In the cold, it’s the first layer against the skin that decides whether you stay dry and warm for the whole hike.
Thermal underwear | The first layer that decides your comfort
Thermal underwear is the base layer of the layering system: the garment in direct contact with the skin, whose main job isn’t so much to warm you as to manage sweat. Good thermal underwear absorbs moisture and carries it outward, keeping the skin dry even during prolonged effort. Neglecting this layer, for example by wearing a cotton t-shirt, is one of the most common mistakes among those new to mountain sports: cotton retains sweat and, as soon as you stop moving, the body cools rapidly, with a real risk of dropping body temperature on the coldest days.
Thermal underwear | How thermal underwear actually works
Contrary to popular belief, thermal underwear doesn’t generate heat: the warm feeling it provides comes from the fabric’s ability to trap a thin layer of air close to the skin and, above all, to quickly wick sweat away. A wet body loses heat much faster than a dry one: this is why thermal underwear works better as a moisture regulator than as a heat source in itself. Understanding this principle helps in choosing the right garment: what matters isn’t just “how warm it feels”, but above all how well it manages sweat during movement.
Thermal underwear | Merino wool, synthetic or blend: which to choose
Merino wool
Advantages: naturally temperature-regulating, resistant to odours even after several consecutive days of use, pleasant against the skin.
Disadvantages: higher cost, longer drying times, pure fibres wear out more easily with frequent washing or intense use.
Synthetic fibres (polyester, polypropylene)
Advantages: dry very quickly, cost less, hold up well to abrasion and repeated washing.
Disadvantages: tend to build up odours more quickly after prolonged use.
Wool-synthetic blend (hybrid)
Advantages: combines the temperature regulation and odour resistance of wool with the greater durability and faster drying of synthetic fabric.
Disadvantages: slightly lower performance than pure materials in their respective strengths, but often a more balanced compromise for everyday use.
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Merino wool, sourced from selectively bred sheep, is naturally temperature-regulating and odour-resistant.
Thermal underwear | Fabric weight: light, medium or heavy
Thermal underwear also varies by weight, similarly to fleece:
Lightweight (up to 150 g/m²)
Advantages: minimal bulk, ideal as a base layer for intense activity or mid-season use.
Disadvantages: limited insulation on the coldest days or during long breaks.
Medium weight (150-250 g/m²)
Advantages: the most versatile, suited to most winter activities.
Disadvantages: can be excessive during very intense effort on days that aren’t especially cold.
Heavyweight (over 250 g/m²)
Advantages: maximum insulation, suited to static activities in intense cold (breaks during cross-country skiing, unheated huts).
Disadvantages: excessive for dynamic activities, where the risk of overheating is higher.
Thermal underwear | Ski thermal underwear: a special case
On skis, the body alternates between intense effort during the descent and long waits on the lifts, where cold sets in quickly. Ski thermal underwear therefore needs to balance two opposing needs: breathability during movement and insulation during periods of inactivity. Long-sleeved, high-neck models, sometimes with different-weight panels between torso and arms, respond well to this dual need: more insulation on the torso, where the cold is felt more standing still, and greater breathability under the arms, where sweating is more intense.
In cross-country skiing, thermal underwear works under constant effort: breathability matters as much as insulation.
Thermal underwear | Long-sleeve top, bottoms or full set: what to choose
Long-sleeve thermal top
Advantages: fully covers the arms, useful under a jacket and fleece on cold days; high-neck versions also protect the nape from wind.
Disadvantages: can be excessive on milder days if not paired with a chest vent zip.
Short-sleeve thermal top
Advantages: ideal for intense activity in mid-season, when you need to manage sweat without overheating the forearms.
Disadvantages: no protection for the forearms from cold or sun.
Thermal bottoms (leggings)
Advantages: essential on cold days under padded ski or hiking trousers, where the legs would otherwise cool quickly during breaks.
Disadvantages: if too heavy, can feel uncomfortable under an already close-fitting outer trouser, creating an annoying “double layer” effect.
Full set (top plus bottoms)
Advantages: ensures consistent material and weight across the whole body, convenient for regular skiers or mountaineers.
Disadvantages: higher overall cost than buying only the piece needed for a specific activity.
Thermal underwear | Thermal underwear vs. technical sports t-shirt: what’s the difference
The two garments are often confused, but serve different purposes. The technical sports t-shirt, designed for activities like running or summer hiking, prioritises breathability and quick drying, often with a looser fit. Thermal underwear, by contrast, is designed for cold: it fits closer to the body to maximise sweat transfer and often incorporates insulating fibres (merino wool or thin fleece) that a simple summer technical t-shirt doesn’t have. Wearing a summer technical t-shirt instead of thermal underwear on a day of skiing or winter hiking means giving up much of the insulation the body needs during breaks.
Thermal underwear | When it’s really needed, activity by activity
Winter hiking
Essential, especially during breaks: the body sweats going uphill and cools quickly as soon as you stop for a rest or to admire the view.
Alpine and cross-country skiing
Fundamental for the same reason as hiking, with the added factor of long waits on ski lifts, during which the body generates no heat but stays exposed to cold and wind.
Mountaineering and high altitude
Here thermal underwear works alongside several layers at once: a close fit and flat seams become even more important to avoid chafing points under a pack and harness during hours of activity.
Summer hiking at altitude
Less essential during the day, but a lightweight thermal top in the pack remains useful for breaks at a mountain hut or a sudden change of weather, since temperatures can drop quickly even in full summer.
Thermal underwear | Men’s thermal underwear
Men’s thermal underwear is generally offered as a top-and-bottom set, with flat seams to reduce chafing under subsequent layers. A close but non-compressive fit is what best guarantees sweat management across the whole body surface: a garment that’s too loose creates air pockets that slow the transport of moisture outward, partly undoing the effectiveness of the technical fabric.
Thermal underwear | Women’s thermal underwear
Women’s thermal underwear follows a closer fit tailored to the female figure, with particular attention to seams around the chest and hips, points where prolonged rubbing can become uncomfortable over a full day of activity. A technical sports bra, when included in the range, should also meet the same criteria of breathability and no rigid seams.
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Thermal underwear | Common mistakes in choice and use
The first mistake, already mentioned, is using cotton instead of technical fabrics: cotton absorbs sweat but doesn’t release it, staying wet for hours. The second mistake is choosing a size that’s too loose, mistakenly thinking “more room” means more warmth: in reality, thermal underwear works best when it fits close to the skin, because direct contact is what allows fast moisture transport.
A third mistake concerns layering itself: wearing two layers of heavyweight thermal underwear on top of each other, instead of a single layer of the correct weight paired with a mid layer like fleece, often worsens sweat management rather than improving it, trapping moisture between the two similar layers.
Thermal underwear | How to wash it without compromising performance
Thermal underwear should be washed at low temperatures, avoiding fabric softener: this product deposits a thin waxy layer on the fibres that reduces the fabric’s ability to absorb and transport moisture, partly wasting the investment made in a technical garment. Tumble drying at high heat should also be avoided on synthetic garments, which can lose elasticity over time; merino wool garments in particular should be washed with gentle detergents and never wrung out forcefully, to avoid felting the natural fibres.
Thermal underwear | Frequently asked questions
What’s the best material for thermal underwear?
It depends on use: merino wool is best for breathability and odour resistance over several days, synthetic for quick drying and affordability, wool-synthetic blend for those seeking a balanced compromise.
How does thermal underwear work?
It doesn’t generate heat on its own: it traps a thin layer of air close to the skin and quickly wicks sweat away, keeping the body dry and reducing the heat loss typical of wet skin.
Does the thermal top really work?
Yes, but only if the right material and weight are chosen for the activity: heavyweight thermal underwear during intense effort can actually backfire, causing excessive sweating.
At what temperature should you wear thermal underwear?
There’s no fixed threshold: the type of activity and intensity of effort matter more than the absolute outdoor temperature. Generally, below 10-12°C a technical base layer starts to make a significant difference compared to a regular t-shirt.
How many sets of thermal underwear are needed for a multi-day trek?
At least two sets, to be able to alternate a dry garment while the other dries overnight: with quick-drying technical fabrics, a hand wash in the evening is often enough to have it ready again by the next morning.
Should thermal underwear be worn directly against the skin?
Yes, that’s exactly its purpose: it works as a base layer and needs to stay in direct contact with the skin to effectively transport sweat to the following layers. Wearing it over another t-shirt largely defeats its purpose.
Thermal underwear | Conclusion
Thermal underwear is the least visible layer of mountain clothing, but often the most decisive for overall comfort during a hike. Choosing the right material and weight for the activity, avoiding cotton, and taking care of maintenance over time are the three factors that truly make the difference between a comfortable outing and one spent battling cold and moisture.
It’s also worth remembering one last practical detail: the right size for thermal underwear is usually your regular size, or at most one size down from your outer garments, precisely because it needs to fit close to the skin without bunching. Anyone torn between two sizes should lean toward the closer fit, saving any extra room for the layers that follow, not the base layer.
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