How undershirts keep you warm in cold & cool in heat
Posted on June 12 2017
You've heard it said that undershirts keep you warm in winter and cool in summer. How can that be? It sounds counter-intuitive, right? The warm in winter part sure, that's easy to get. An extra layer, insulation from chilly blasts of air, trap a warm layer of air against the skin, etc., that all makes sense. But how does an extra layer work to keep you cool in summer?
The answer lies in the what happens when you introduce water in the form of sweat. The theory assumes that the wearer of the undershirt won't sweat in winter but will sweat in summer. If you wear an undershirt in summer, and you sweat, the water gets absorbed by the undershirt (assuming it's made of a moisture absorbing fabric), and that changes how your extra layer behaves and feels.
To fully understand why we are cooled down we need to take a look at the science of how water and heat interact. There are two factors to consider.
1. The heat capacity of water
The capability for a molecule to absorb heat energy is called heat capacity. Water has a high heat capacity, in fact, it has the highest specific heat capacity of any liquid. Specific heat is the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie or 4.184 Joules. As a result, it takes water a long time to heat and a long time to cool.
In other words, water is good at absorbing lots of energy (heat) without rising much in temperature. It's one reason why your body can stay at a fairly constant temperature because the human adult male is comprised of approximately 60% water.
2. Latent heat of water vaporisation
A high amount of energy is required to transform one gram of liquid water into water vapour; this energy requirement is called the heat of vaporisation.
This process occurs on the surface of water. Even when below its boiling point, water's molecules can acquire enough energy from each other so that some surface water molecules can escape and vaporise; this process is known as evaporation.
In other words, when water evaporates it absorbs energy - 2.260 joules per gram of water (at normal atmospheric pressure) to be precise and the environment where the evaporation takes place is cooled.
That's how sweating keeps you cool. Sweat absorbs energy from your skin so it can evaporate. As the sweat (mainly water) evaporates, it leaves the body cooler.
How does clothing help this cooling process?
The same principle applies when you are wearing garments specifically designed to help you manage your body temperature.
In the case of sports clothing, the garment is usually made from a sweat-wicking fabric. This material is designed to lift the water (sweat) away from your body and to the outside of the garment to vaporise into the air. This helps keep the body cool in heavy sweating activities such as sport by the processes mentioned above.
An undershirt works slightly differently as it's designed to be worn under other layers to prevent your sweat from showing. Sweat is absorbed into the undershirt and will gradually evaporate from the surface of the material, extracting heat energy from the shirt (and your body) as it does. Of course, the undershirt is designed to be close fitting to the skin, so as it cools down, so does your skin. If you've ever worn a wet tee-shirt will know how effective this process can be. Of course, the process with an undershirt is a lot less dramatic and much more comfortable, but you get the idea.
So, that's why wearing an extra layer, such as an undershirt, can keep you warm in winter and cool in summer.