellulose-based. Our woven items are 100% hemp. Our knitted items (like the White Sands hooded sun henley) are a mix of hemp and tencel, which is a cellulose fiber.

The Temperature Ceiling

When it’s cold, your body is trying to keep heat in. Wool’s heat-releasing reaction adds warmth to a system that’s losing it. The way it works is in tune with what your body is doing.

But when it’s hot (especially above 90°F - which is close to the ~93°F / 33ºC of human skin temperature), your body's situation is reversed. Two of the three main ways your body loses heat (convection and radiation) pretty much stop working when the air around you is as hot as your skin. When you're in strong sun, the heat you get from your surroundings can actually be more than your body is giving off as heat. This means the environment is actively warming you up and the only way to cool down is through sweat. You sweat, it gets to your skin, turns into a gas, and takes heat with it. That’s all there is to your body’s temperature balance at that point.

Now, if you put wool on, the wool soaks up that sweat vapor inside the fibers, and as it does, releases 1.1 kilojoules of heat for every gram of sweat it holds. Your body was trying to get rid of heat using the moisture, but the wool has turned it back into heat. Because wool holds a lot of moisture (more than any other fabric commonly used), it keeps absorbing sweat and creating heat as long as you continue to sweat, and the fabric gets heavier. This effect gets worse and worse as time goes on. So basically, in the summer, you’re wearing your own Finnish sauna… except they’re dry, so never mind.

Cellulose fibers (like linen or hemp) don’t have this problem in high temperatures because they don’t produce much heat when they soak up moisture. Hemp, for example, takes in moisture, spreads it around near the surface where the breeze can get to it, and lets the sweat evaporate as it should. It’s a way for your body to get cool, and doesn’ and doesn't work against it by adding heat.

Shepherds Already Know This

If you're finding the cellular explanation a bit hard to grasp, no worries whatsoever, it’s a bit nitty gritty, but we wanted to show that we’ve dug into this very deeply when figuring out how to create heat dissipating clothing. But to make it a bit simpler to grasp, think about the people who deal with wool every day: sheep farmers and shepherds.

All over the US, sheep are sheared in the spring - usually February to May - and specifically before summer gets going. They do this deliberately. Farmers leave around an inch of wool to protect against the sun's rays and to block some of the sun's heat, but they get rid of the full weight of the winter coat. Agricultural research from universities shows that sheep with about an inch of fleece manage the summer heat better than sheep who are either fully covered in wool or have just been completely shorn. That shorter wool blocks the sun, but doesn't trap too much heat.

If you leave all the wool on all the way through July, you can clearly see the consequences: the sheep get heat stressed, move about less and eat less, are more likely to get parasites, and can even be in real danger from the heat. Domestic sheep have been bred over many years to continually grow wool. Unlike their ancestors in the wild, they can’t get rid of it themselves. The farmer has to make that decision for them, depending on the time of year.

We feel that the most honest way to look at wool is to view it as a truly excellent three-season fabric. And an excellent one at that. We’d happily wear it from October to April.