1. Warmth: With many crimps per inch, wool forms a countless number of almost microscopic air pockets in between individual fibers. While the overall fabric is breathable, these miniscule pockets trap body-warmed (“dead”) air. As with sleeping-bag insulation, trapped air keeps a person warm.
What about damp conditions? A property unique to wool is its ability to release small amounts of heat as it absorbs water. This effect is known by the arcane term “heat of sorption.” Energy, in the form of small amounts of heat, is produced through the work of moisture-absorption (and condensation) by wool fibers.
Interestingly, as noted in the next section, this effect is reversed as the fiber dries. Thus by absorbing and releasing controlled amounts of moisture and heat, merino wool buffers the body’s microclimate in changing environmental conditions. This leads to manufacturer assertions that merino wool delivers “natural air conditioning” —a lofty claim, yet one that seems to have merit.
2. But not too much warmth (in other words, temperature control in changing conditions): Everyone associates wool with warmth. But can it really give a person the best of both worlds, providing comfort in both warm and cool conditions?
So it appears. (After all, sheep manage pretty well in warm weather, perhaps a survival attribute developed over centuries.) But wouldn’t the body-warmed trapped air, which warms me when it’s cool, cause me to overheat when it’s hot? No, wool proponents tell me, because wool tends to work like an insulated mug:
Wool fibers, while highly breathable, buffer skin from air heated by the sun the same way they can trap warmed air and keep it close to skin in cool conditions.
By efficiently managing the humid microclimate created between skin and clothing (see details in the Moisture Management section below), wool keeps skin regulated for comfort in hot conditions.
Based on my anecdotal experience, I can’t argue with those assessments.
3. Moisture management: Wool has the capacity to both absorb perspiration (in vapor form) while also resisting water (in a liquid state such as light rain). Admittedly, this is another of those you-can-have-it-both-ways claims that sounds too good to be true, but wool proponents insist they have the science to back it up.
While the exterior layer of a wool fiber is hydrophobic (water-resistant, that is; see next section for an explanation), its inner layer, its cortex, is hydrophilic (water-loving). The cortex can absorb about to one-third of its weight in moisture (estimates range from 27% to 36%), typically without feeling damp. It may, however, feel a few grams heavier in this condition.
On a perspiring person, in the high-humidity zone between fabric and skin, sweat in the form of vapor is absorbed by the wool fiber’s cortex. Meanwhile, the hydrophobic exterior layer of the fiber next to your skin minimizes the chance that wet fabric will come in contact with your skin.
The moisture absorbed inside the cortex? It is impacted by air and sunlight on the outside of the garment. By being exposed to a lower-humidity zone, moisture is released through evaporation, and the fabric feels drier. The wearer thus feels comfortable.
Synthetic fabrics “wick” moisture by transporting it through very narrow air spaces between hydrophobic (water-avoiding) fibers. This requires sweat vapor to condense before it is moved through the fabric—an extra step, wool proponents assert, that makes synthetics slower than wool’s moisture-management process.
4. Water-resistance: The scaly exterior layer of a wool fiber is called the cuticle and is overlaid with the epicuticle, itself coated with lanolin, a waxy, water-shedding film. The epicuticle and its waxy coating is what confers wool’s resistance to mist and light rain. This structure also helps make the fiber resistant to stains, mildew and mold.
