There is no single "right" temperature for when to take thermal underwear out of the closet. For some, it is needed as soon as it reaches +10 °C due to wind, rain, and prolonged inactivity. Another person may be moving actively at 0 °C and feel perfectly comfortable in a light base layer.
Therefore, relying solely on the number in the forecast is not a very good idea.
Consider three things: the temperature, your activity level, and what you will wear over it. Then thermal underwear will work as part of a layering system, not just as another warm layer.
What temperature to wear thermal underwear
As a general rule, you can use the following ranges:
- from +10 to +15 °C - light thermal underwear may be appropriate for prolonged time outdoors, wind, damp weather, or active movement, when it is important to wick away sweat quickly;
- from 0 to +10 °C - thermal underwear becomes a practical solution for most prolonged outdoor activities;
- from 0 to -10 °C - medium-weight thermal underwear is usually needed, and the level of insulation is adjusted with a second layer;
- below -10 °C - look for warmer models, but do not forget about fleece, an insulation layer, and wind protection.
This is not a rigid temperature chart. At +5 °C with strong wind and humidity, it may be less comfortable than at dry -2 °C. And a person walking briskly with a backpack generates significantly more heat than someone standing or sitting for several hours without active movement.
Therefore, it is better to ask the question a little differently: at what temperature and for what task is thermal underwear needed?
Active movement does not require the warmest set
One of the typical mistakes is seeing a minus in the forecast and immediately choosing the thickest thermal underwear. During active movement, the body heats up quickly. If the base layer is too warm, the person will start sweating heavily. Then the pace slows down or a long stop begins - and the wet clothing cools down.
For hiking, fast travel, physical work, or high-activity duty, it is often more logical to choose lighter thermal underwear that wicks away moisture well. Heat is added with subsequent layers.
For example, at a temperature of around 0 °C during continuous movement, a functional base layer, fleece, and outer protective clothing may be enough. But for several hours of almost no movement at the same temperature, the system will be different.
If there is little movement - insulation becomes more important
Shifts, observation, fishing, working outdoors without continuous movement - here the body produces less heat. In such a situation, light thermal underwear designed primarily for moisture wicking may not be enough. A thicker option and proper insulation on top are needed.
This is especially noticeable at sub-zero temperatures. Standing at -5 °C and walking several kilometers at -5 °C are completely different experiences. Although the weather forecast is the same. For a soldier, this is a critical point. If there are active movements during the day, followed by prolonged time at a position, the clothing should allow you to quickly change the level of insulation. One very warm set does not provide such flexibility.
Is thermal underwear needed at above-freezing temperatures?
Yes, sometimes it is needed. Thermal underwear does not necessarily have to be thick and winter-specific. Light models are used primarily for moisture control. Imagine +8 °C, a long outing, a backpack, or equipment. Under a regular cotton T-shirt, the back gets wet quite quickly. Cotton absorbs moisture and takes longer to dry.
A functional base layer behaves differently: its job is to help wick moisture away from the body. That is why at above-freezing temperatures, base underwear is usually needed: it is thin and light, wicks away moisture, and works as the first layer. Thermal underwear is worn over it and is responsible for insulation. Nothing with short sleeves can be worn under base underwear; this is important.
Therefore, light thermal underwear can be used in autumn and spring, even when it is consistently above freezing outdoors. Here, it is more important not "how cold it is," but how long you will be outdoors and how actively you will move. For above-freezing temperatures, men's summer thermal underwear is suitable, as well as women's light-weight thermal underwear.
What thermal underwear to choose for 0, -5, and -15 °C
At a temperature of around 0 °C for active movement, you should usually start with a light or medium base layer. If activity is low - add insulation.
At -5 °C, the entire system matters more. Thermal underwear + fleece jacket + protective outer layer is often more practical than one overly thick item under a jacket. At -15 °C and below, a more serious approach to insulation is needed. Thicker thermal underwear may be appropriate, especially with low activity. But it still does not replace an insulated jacket, fleece, or another mid-layer.
And one more thing. Wind, humidity, and precipitation can greatly change the perception of cold. If the outer clothing is drafty, one warm thermal underwear set will not solve the problem.
Thermal underwear for soldiers: look at the duty scenario
For duty, it is especially important not to buy a set solely on the principle of "for winter."
First, determine the task. If there is a lot of movement with equipment - a base layer is needed that works well with moisture and does not overheat. If most of the time is spent in a static mode - more insulation is needed. If these modes alternate, it is better to build the clothing in layers.
There are also practical details. Thermal underwear should fit well under the uniform, not bunch up under body armor, not press against the waist, and not restrict arm movement. That is, the temperature is only the starting point. Sets for duty are assembled in the military thermal underwear section on the Camotec website.
Don't look for one temperature - choose a system
In short: light thermal underwear can start being used from approximately +10 °C, if activity and the weather require it. Closer to 0 °C, it becomes relevant much more often. At a stable minus, you need to think not only about the base layer, but about the entire insulation system.
Check the forecast. Assess how much you will move. Think about what you will wear as a second and outer layer. This makes it much easier to choose thermal underwear that will work for your task, rather than just sitting in the closet as "very warm."
At Camotec, you can choose thermal underwear for duty, active recreation, and everyday use, taking into account the season and level of exertion. If you are torn between several options - describe the conditions of use, the approximate temperature, and your activity level to the manager. We will help you figure out the set and size.
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