The performance of Merino Wool is outperformed by synthetic base layers

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I’ve stopped wearing merino base layers. The change wasn’t deliberate, sudden, or driven by a deep concern for the wellbeing of rare sheep. The evolution of synthetic next-to skins has genuinely improved performance and it occurred naturally over time.

A unique mix of performance attributes have always made wool the go-to long underwear. It remains warm when wet, resists odor, actively wicks moisture away from your skin, and helps regulate temperature, keeping you warm when you’re resting and cool when you’re moving.

And the ways in which wool does all that can—if you forget that it’s a natural material for a moment—sound surprisingly high-tech. Take wool’s ability to actually Generate Heat, for example. While its scaly outer layer repels water droplets, a wool fiber’s hollow interior is actually hydrophilic: it attracts and absorbs water vapor. Once water is inside these fibers, its rough texture actually separates the hydrogen and oxygen molecules. This creates a chemical reaction that produces a small amount heat.

Merino carries many of the same performance attributes as regular wool; it’s just softer, so wearing it next to your skin usually feels a bit less itchy. Synthetic fabrics are becoming just as soft and cozy, with incredible technical attributes that go beyond the marketing hype. Consider me a convert.

Old Faithful

Sitka Core Lightweight (from $79

Sitka Core Lightweight tights with a T-shirt was my first introduction to synthetic base layer technology. Founded as a hunting brand that’s now expanding into general outdoors apparel, Sitka tries to prescribe precise layering systems to suit specific activities and weather conditions. The brand uses synthetic fabrics to engineer how heat and moisture move through multiple layers of clothing.

The Core Lightweights are made of very thin, waffle-pattern polyester in four-way stretch. The waffle texture can trap a bit of air on your skin making them feel warmer than they would. The fabric is so thin it can dry instantly even when it is completely submerged in water. This instantaneous drying can also be facilitated by the hydrophobic properties of polyester fibers. They are only capable of absorbing 0.4 percent of their body weight in moisture. To put it in perspective, cotton can absorb 24 to 27 times its body weight in water. Wool, however, can absorb 30 percent. And because water isn’t absorbed, its surface tension draws it across those fibers, spreading it out and allowing it to evaporate rapidly.

The Core Lightweights are great for working up a sweat. Rapid evaporation creates a cooling effect similar to wool but with less moisture retention.

How about odors? The perfect environment for naturally occurring bacteria to thrive and multiply quickly in warm, moist environments such as those created by sweating in base layers is warm, humid environments. To discourage bacteria growth, Sitka employs a fabric treatment composed of silver chloride that’s made by Swedish firm Polygiene. Silver ions can penetrate bacterial cell membranes and then bond with the DNA inside. This prevents them from replicating.

In short, the Core Lightweights provide insulation when you’re holding still, work to actively cool you through evaporative heat loss when you’re moving, and don’t stink—just like wool. They have a distinct advantage over wool in that polyester sheds moisture much faster than wool and is more durable. I’ve been wearing the same set of these (pictured) regularly for at least the last eight years, and they remain as good as new. Over the same period, dozens of ultralight layers of merino have also gone.

The Warm Ones

Forloh Deep Space (from $99)

I’ve been writing a lot about hunting apparel lately, because there’s currently a lot of technical innovation happening in that space that’s relevant to all outdoor enthusiasts. Forloh gear is not only made in America but also makes use of advanced technologies that may not be available to larger companies.

Deep Space layers are made out of a thick, heavy Polyester fabric and Spandex. This makes them tangible warmer and more flexible than Core Lightweights. Forloh then coats those fibers in Trizar, a ceramic material designed to reflect heat back toward the wearer’s body. It’s similar to the metallic linings used in some outerwear, except it’s woven into the fabric itself, so it’s not crinkly, won’t wear off, and doesn’t limit wicking. Forloh claims it makes the base layers feel 2 degrees warmer than they would without the coating.

A Polygiene silver chloride treatment is used in the company, as well as a new technology. Odor Crunch, a fabric treatment made with silica beads, is available. These particles bind to odor-causing molecules such as oxygen, nitrogen, or sulfur and then, through a chemical process, break them down into carbon dioxide, water. Polygiene makers say it deals with externally-sourced odors—wood smoke or cooking smells—as well as body odors.

Deep Space base layer layers, just like wool, provide more warmth than the trapped air and protect against odor-causing bacteria growth. That they’re quicker drying, able to shed externally-sourced odors, and very stretchy gives them a real advantage.

The Athletic Option

Beyond Bask L1 (from £70)

You’ll be familiar with Polartec Power Grid fabric from mid-layers like Patagonia’s ubiquitous R1. This material has about the same weight as a regular polyester fleece sweater. However, it features a grid pattern where squares of loft trap and insulate air, while thin dividing lines allow for easy breathability.

Basks are made with a lighter, thinner and lighter-weight version of Power Grid (pictured at the top). You can read a book through one of these. Beyond Clothing, based in Seattle, is trying to transfer the lessons from clothing designed for Special Operations Forces apparel to civilian athletes.

The material is so breathable, its more reminiscent of and constructed like Polartec Alpha, the fabric technology company’s most breathable mid-layer. Both have a mesh chassis that holds a small loft of polyester fibres together. The loft traps an amazing amount of warm air if you sit still. You can move freely and the loft will not hinder you as moisture and heat disappear outside. Pair the Basks with an Alpha mid-layer—Beyond makes several, but my all-time favorite remains the Rab Alpha Flash—and you’ll be uniquely well equipped for athletic endeavors in cold weather.

The inability of the polyester material to absorb water makes it able to wick moisture away from the skin, while still remaining dry. Power Grid’s lofted Polyester fibers have been engineered to be thin next to your skin and then expand as they travel outwards. It draws water away from the skin, spreads it, and encourages it to evaporate faster.

Basks were my first purchase in January. I have been wearing them almost every day since. For such a minimal, thin material, they’re surprisingly robust. According to the instructions, wash them in cold water and dry them on a flat surface. I’ve been afraid of poking holes in them with my fingers as I pull them on, but so far have had no such problems. Polartec doesn’t apply any odor-resistant treatments to Power Grid, but I’ve been able to wear the Basks daily for up to a week without them stinking.