For all our opposable thumbs, ability to reason, and interstellar ambition, we humans have a pretty narrow environmental survival zone. We do fine in warm, dry conditions — say, inside a temperature band that ranges from 50 degrees F to about 95 degrees F. But outside that environment, our bodies have trouble maintaining a core temperature.
The human body is a thermal engine that produces heat as it converts calories to energy. The by-product of the conversion keeps us warm when it's cold out, but as we use more energy, the body dumps excess heat in a bid to maintain its perfect internal temperature. The body's primary method of shedding excess heat relies on the principal of evaporative cooling; specifically, the body produces sweat on the skin's surface to speed cooling.
This process works fine when we're naked and sweat can evaporate. But when the skin is covered by insulation, two things can happen. First, sweat gets trapped against the skin, and, second, that moisture soaks the surrounding clothing. The result is evaporative cooling combining with conductive and convective cooling actions to produce a rapid cooling effect that, if unchecked, leads to hypothermia in cold climates.
So, the trick to staying warm has as much to do with managing sweat as it does with providing insulation. Maintaining a buffer of air next to the skin (called a microclimate in marketing circles) is the key to warmth, comfort, and, ultimately, survival.
When we think of a baselayer, we think its job is to provide insulation. But its primary role is actually to counteract the effects of cooling by managing moisture. In a proper layering system, a sweater or other lofty midlayer provides the primary insulation, while the baselayer keeps the skin dry. It's a team effort.
Since physics tells us air pressure forces moisture in the direction of cool, dry air, we need to maintain a small buffer of air that's warmer than the outside air, but cooler than skin temperature, to keep moisture moving in the right direction.
There are a bunch of fibers out there that provide loft and more than a few that can wick moisture away from the body. But none does both as well as wool. For all its itchiness, wool is a crazy combination of features that reads like a textile engineer's wish list. For example, it's both hydrophobic and hydrophilic. Weird.
Wool's major downside as a clothing fabric is its itchiness. The itch comes from the scaly surface of coarse fibers found on most sheep. But there's a genetic strain of sheep that produces a fine wool fiber that takes itch out of the equation. That family of sheep is known as the merino, and its bounty is nature's itch-free, super fiber: merino wool.
Merino wool does well enough as a thermal insulator. Its ultrafine fibers are naturally crimped so that when they're spun into a yarn, at a microscopic level, they interlock in a zigzag orientation that creates lots of small air pockets. Those air pockets hold air that's warmed by heat radiating from the skin. This quality is also responsible for wool's ability to stretch.
Where merino excels is at the important task of moisture management. A wool fiber's outer surface allows moisture vapor to pass into the fiber's hydrophilic core where it's trapped until it's released into a cooler, drier environment. Each fiber can hold as much as 35 percent of its own weight in liquid in the fiber's cortex, or innermost core. Without oxygen in the cortex, there's no way for stinky bacteria to breed, so smells are held at bay.
Now for the science fiction; wool fibers are hydrophobic, too. They repel standing water and prevent, to a certain extent, the kind of wet-out that cotton is known for. Since the wool yarns can maintain a matrix without collapsing like wet cotton, they continue to trap and warm some air … as long as you're still putting out heat. This explains wool's warmth-when-wet ability. Along this line, we've also read some science journals that suggest whenever wool absorbs moisture into its cortex, the chemical reaction produces a tiny bit of heat. We can't disprove this idea, but we've never felt a perceptible boost in warmth when jumping into a frosty lake while wearing merino, either.
We already touched on odor control, but wool is also quiet and nonreflective. And, lastly, wool's capacity to hold moisture means it's flame resistant. It's not flameproof, but its self-extinguishing, no-melt, and no-drip qualities make it far better than synthetics in fire-prone environments.
The base fiber of a merino yarn is tiny sheep hairs, chemically treated, washed, and dried, then either spun into yarn alone or combined with other fibers to make an intimate blend. These merino yarns, either alone or alongside yarns of other fibers, are knit into a cloth. The thinner the fibers, the more supple and itch-free the yarn. Synthetic fibers are sometimes mixed in to add other properties to the finished product. For example, merino isn't the most durable fiber, so it's often blended with nylon to add strength to the fabric. Wool garments are also known to steadily shrink when washed over their lifetime, and they don't dry quickly. Merino blends are formulated to help with these issues, too.
A textile that's managed to capture merino's benefits while mitigating its shortcomings is Polartec Power Wool. “The idea behind power wool was to put the wool where it makes the most sense, next to skin,” says Karen Beattie, Senior Product Manager with Polartec, “and then mitigate the negatives using synthetics.”
Power Wool combines merino wool with synthetics such that only wool is in contact with skin and a smooth polyester fabric faces out, providing more surface area than wool alone for moisture to evaporate. The synthetic inclusion makes Power Wool more durable than 100-percent merino, and means it doesn't shrink, dries faster, and layers better thanks to the smooth outer fabric.
Clothing makers use fabric weight to describe the amount of warmth a garment provides. This is somewhat useful when comparing garments of similar fabric construction, such as flat knit jerseys. But in general terms, fabrics under 100 grams per square meter feel like a light T-shirt, 100-200 GSM baselayers feel like heavyweight T-shirts, and 200 GSM fabrics feel like a light sweatshirt. Anything around 100 is appropriate for warm weather; 150 -180 is a midweight and ideal for mid to high output endeavors in cold climates. 200+ fabrics are best for sedentary activities in cold temps or high output activities in frigid weather.
Once past the fabric level, the most important aspects of the baselayer are a close fit (the fabric has to be in constant contact with the skin to wick moisture), seam quality and location, durability, and the ability of the garment to stay in place. Oh, and dudes might like a pee slot. But every added seam means more expense and bulk. There's no free lunch.
We pulled a bunch of merino and merino blend baselayers together to learn what we can about the way fabric choices, garment constructions, and other features combine to keep us warm and dry.