Winter Base Layers

Winter underwear, also referred to as a thermal undershirt, thermo shirt, thermal clothing or ski jumper, is the base layer that you wear directly on the skin during active outdoor activities in the cold. The function is twofold: to transport perspiration away from the skin so that you stay dry, and at the same time to retain heat to prevent cooling down. Cotton does not fulfil these functions: cotton absorbs sweat and holds onto it, which quickly leads to the cold and damp feeling that winter sports enthusiasts know when standing still or at a lower pace. Technical thermal material works the other way around: it draws moisture away from the skin to the outside of the fabric, where it can evaporate. That principle makes thermal sports underwear the most impactful investment in your winter comfort layer.

The range consists of two types: a close-fitting thermal undershirt with long sleeves for use as a purely functional base layer directly on the skin, and a ski jumper with a half-zip that can also be worn as a light visible outer layer at lower exertion intensity. Both are available in versions for women and men in several colours. The choice depends on how you want to use the layer, how intensively you do sport and whether you also want to be able to wear the ski jumper standalone at varying temperatures.

Which winter underwear suits you?

The choice depends on your activity, the exertion intensity and whether you wear the base layer exclusively as a bottom layer or also standalone.

You skate outdoors or do cross-country skiing and want a first layer that draws away perspiration during intensive exertion: choose a close-fitting thermal undershirt with long sleeves. That layer lies directly on the skin and is responsible for the moisture management in your clothing system. Wear it under your skating jacket or skating trousers on the ice or under your cross-country skiing clothing on the trail for direct comfort during active exertion.

You want a base layer that you can also wear without a top layer during light exertion or when it is a little less cold: choose a ski jumper. The half-zip lets you set the ventilation depending on your pace and the outdoor temperature. A ski jumper fits more closely than a casual winter sweater but is less enclosing than a purely functional undershirt and is presentable enough to wear as a light standalone layer during less intense activities.

You train outdoors in winter for triathlon, cycling or running at varying intensity: choose a thermal undershirt. The fast-drying property of technical material is extra valuable during interval training: at high exertion the body sweats, at lower speed or pauses it cools down quickly. Always wear it as a first layer under a winter sports jacket as the outermost layer at cold outdoor temperatures.

You take part in several winter sports activities and want one base layer that works in all situations: choose a thermal undershirt in a neutral colour that fits under all your winter sports clothing. The same base layer works under skating clothing, cross-country skiing clothing and a winter sports jacket. Always choose a model with long sleeves: cold wrists are a frequently heard complaint among athletes who opt for a shorter sleeve during outdoor activities in winter.

You are switching from cotton to technical material for the first time and want to know what the difference is: start with one thermal undershirt and wear it during your first winter outdoor training session. The difference is immediately noticeable: dry skin during exertion, no cold and wet feeling when standing still. Have a look at the complete winter sports range for the other layers you need for complete winter equipment.

Thermal undershirt or ski jumper?

This is the only real choice in this range. The difference is small but practically relevant for how you want to use the layer.

A thermal undershirt with long sleeves is a purely functional garment: close-fitting, invisible under other sportswear, specifically designed for maximum moisture transport and thermal retention directly on the skin. You almost always wear it under another layer. It is the type of base layer that you do not see but always feel when it is there, and even more so when it is not.

A ski jumper is a close-fitting thermal shirt with a half-zip at the neck. The half-zip is a practical detail: you can ventilate by opening the zip without taking off the jacket or the shirt, which is handy during activities with varying intensity. A ski jumper is also visibly presentable enough to wear as a standalone light outer layer during less cold activities or recreational use outside of sport.

Choose a thermal undershirt if you are exclusively looking for a functional base layer that disappears under other clothing. Choose a ski jumper if you also want the option to show the layer or if the half-zip offers you a practical advantage during activities with varying exertion intensity.

What should you pay attention to when buying?

Synthetic material and drying speed

The products in this range are made of synthetic technical material. Synthetic thermal underwear dries faster than merino wool, is more resistant to repeated washing and retains its moisture-wicking properties during intensive use. It is the choice for athletes who use the base layer several times a week and want to wash and dry quickly between sessions. Merino wool also regulates temperature well and has less odour formation during prolonged use, but dries more slowly. For active winter sports, synthetic is in most cases the more practical daily choice.

Fit: close-fitting but not constricting

A thermal undershirt only works well if it fits closely enough to have constant contact with the skin: loose materials that come away from the skin cannot carry out the moisture transport. The fit must be tight but not so tight that it restricts breathing or causes pressure points during prolonged wear. Check whether the undershirt fits well with the arm posture of your sport: in skating and cross-country skiing the arms go far forward and back, and a sleeve length that is too short then pulls the shirt up. Always choose the size based on chest circumference, not on length alone.

Ski jumper as standalone or as a base layer

The ski jumper combines the basic functions of a thermal undershirt with a slightly freer fit and the half-zip. If you wear it exclusively as a base layer, then choose the size that fits most closely for optimal moisture wicking. If you also want to wear it standalone, then choose one size larger for a more comfortable fit as a visible layer. The half-zip is most usable during activities with varying intensity: open at higher pace for ventilation, closed when standing still or cooling down after training.

Care for long-lasting performance

Thermal sports underwear loses its moisture-wicking function if it is not washed properly. Never use fabric softener: it lays a film on the fibres that hinders the transport of moisture. Wash at 30 or 40 degrees, turned inside out, and dry flat or hanging in the air instead of in the dryer. Rinse the garment with cold water first after each use to remove sweat and chlorine, even if you only wash it the next day. With good care, synthetic thermal underwear lasts several years without loss of function.

Our advice: always start with a good thermal undershirt as the basis for your winter sports equipment. It is the most affordable garment in your layering system but has the greatest direct influence on your comfort in cold and exertion. Consider buying two items if you train several times a week, so that the base layer is always dry and fresh for the next session. Never wash with fabric softener and the moisture-wicking properties remain intact for years.

Who is winter underwear suitable for?

• Skaters and cross-country skiers who need a functional base layer that draws away perspiration during intensive exertion and retains heat when standing still or at a lower pace.
• Outdoor athletes in winter who run, cycle or do other activities at cold temperatures and always wear a thermal base layer as a first layer under their equipment.
• Triathletes and multisport athletes who train outdoors all year round and also do outdoor sessions in winter where a fast-drying thermal base layer makes the difference in comfort.
• Recreational winter sports enthusiasts who want to stay comfortably warm during skating outings, walks or other winter activities and buy thermal sports underwear for the first time.
• Anyone who switches from cotton to technical material and wants to experience straight away what the difference is of a good functional base layer during active exertion in the cold.

Need help with your choice?

Would you like to buy winter underwear but you are not yet entirely sure which model and which size best suit your sport and use? Then we are happy to help you further. You can reach our triathlon specialists by phone on +31 (0)413-256800, send an email or drop by our triathlon shop in Volkel, the Netherlands.

Frequently asked questions about winter underwear

What is the difference between a ski jumper and a thermal undershirt?

A ski jumper is a thermal sports shirt with a half-zip at the neck that makes ventilation possible by opening the zip at higher exertion intensity. A thermal undershirt without a zip is a purely functional base layer that is worn close-fitting under other clothing for maximum moisture transport. A ski jumper can also be worn as a light standalone outer layer; a thermal undershirt is less suitable for visible use. The moisture-wicking and thermal function are comparable for both.

May I wear cotton as a base layer during winter sports?

Cotton is unsuitable as a base layer during active winter sports. Cotton absorbs perspiration and holds onto it, which makes the fabric feel heavy, wet and cold as soon as you stop or move less intensively. In cold weather this can lead to rapid cooling of the body. Always choose technical synthetic material as a base layer during outdoor activities in winter, even if it only concerns a short recreational activity.

How warm is a thermal undershirt?

A thermal undershirt is not a thick insulation layer but its value for warmth lies in keeping the skin dry. Wet skin cools down considerably faster than dry skin, even at the same outdoor temperature. By keeping the skin dry, the thermal undershirt effectively ensures that the layers over it can carry out their warmth function optimally. Add an insulating mid layer for extra warmth in extreme cold.

Which size of winter underwear do I choose?

Choose the size that fits most closely to your chest circumference, not to your length. Thermal underwear must be close-fitting enough for direct contact with the skin; a size that is too loose reduces the moisture transport and gives creases under the outer layers. Take the smaller size if in doubt if you only want the base layer function, or the larger size if you also want to wear the ski jumper as a light standalone layer.

How do I wash my thermal undershirt?

Wash thermal sports underwear at 30 or 40 degrees, turned inside out and without fabric softener. Fabric softener lays a film over the fibres that affects the moisture-wicking capacity of the material during repeated use. Preferably dry flat or hanging in the air. Rinse with cold water first after each use to remove perspiration and chlorine, even if you only wash it the next day.