Originally, humankind started layering clothes for thermal comfort. Today stylish layering serves to create interesting outfits too. Like one can ruin a stylish look with layering mistakes, one can also achieve the opposite of improved wear comfort when layering for thermal comfort. Read what you need to know to do it right.
What Is the Role of Thermal Resistance for Thermal Comfort?
At ambient air temperatures below our skin temperature, our naked body looses heat to the environment. Clothing has served to reduce this heat loss for thermal comfort at temperatures below 9oC (48.2F). When we are dressed the total thermal resistance to transfer of heat from the body to the environment consists of the
- Resistance to heat transfer from the fabric surface to environment,
- Thermal resistance of the fabric material itself, and
- Thermal resistance of the air trapped inside the fabric.
The heat transfer can be via radiation, convection, and conduction. Thermal resistance increases with fabric thickness.
What is Thermal Conduction?
Conduction is heat transfer by the molecules of the fabric fibers. Think of the iron pan that is on the hot stove. Despite the handle is not in contact with the stove it gets hot. Conduction can also occur between molecules of different, but adjacent material. Just think of what happens when you touch the hot handle with your bare hand.
A high thermal resistance means a low conduction. The thermal resistance is a material dependent property, while conduction depends on the environmental conditions. This process varies with time.
Definition of Thermal Convection
Typically, our skin is warmer than the ambient air. The air in the immediate vicinity of the skin gets warmed by the skin. At same pressure and moisture, warm air is lighter than cold air. Therefore, it rises.
Ventilation by wind or body motion can accelerate the transport of the slightly warmer air away from the skin. Consequently, the heat loss by convection increases.
Radiation
Every body emits radiation according to its temperature. In case of our body, the body heat corresponds to infra-red radiation.
The main contributors to heat transfer of the system body-fabric-environment are convection by air and radiation.
How Does Insulation by Fabrics Works
At a given fixed weight, thermal insulation increases with fabric thickness because of the increased amount of entrapped air. However, when fabric thickness remains constant, thermal insulation decreases with increasing fabric weight because of the reduced amount of entrapped air.
An increase in the fabric’s weight-to-thickness ratio increases the effective thermal conductivity due to the increased packing density. This increase in fiber-to-fiber contact increases the mean free path photons can travel, for which less heat flows thru the pore-channels of the fabrics.
Explanation of How Layering Clothes for Thermal Comfort Alters Insulation
Layering creates a thin air layer between the two layered fabrics. When this layer is thin enough and stagnant, the high thermal resistance (low conductivity) of air adds to the insulation of the two fabrics. However, when the air layer is too thick convection can build in the air layer leading to a faster loss of body heat. Furthermore, any physical activity means ventilation. Consequently, the temperature at the outside of the inner layer decreases faster. As a result the heat flux from the inside to the outside of the inner layer rises meaning a faster loss of body heat.
When the two fabrics are squeezed, the insulation by air is lost. Moreover, the direct contact of the two fabrics’ fibers leads to direct heat transfer by conduction between the fabrics because of the lack of the air barrier.
Even Slight Body Movement Enhances the Impact of Cold Gaps
The openings of sleeves and the legs of pants permit cold outside air to enter the air layer between fabrics as well as between the finer fabric and body. Body motion accelerates the mixing of the cold intruding outside air and relatively warmer air between the clothing layers. Therefore, winter coats for cold climate regions (and thermal comfort) should have closures or tightly knit cuffs at the wrists to prohibit cold air to enter.
Furthermore, long underwear and thermal pants should have tight cuffs too to avoid cold gaps in clothing.
Photos of me: G. Kramm
Diagrams: N. Mölders
© 2013-2023 Nicole Mölders | All rights reserved
It’s really interesting to read about the science behind thermals!
Emma xxx
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