Why Wool: The Extraordinary Properties of Merino Wool

Wool in my collection.

Imagine yourself a primitive man, stone tipped spear in hand, slogging through the rain of prehistoric Britain on the hunt... Fast forward a few thousand years. You're a Roman Legionary, marching north toward the snowcapped Alps to meet the barbarian hordes at the empire's edge... Now it’s January 1915, and you along with the rest of your brave crew of explorers are stranded on a vast ice-flow in the Antarctic, after your ship froze solid and sank... At last its 2017 and you're a backpacker on the PCT, braving the High Sierra despite the earlier-than-expected snowfall, and warnings from other hikers to turn around.

What do all these seemingly unrelated scenarios throughout history have in common?

Wool, the super-fabric spun from the fleece of sheep (and other animals) that has been keeping humans warm in the outdoors since the Stone Age. It is a fabric both ancient and advanced, as no man-made synthetic material yet has been able to duplicate all of wool's extraordinary properties. Indeed, wool has been the fabric of choice for military's, explorers, adventurers, and outdoorsmen since ancient times. 

If you’re out of the loop, you might associate wool with childhood memories of itchy socks or smelly old army blankets. I'm here to tell you that today’s Merino wool garments are the crème-dela-crème of next–to-skin comfort. Merino are a breed of sheep prized for their fine wool, which is considered the softest in the world. But its more than just soft. Merino wool is both water resistant and water absorbing, and will retain its insulating properties when wet. It’s flexible, durable, and stain resistant. It’s antibacterial, anti-microbial, and hypoallergenic. It’s a flame retardant and provides protection from UV rays. I know, it sounds like some super space age material that might be used for the construction of rocket ships, but I assure you, I'm talking about all-natural wool that comes from some extremely stalwart mountain dwelling animals. 

It starts with the impressive, but complex nature of the wool fiber itself. The crimped shape of the fibers creates millions of air pockets when packed together, which trap heat escaping from the body, significantly reducing convection. The fibers are so durable that folding or compressing them will not affect the pockets of air, so the insulating property works equally well whether you're hunkered down in the tent or making a bid for the summit. 

What really makes wool such an amazing fiber is its ability to manage moisture. The naturally hygroscopic fibers absorb 30% percent of their own weight in moisture (10 times more than any synthetic) without feeling damp. So, when you sweat, the moisture from your skin is absorbed into the wool, and pulled to the surface where it evaporates, keeping a dry layer of air next to the skin. Since evaporation of perspiration is how the body naturally cools, the process works to keep you warm in cold conditions, and cool in warm conditions, making wool an ideal fabric for any environment. Studies have shown that just sleeping with a wool blanket can significantly lower the next-to-skin humidity 71% of the time. This incredible quality also allows wool to retain its insulating properties when wet. One study showed that a merino wool garment can absorb 60% of its weight in water before it feels wet.

Bottom of the Grand Canyon in a wool shirt.

Another benefit of wool's water absorbing super power is something called “the heat of sorption”. In this process absorbed moisture in cold conditions release stored energy in the wool fabric that generates heat. One source suggests that just a single kilogram of Merino wool generates heat of sorption “equivalent to the output from an electric blanket over eight hours”.

All this means that wool is not only superior at moisture management, it also excels at thermal regulation. Your core temperature won’t fluctuate as much. You won’t overheat, and you'll sweat less. Another study by the University of Otago in New Zealand concluded that when compared to other fabrics “Merino wool was better at minimizing body regulatory fluctuations which may result from changes in exertion or the environment. Wearing Merino wool fabric appears to lead to generally lesser physiological stress during exercise in both hot and cold conditions.”

Wool fibers also contain a natural wax called lanolin that is excreted by sheep through the skin as a water repellent. Lanolin not only helps you stay dry, it has natural antibacterial and anti-fungus properties as well. The absorption of wool, combined with the benefits of lanolin, and the fact that bacteria has difficulty growing on the scaly wool fiber itself, means that your Merino wool garments will remain odor free even after extended use.

Did I mention wool was a flame retardant? Because so much water can be absorbed into the fabric, wool will not catch fire easily, and is self-extinguishing when it does, which means you won’t burn a hole in your shirt if you get tagged by an ember standing by the fire. Wool can also can protect you from dangerous ultraviolet rays. Studies of 236 clothing textiles from the University of Bochum in Germany found wool to be the only fabric to pass European standards for UV protective clothing.

No doubt wool is an incredible fabric, and the reason humans have been using it so long is obvious. In today's outdoor world, Merino wool clothing remains the ideal choice for next-to-skin layers. In quality Merino wool garments, the comfort and performance are second to none, but beware: Merino wool is easily mixed with other fabrics. An item claiming to be made from Merino wool might only be 50% Merino in actuality. Often times these garments will be mixed with a synthetic and claim to possess the benefits of both. Just read the fine print, shop smart, and trust your name brands. It’s not a coincidence that brands like Smartwool and Icebreaker sit at the top of the Merino wool food-chain. They know their wool!

Perfect conditions for wool active-wear.

Disclaimer - The article is my original work. It first appeared in The Mountain Blog in a 2013 article called "Wool the Super Fabric". Recently when I tried to reference the article I could not locate it online. It in fact appears as if The Mountain Blog is now defunct, as I could not find any reference or mention of it on Mountain Gear's website. I decided to reprint the article here with minor changes as a reference for people searching for this information.