Quick answer
A base layer's only job is to move sweat off the skin — and that's the one thing cotton cannot do. For genuinely cold or active days, superfine merino wool is the best next-to-skin layer for most children, with synthetics a cheaper, faster-drying alternative. Cotton is still the right choice for everything else: everyday wear, sleep, and mild weather where nobody is going to work up a sweat.
Every autumn the same question comes back around: does a four-year-old really need thermal underwear, or is a long-sleeve tee under a jumper fine? The honest answer is that it depends entirely on how wet the child is going to get — from rain, from snow, or from their own sweat. And since we make organic cotton clothing, it's worth saying plainly up front: cotton is a wonderful fibre, but it is not a base layer fibre.
What a base layer is actually for
The standard cold-weather approach is three layers, each with one job: a base layer that sits against the skin and wicks sweat away from it, a mid layer (usually fleece or knit) that traps warm air, and an outer layer that blocks wind and rain. Most parents get the mid and outer layers right and then undo the whole system with a cotton vest underneath.
Here's why that matters. Children run hot and move constantly — a walk to school turns into a run, a sledge gets dragged uphill. They sweat, and then they stop. If that moisture is still sitting in the fabric against their skin, they get cold fast, even under a very good coat. A base layer isn't about adding warmth so much as keeping the warmth you've already built from being cancelled out by damp.
Weight follows the season: lightweight base layers suit cool autumn and spring days, midweight the shoulders of winter, and heavyweight is really only needed for sustained below-freezing conditions. In the Netherlands, lightweight or midweight covers almost the entire year.
Merino, cotton and synthetic, side by side
| Merino wool | Synthetic | Cotton | |
|---|---|---|---|
| Moves sweat off skin | Yes — wicks and releases it as vapour | Yes — wicks fast | No — absorbs and holds it |
| Warm when damp | Yes | Partly — dries fast but feels clammy | No — feels cold and clammy |
| Smell after a long day | Naturally resistant; 2–5 wears between washes | Holds odour, often permanently | Neutral, but wash every wear |
| Cost | Highest | Lowest | Low to mid |
| Sheds microplastics | No | Yes | No |
| Best for | Cold, active, multi-day, sensitive skin | Sport, growth spurts, tight budgets | Everyday wear, sleep, mild weather |
Where cotton genuinely belongs
None of this makes cotton a poor fabric — it makes it a poor base layer. For the roughly nine months of the year when a Dutch child is walking to school, sitting in a warm classroom, and playing in the park rather than crossing a mountain, absorbency is a feature rather than a fault. Cotton is soft, it doesn't shed plastic in the wash, it takes hard wear and repeated washing, and it holds up well enough to be handed down.
The practical split we'd suggest: keep cotton for the everyday wardrobe and for sleep, and own one or two proper base layer sets per child for the genuinely cold, genuinely wet, genuinely active days. That's usually all a family needs — a wool base layer worn two or three times a week doesn't need duplicates.
But isn't wool itchy?
This is the objection we hear most, and the research on it is clearer than most parents expect. The prickle people associate with wool is caused by fibre diameter, not by wool itself. Coarse wool sits somewhere around 25–35 microns, thick enough to stay rigid against the skin and trigger the nerve endings that register prickle. Merino is typically 15–19 microns — the fibres bend on contact instead of pushing back, so that sensation doesn't happen.
A review of a century of research by an international group of clinicians concluded there's no credible evidence that wool is an allergen; where wool causes irritation, it's the coarseness of the fibre doing it. More usefully for parents of children with eczema, clinical trials have found that superfine merino worn next to the skin reduced the severity of mild-to-moderate atopic dermatitis in children, with benefits reported after a few weeks of consistent wear. The threshold cited for those benefits is a mean fibre diameter of 17.5 microns or finer, so it's worth checking whether a brand publishes its micron count.
If your child has genuine sensory sensitivities rather than eczema, the usual caveats still apply: seams, labels and cuffs matter as much as the fibre. We wrote about that in more detail in our fabric and fit guide.
Buying well, and making it last two children
Merino base layers are expensive enough that the maths only works if they survive more than one child. A few things help.
- Look for a welfare certification. The Responsible Wool Standard (RWS) and ZQ both audit farms annually and both prohibit mulesing. Neither standard is perfect — both still permit tail docking — but either is a meaningful step above uncertified wool.
- Fit close, not tight. A base layer works by sitting against the skin, so this is the one garment where our usual case for oversized cuts doesn't apply. Buy true to size and expect one to two seasons rather than three.
- Wash cool and rarely. Wool is odour-resistant; two to five wears between washes is normal. Use a wool or delicate cycle at 30°C or below with a wool-safe detergent, and turn garments inside out to reduce pilling.
- Skip fabric softener. It coats the fibres and blunts exactly the breathability and wicking you paid for.
- Never tumble dry. Heat and agitation, not water, are what shrink wool. Dry flat on a towel, reshaped, away from radiators and direct sun.
Treated that way, a merino set genuinely does pass down. If yours has been outgrown, our Twice Loved collection exists for exactly that kind of garment.
FAQ
At what temperature does my child need a base layer?
There's no single cut-off, because activity matters more than the thermometer. As a rough guide, lightweight base layers make sense from cool autumn days onward, midweight through the coldest stretch of winter, and heavyweight only for sustained sub-zero conditions. A still, dry 8°C school run rarely needs one; a wet, windy 8°C bike ride often does.
Can a cotton long-sleeve work as a base layer in a pinch?
For short, dry, low-effort outings, yes — plenty of children wear one all winter without trouble. The problem only appears once they sweat or get wet, because cotton holds that moisture against the skin instead of moving it away. If the day involves running, snow or rain, choose wool or synthetic.
Are synthetic base layers a bad choice?
Not at all — they wick well, dry quickly, stretch generously and cost a fraction of merino, which matters during a growth spurt. The trade-offs are that they hold odour, feel clammier when damp, and shed microplastics in the wash. A filter bag helps with the last one.
How many base layer sets does one child need?
One or two. Because merino can be worn several times between washes, most families find a single set covers ordinary winters and a second is only useful for skiing holidays or long stretches of outdoor play.
My child's merino top has shrunk. Is it ruined?
Often not. While it's still damp, lay it flat on a towel and gently stretch it back towards its original dimensions, then let it air dry completely. Prevention is easier: keep it out of the tumble dryer and off hot washes.
Does merino help with eczema, or is that marketing?
There is real clinical evidence behind it. Trials in children with mild-to-moderate atopic dermatitis found reduced severity when superfine merino was worn next to the skin, with 17.5 microns or finer cited as the threshold. It's not a treatment, and it won't suit every child — but it's a fair thing to try, ideally alongside your GP or dermatologist's advice.