​​Wearing an undershirt despite the summer heat: Why base layers make sense in summer

In fact, the situation is more complex. If you look around at the professionals in the peloton, you’ll often spot sleeveless Mesh base layer under your jersey. That’s no coincidence. The question, therefore, is not whether performance underwear makes sense in summer, but when and which type to wear.

The science behind it: Why do we sweat in the first place?

The human body has an extremely effective air-conditioning system: sweat. When sweat evaporates from the skin, heat energy is dissipated. It is precisely this evaporation effect that is the most important method of thermoregulation during physical exertion.

A simple example: it takes around 2.4 kJ of energy to evaporate 1 gram of water. This energy is mainly drawn from the body. Even just a few hundred grams of sweat evaporating per hour can therefore provide a considerable cooling effect.

The problem is that sweat must be wicked away from the skin as quickly as possible and spread over a large surface area. If it remains on the skin as a film or completely saturates the jersey, the rate of evaporation decreases. The result is that the subjective sensation of heat increases.

What is the purpose of a functional undershirt in summer?

This is where functional cycling base layers come in. A summer base layer isn’t meant to keep you warm. Its purpose is, in fact, quite the opposite:

• Absorbing sweat from the skin
• Spread the moisture over a large area
• accelerate the evaporation of sweat
• Keeping the inside of your cycling jersey drier
• Reducing friction between the skin and the jersey

In particular, very breathable mesh undershirts or mesh vests provide a larger surface area for evaporation than the skin alone. This can enhance the cooling effect.

A study into the thermal and moisture regulation of cycling clothing also showed that the choice of material and moisture management have a measurable impact on thermal perception and sweat evaporation during exercise in hot environments.

When is it a good idea to wear functional underwear in the height of summer?

1. On long, strenuous cycle rides

The longer the exertion lasts, the more sweat the body produces. A lightweight, functional base layer for the summer can prevent your jersey from becoming completely soaked and sticking to your skin.

Road cyclists and marathon mountain bikers, in particular, often benefit from a drier inside of their jersey and a more stable microclimate directly against their skin.

2. In the case of varying loads

In the mountains, steep climbs and rapid descents follow one another constantly. Anyone who has worked up a sweat climbing a long pass and then hurtles down into the valley at 60 km/h will be familiar with that unpleasant feeling of getting chilled.

A base layer for cycling helps to mitigate these fluctuations to some extent, as it distributes moisture more evenly, thereby reducing peaks in evaporation and cold.

3. When wearing close-fitting aero jerseys

Modern aero jerseys literally cling to the body. A thin mesh base layer creates a minimal gap between the skin and the jersey fabric. This allows air to circulate more freely and reduces direct friction between the fabric and the skin.

4. For riders who sweat a lot

Some people lose well over a litre of fluid per hour. In such cases, a breathable performance top can help to wick away moisture more effectively and make the garment more comfortable to wear.

When can you do without a performance shirt?

Not everyone benefits equally from wearing a functional undershirt in summer. This is particularly true for:

• short evening runs
• of moderate intensity
• extremely dry heat
• very light and airy summer jerseys

the effect is often minimal. In some situations, riding without an extra layer simply feels more comfortable.

What’s more, each additional layer initially increases the distance over which moisture can evaporate. So if the fabric and ventilation are not optimally matched, an undershirt can actually act as an extra layer of warmth.

The pros and cons of functional underwear in summer

Advantages

• Improved moisture management: Sweat is distributed more quickly and evaporates more efficiently.
• A more comfortable skin environment: The jersey doesn’t cling quite so tightly to the skin.
• Less cooling during descents: This is particularly noticeable in the mountains.
• Reduced odour formation: This is particularly true of merino wool.

Disadvantages

• Extra layer of clothing: It may initially feel warmer, subjectively speaking.
• Depending on quality: Cheap cotton T-shirts or thick synthetic T-shirts often detract from the effect.
• More effort: Extra washing after every ride.
• Not something everyone notices: The benefits depend heavily on sweat production, intensity and weather conditions.

Which material is best suited?

Polyester and polypropylene

Polypropylene is regarded as one of the most effective materials for high-intensity activities involving a lot of sweating, as it absorbs virtually no moisture. Polyester is a close second and therefore dominates the market for functional cycling underwear.

Features:

• very light
• extremely fast drying
• low water absorption
• high moisture-wicking properties
• durable

Mesh fabric

Mesh structures are less a material than a construction.

Features:

• maximum ventilation
• large evaporation surface area
• minimum weight
• particularly high air permeability

That is why many manufacturers opt for mesh undershirts with large mesh panels when it comes to functional underwear for hot weather. These types of undershirts are ideal for temperatures above 25 degrees.

Merino wool

It sounds paradoxical, but it works surprisingly well. Merino fibres can absorb and retain moisture and maintain a stable microclimate. Recent studies show that merino enables very consistent thermoregulation, particularly during varying levels of physical exertion, and absorbs and retains moisture significantly better than polyester.

Features:

• natural odour resistance
• effective temperature management
• high level of comfort
• still comfortable even when damp

Disadvantages:

• dries more slowly
• more expensive
• slightly less robust

Ideal for: Alpine cross-country tours, bikepacking and multi-day trips.

Blended fabric

Merino-synthetic blends combine the best of both worlds:

• longer shelf life
• faster drying
• effective odour control
• balanced thermoregulation

Conclusion: Is functional underwear a good idea in summer?

The common assumption that ‘the less fabric, the cooler’ is only true to a limited extent when it comes to cycling. A high-quality, thin performance base layer for hot days can be useful even at temperatures above 30 degrees – provided it is a genuine summer model with open mesh fabric, high breathability and excellent moisture-wicking properties.

Particularly on long road cycling tours, during intense training sessions or in the mountains, a lightweight mesh base layer for cycling can improve your skin’s microclimate and help sweat to evaporate. On the other hand, if you’re just cycling at a leisurely pace across flat terrain or are already wearing a very breathable summer jersey, you can usually do without one without any problems.

In short: The best performance underwear for summer doesn’t keep you warm – it helps the body cool itself naturally and makes cycling more comfortable.