If you have shopped for thermals, you have faced the same choice: a synthetic thermal for around twenty or thirty dollars, or a merino base layer for considerably more. The price gap is large enough that it deserves an honest explanation rather than a sales pitch.
Both fabrics work. They work differently, and they suit different situations. Here is the actual comparison.
How each one handles moisture
This is the real difference, and it is not about warmth.
Synthetics — polyester, polypropylene and the branded thermal fabrics — are hydrophobic. They do not absorb water. Instead they wick it along the surface of the fibre to the outside of the garment where it evaporates. This is fast and effective, and it is why synthetics dominate high-output sport.
Some thermal fabrics go further and use the moisture actively: they are engineered so that body vapour is converted to heat within the fabric. It is clever technology and it genuinely works, particularly in dry cold.
Merino absorbs moisture vapour into the core of the fibre — up to roughly a third of its own weight — before the surface feels wet at all. It also releases a small amount of heat as it absorbs. The effect is slower than synthetic wicking but more even, which is why merino feels stable across changing conditions rather than fast in one direction.
Where synthetics genuinely win
It would be dishonest to pretend otherwise. Synthetics are:
- Cheaper, by a wide margin
- Faster drying — meaningfully better if you are getting properly wet
- More durable — they resist abrasion and pilling better than fine merino
- Easier to care for — they tolerate a hot wash and a dryer
If you ski once, in dry cold, and you want a thermal that survives being thrown in a machine with everything else, a synthetic thermal is a sensible purchase. There is no shame in it.
Where merino wins
- Odour. This is the big one. Synthetics harbour the bacteria that cause smell, which is why a synthetic thermal is usually a one-wear garment. Merino resists them naturally and can be worn several days running. On a week-long trip that is the difference between seven base layers and two.
- Temperature range. Merino regulates in both directions, so the same garment works in a cold cabin and a warm restaurant. Synthetics are better at shedding heat than holding it.
- Next-to-skin feel. Fine merino — under about 19 micron — bends against the skin rather than pressing into it. Synthetics can feel clammy once damp.
- It is a natural fibre. Biodegradable, renewable, and it does not shed microplastics in the wash.
The micron point, because it matters
If you have tried wool and found it itchy, you almost certainly tried coarse wool. Micron measures the diameter of the fibre. Above roughly 24 micron the fibre is stiff enough to press into the skin and trigger the itch response. Fine merino sits well below that.
So “wool is scratchy” is really “that wool was too coarse”. Worth knowing before you rule out the category on one bad jumper.
Weight, and what to actually buy
Base layers are sold by fabric weight in gsm:
- 150–170gsm — light. Good for high-output activity and mild conditions
- 200gsm — the versatile middle. Warm enough for a cold morning, not so warm you overheat in a mountain restaurant. This is what we make
- 250gsm+ — heavy. Genuinely cold, low-output conditions only
Most people over-buy on weight and regret it the first sunny day on piste.
The honest verdict
Buy synthetic if you are on a tight budget, you ski rarely, or you are doing something high-output and wet where fast drying matters more than anything else.
Buy merino if you are travelling, if you want to pack light, if you will wear it for several days between washes, or if you want one garment that handles a plane, a mountain and a cold morning at home. The cost per wear closes the gap faster than the sticker price suggests — particularly if you are buying two merino sets instead of seven synthetic ones.
Ours is 200gsm Australian merino, mulesing-free, cut and sewn rather than knitted to a generic block. See The Merino Base Layer Edit, or read more on why merino works so well for travel.