Wool and synthetic base layers fail in opposite ways

Everything here earned its place by changing an outcome. Nothing about base layer fibres is included to round the number up.

What a base layer is for

The layer against skin has one job that really matters: moving sweat away from the body fast enough that evaporation does not happen on your skin. Evaporation is a cooling process, so any moisture left on the skin surface removes heat at exactly the moment the body has stopped generating much of it.

That is why the dangerous moment on a hill walk is the stop at the top rather than the climb, and why a base layer is a safety item rather than a comfort one. Warmth is supplied by the layers outside it, which trap air; the base layer contributes little insulation and should not be chosen as though it did. This framing explains most base layer decisions, including why a soaked cotton shirt under an excellent jacket still leaves a walker cold and miserable.

How wool behaves

Wool fibres absorb a substantial amount of water vapour into their internal structure while the surface still feels dry to the touch. That buffering slows the arrival of clamminess and smooths out the temperature swing between working hard uphill and standing still in wind. The same absorbency means wool holds more total water and takes longer to dry once genuinely soaked, which is its main practical weakness on wet trips.

Wool resists odour markedly better than most synthetics, which matters a great deal on consecutive days without washing and not at all on a single day out. Fine wool garments are less durable than synthetic ones of similar weight, and abrasion where pack straps sit is usually where they fail first.

How synthetics behave

Synthetic fibres absorb almost nothing, so sweat is moved along the fibre surfaces and spread across a wider area where it can evaporate more quickly. The result is fast drying and a garment that becomes usable again quickly, which is the decisive advantage on trips involving repeated soakings. The absence of absorption also makes the transition from dry to wet abrupt, so the clammy phase arrives sooner even though it also ends sooner.

Odour is the well-known drawback, because the fibre surfaces host bacteria readily, and treatments that suppress this generally lose effect with repeated washing. Against that, synthetics are cheaper, stronger and far more resistant to abrasion, which is why they dominate hard-wearing and high-output clothing.

Fit and weight do more than fibre choice

A base layer must sit close to skin to move moisture at all, since a loose garment leaves an air gap and sweat evaporates before it ever reaches the fabric. Too tight is also a problem, because a compressed layer restricts movement and can chafe badly under a pack hipbelt or a pair of shoulder straps.

Locally, fabric weight determines how much moisture a garment can hold and how quickly it clears, so a heavy layer worn in warm conditions is simply the wrong tool. Sleeve length, collar height and body length decide how much skin is covered and how much heat can be dumped by opening a neck on a climb. Anyone comparing fibres while ignoring weight and cut is arguing about the smaller variable and quietly ignoring the larger one.

Blends and treatments

Most base layers on sale are blends, aiming for the buffering and odour behaviour of wool with enough synthetic content to add strength and shape retention. Blends behave as intermediates rather than as a solution to both problems, and the proportions matter considerably more than the marketing language does.

Antimicrobial treatments on synthetics reduce odour to varying degrees, and manufacturers are usually clear that the effect diminishes across the life of the garment. Some blends add a small amount of elastic fibre to improve fit, which helps performance and slightly reduces drying speed and long-term durability. Reading the composition label tells you more about how a garment will behave in use than any name printed across the chest.

The exception is usually weather, which overrides most planning.

Matching the fibre to the trip

For a single hard day at a high output rate with a shower afterwards, synthetic makes obvious sense and its weaknesses never get the chance to appear. For several days without washing, in cool conditions and at a moderate pace, wool or a wool-rich blend makes those days considerably more pleasant for everyone. In cold conditions where sweat management becomes a safety question, many walkers use a thin synthetic layer against skin with wool above it to get both behaviours.

On very wet trips where nothing dries properly, drying speed outranks every other consideration, which argues for synthetic regardless of personal preference. The honest conclusion is that this is a matching problem rather than a ranking, which is why experienced walkers own both and do not argue about it.

Everything above, in order of what to do first

1. What a base layer is for. The layer against skin has one job that really matters: moving sweat away from the body fast enough that evaporation does not happen on your skin.
2. How wool behaves. Wool fibres absorb a substantial amount of water vapour into their internal structure while the surface still feels dry to the touch.
3. How synthetics behave. Synthetic fibres absorb almost nothing, so sweat is moved along the fibre surfaces and spread across a wider area where it can evaporate more quickly.
4. Fit and weight do more than fibre choice. A base layer must sit close to skin to move moisture at all, since a loose garment leaves an air gap and sweat evaporates before it ever reaches the fabric.
5. Blends and treatments. Most base layers on sale are blends, aiming for the buffering and odour behaviour of wool with enough synthetic content to add strength and shape retention.
6. Matching the fibre to the trip. For a single hard day at a high output rate with a shower afterwards, synthetic makes obvious sense and its weaknesses never get the chance to appear.

The takeaway

Pick the fibre for how long you will wear it and how often it will get soaked. Everything after that is preference.

Go slowly enough to notice, and most of this stops being advice and starts being obvious.