Hidden Mysteries in Autumn/Winter Clothing: About Human Thermal Comfort

Introduction

Every autumn and winter, a “warm” war erupts in the textile and clothing market. High-tech brands compete with their cold and warm clothing. They claim to be three times as warm. And they have a self-heating feature and a constant 37℃ temperature. They use super warm materials and “warm black technology.” They say they use aerospace science and tech. They’ve tested their gear from Mount Everest to the South Pole, in -50℃ extremes! And, its products’ authenticity is in doubt. The publicity is unscientific and unpersuasive. Consumers have serious doubts.

Autumn and winter clothing must be warm above all else. It must also be comfortable and functional. There are few product standards for thermal insulation. It has no tests or requirements in cotton, down jackets, coats, windbreakers, and quilts. These products aim to protect against the cold. But, they lack guidance on testing their insulation. A few products exist but are not professional or convincing. Very few standards listed insulation rates and other indicators. So, guidance documents apply to them. There is also a lack of theory behind the development of these indicators. Actual human requirements are not considered. So, the indicators have little practical value. In practice, we rely on experience and feelings. We trust those with life experience to know what to wear for the weather. But, there are often unexpected hot and cold spells. This is especially true in bad weather, like low temperatures and high winds. Also, our activities can change. So, we can’t always rely on our experience. This has led to two skins and high demand for them. With no standards to follow, market confusion is inevitable.

1. Clothing Ergonomics and Thermal Comfort Equations

1.1 About clothing ergonomics

Humans strive to create a warm, comfortable environment. So, what is comfort? How to achieve comfort? We must know what to wear for each occasion. Then, we can make ‘comfortable clothing’ for them. We must also know how to achieve them. What environmental conditions can a garment be suitable for?

This begins with garment ergonomics.

To discuss clothing ergonomics, we must first understand ergonomics, or human factors. Ergonomics studies how to design ‘human-machine/object-environment’ systems. It aims to align them with human body and mind. The goal is to best match the systems to humans under various conditions. The research aims to help people work and live better. It seeks to enhance their safety, health, and comfort. Ergonomics has a very wide research scope. Related to this paper is clothing ergonomics, a branch of ergonomics. Clothing ergonomics is a field. It studies the link between the “human body, clothing, and environment.” It’s a new science focused on clothing comfort. Clothing ergonomics aims to meet diverse human needs. It provides designers with key metrics to maximize comfort and health. The focus is on humans, serving their bodies, and studying clothing systems. The research includes: human thermal comfort, the principle of action, and using clothes for comfort. It also covers how to design clothing for the body. Clothing ergonomics is the core of textile and apparel science and technology.

Clothing, the human body, and the environment form a system. It plays an important role in regulating comfort. Clothing comfort is a broad evaluation of its effects. It includes many factors. These are the physical properties and characteristics of the clothing materials. They also include people’s activities, the environment, and their physiological and psychological states. It also relates to people’s activities, the environment, and their minds. Clothing comfort includes thermal, humidity, contact, and visual comfort. Of these, thermal and humidity comfort is most important. This paper focuses on thermal comfort, a part of ergonomics. Human activity converts 80% of the energy into heat. Clothing causes the skin to lose 90% of this heat. So, clothing and its materials are vital for keeping body temperature.

1.2 Thermal comfort for the human body What is thermal comfort?

ISO 7730 defines thermal comfort as a state of mind. People find their thermal environment satisfactory. They should not be too cold to shiver or too hot to sweat. The body must balance heat produced and heat lost. ‘Comfortable clothing’ keeps the body at a normal temperature for a given situation and activity. This is the ‘comfortable clothing’ required under the condition.

Body temperature shows our health. It should be 36-37℃. Skin temperature reflects it. A carcass’s skin temperature is about 33℃. A normal body temperature ensures our physiology. It’s the most comfortable state for our senses. In a quiet state, our senses and skin temperature match Table 1.1.

Table 1.1 Relationship between sensory perception and skin temperature (in quiet)

People feel comfortable under normal circumstances.

This is because of a mechanism that regulates body temperature. It maintains a balance of heat with the outside. It keeps the body at a normal temperature. Japanese scholars, like Shoji Kou, represent this mechanism in the balance diagram. On one side of the scale are heat-producing factors (chemical processes). They include our metabolic heat from food, like sugars, proteins, and fats. And they also include disease, gland activity, muscle tension, and chills. They include exercise and some mechanical effects, too. All of these can produce heat. On the other side are heat-dissipating factors (physical processes). Heat conduction, convection, radiation, and evaporation dissipate heat. They, the environment, air temperature, wind, and radiant bodies, keep our body temperature in check. Also, the environment, temperature, wind, and radiation will cause heat loss in the body. So will clothing, surface area, breathing, sweating, and blood flow.

The human body constantly emits heat into the environment.

It does this through radiation, conduction, convection, and evaporation. Clothing and the environment affect this. The body loses heat through sweat evaporation, too. This is in addition to radiation, conduction, and convection. If a blockage occurs in the heat dissipation path, heat will build up. Also, if the body absorbs heat from a hot environment, it will build up heat. This will raise the body temperature. Conversely, if heat dissipates too much or is not produced, a heat deficit will occur. This will lower the body temperature. Extreme body temperatures cause discomfort. They can lead to illness or death. Thermoregulation is the regulation of body heat. It manages heat production, loss, and exchange with the outside. To maintain a constant body temperature, we can rely on two things. First, the body’s automatic regulation. Second, external regulation, like adjusting our clothing and the environment. We must combine both methods. We want to change dress attributes to ensure comfort in spring, autumn, and winter. In cold, windy weather, the body needs to dissipate heat. Our focus is on choosing the right dress to avoid wind chill. This is the goal of both research and product development.

1.3 Thermal balance

The most basic condition for human thermal comfort is to maintain thermal balance. This is the balance between the heat produced by the body and the heat lost to the environment.

The body’s heat exchange through clothing includes:

Conduction

Convection

Radiation

Sweat evaporation

The latent heat effect of clothing depends on two factors. They are its wet resistance and the vapour pressure difference with the skin. The dry heat exchange (sensible heat) depends on two things. They are the thermal resistance of the clothing and skin. And, the clothing’s surface temperature.

Thus, the heat exchange through the clothing is its total thermal and wet resistance.

Clothing helps the body regulate its temperature. It does this through its thermal and moisture resistance. It is part of the body-clothing-environment system. In cold environments, poor clothing insulation may cause heat loss. This can lower the body temperature. In high summer temperatures (over 35℃), the body can gain heat from the outside. This can happen through conduction, convection, and radiation. At this point, the only way to cool down is by evaporating water. In cool and cold climates, the body loses heat by cooling the skin. Convection and radiation to the environment account for most of this. Evaporation is less than 1/5 of total heat loss. In hot, humid weather, the body and environment are nearly the same temperature. Sometimes, the environment is even warmer. The body can only cool itself by evaporating sweat. But, this raises the humidity inside the clothing, causing discomfort. So, the ideal garment should both insulate and allow moisture to escape.

To achieve thermal balance, change any of these factors. It will create thermal comfort.

For example, when the body feels cold, you can:

① Increase its metabolic rate (eat more, be more active).

② Raise the ambient temperature.

③Reduce the wind.

④Add more clothing.

⑤ Improve clothing insulation.

But, human will and activity limit these changes. You can’t always eat a lot or increase activity. Environmental factors are even less controllable, except with A/C or heat. Outdoors, you can only adapt to nature.

So, the easiest and most realistic solution is to change the clothing system. It should regulate body heat.

2. Heat and humidity transfer of textile materials and clothing

For spring, autumn, and winter clothing, insulation is key in cold weather. It must be high. What indicators measure clothing insulation? How can we test it? What factors affect it? How can we improve it?

Heat transfer is key for clothing, sleeping bags, and other gear. This includes hats, scarves, gloves, socks, and shoes. It depends on their materials. It determines thermal insulation. The product’s structure and style also matter.

2.1 Heat and moisture transfer of textile materials

Textile materials here include all kinds of fabrics, coatings, and foams. They also include composites and flocculent fillings, and their combinations. For convenience, ‘materials’ are abbreviations for textile materials, except where noted.

2.1.1 Main performance indexes of heat and moisture transferability

Thermal resistance (Rct)

It is the ratio of the temperature difference to the heat flow per unit area. That flow passes through the material. This indicates that the textile is under a stable temperature gradient. It measures the dry heat flow through the specified area. One or more forms of conduction, convection, or radiation may transfer this dry heat flow.

The heat flow per unit thickness through a unit area. This is for a unit temperature difference between the material’s two sides. Thermal conductivity is, by definition, the sum of conduction, radiation, and convection. It is the inverse of thermal resistance per unit thickness.

Wet Resistance (Ret)

It is the ratio of the difference in water vapor pressure on both sides of a material to the heat flux per unit area through it. This is the evaporative heat flux through a textile. It is under a steady vapour pressure gradient. Diffusion and convection may form this evaporative heat flux.

Thermal and moisture resistance are the most important. They indicate a material’s ability to transfer heat and moisture. They are the key indicators of comfort with temperature and humidity.

The heat transfer coefficient and thermal conductivity measure thermal resistance. They are not the same as thermal conductivity in physics. In physics, thermal conductivity refers to a material’s property. This includes a single fibre, feather, coating film, and leather. The heat transfer coefficient and thermal conductivity refer to the heat transfer of fibre-processed materials, such as woven and nonwoven fabrics. Products made from the same fibre differ in structure, tightness, and thickness. This affects their heat transfer performance. The same fibre material processes textile products. Their structure, compactness, and thickness differ. This causes their heat transfer properties to vary. So does their heat transfer coefficient.

So, it can be simply understood as follows:

In physics, thermal conductivity is a material’s inherent property. A textile’s heat transfer performance relies on two factors. They are its heat transfer coefficient and its thermal conductivity. The textile’s heat transfer coefficient depends on the fibre’s thermal conductivity. But, it is also affected by post-processing and other factors.

People want to improve heat retention by changing these factors. They want to adjust the textile’s structure to get a desired thermal resistance.

That is, the thermal resistance. It aims to improve insulation or coolness.

Wet resistance measures how well sweat evaporates. It depends on heat loss and water vapor transmission. And it is not just a test of vapor transmission. It depends on the vapor pressure difference and heat dissipation on both sides. A smaller pressure difference means greater heat dissipation and lower wet resistance. This also means lower resistance to moisture and heat.

The formulas for moisture permeability and its index say that lower wet resistance raises both. It increases the permeability index and the rate. This makes the human body more comfortable.

The moisture permeability index and rate are as follows: they rise as moisture resistance falls. Thus, the more comfortable the human body is. Otherwise, it feels stuffy. Our life experience supports this principle. The conventional moisture permeability only measures the ability to pass through moisture vapour. It does not show heat dissipation or vapour pressure. So, it is not a direct index of comfort.

2.1.2 Detection Methods of Textile Material Heat Transfer Performance

There are textile heat transfer testing methods, both domestic and foreign. They differ by the shape of the test body: flat plate, double plate, drum, and tube. They also vary by test method: constant temperature and cooling. Other differences include the micro-environment and the heat source. The micro-environment can be static or ventilated. The heat source can be dry heat or evaporation.

The two most common methods are:

the static constant temperature flat plate.

the ventilated constant temperature evaporation hot plate.

▲ Constant Temperature Plate Method (Static)

The test instrument uses the static thermostatic plate method. It has a hot plate, a climate chamber (open at the top), and a temperature control system. It measures heat and other components. The hot plate can maintain a specified constant temperature. The test plate (with a square side of at least 200mm) and a protection plate make up the hot plate. The climate chamber must maintain a static wind (air velocity ≤ 0.1m/s). The temperature measurement point is from the test plate to the test chamber. The temperature measurement point in the chamber is over 150mm from both the test plate and the top.

The test specimen covered the electric test plate. It and the thermal ring of the test plate stayed at a constant 35 ℃. Thus, the heat of the test plate could only dissipate through the test specimen.

We needed to measure the heat required over time.

This would test the plate’s ability to maintain a constant temperature. We also needed to measure the temperatures on both sides of the test specimen. This time is the test specimen plus the thermal resistance of the air layer.

Next, we needed to find the thermal resistance of the air layer on the test plate’s surface, without the specimen. Lastly, we had to find the test specimen’s thermal resistance. We did this by subtracting the air layer’s resistance from the total resistance.

The purpose of this method is to reduce the interference of changes in the surrounding environment (e.g., temperature and humidity, airflow, etc.).

However, the climate chamber is semi-closed. The heat from the test gradually accumulates in the chamber. The chamber’s temperature rises. It creates a gradient from the test plate to the top. This is especially serious after a long test. This affects the heat dissipation through the specimen in the test. The chamber’s measurement point no longer shows the specimen’s surface temperature. Also, the air plate test at a certain time can no longer reflect the thermal resistance of the air layer at each time. The method will cause an unavoidable bias in testing multiple specimens. Measuring each test specimen for an air plate thermal resistance will close this gap. However, the test environment for each specimen is always changing. Inconsistent results can arise that we cannot fix.

In 2011, ASTM D 1518 restricted the heat transfer test to the cotton tyre test, due to flaws in the method. No one has updated the 2014 edition. Some say the standard is now obsolete. ASTM F1868 covers testing thermal resistance.

Also, ISO5085-1:1989 and ISO5085-2:1990 use two methods to test the thermal resistance of textiles. They cover low and high calorific value ranges. The first is a double hot plate (under pressure) and the second is a single hot plate. Researchers do not widely use these methods, and experts consider the standard outdated.

The static thermostatic plate method for heat transfer tests is being phased out. It has fatal flaws. We cannot detect how wet heat performance links to comfort. It is now replaced by the ventilated evaporative heat plate method. It tests both dry and wet heat transfer.

▲ Constant temperature type evaporative heat plate method (ventilated)

The ventilated thermostatic evaporative heat plate method tests textiles. It checks their heat and moisture transfer. The team created it in response to a demand for research on garment ergonomics and comfort. Researchers based their method on a skin-simulating plate tester. Scientists at the Hohenstein Institute in Germany developed it. In 1993, the organization published the international standard ISO11092:1993. It gives a test method for the heat and moisture transfer properties of textiles. The method is a basic test for the thermal and humidity comfort of textiles. Various countries widely adopted it once they released it. The EU and European national standards are, in general, equivalent to this standard. The U.S. has released ASTM F1868, which references this method.

The evaporation hot plate method looks like the flat plate method. The hot plate is below the test plate (minimum 200mm square). It has a protection board and a hot plate assembly. The test plate must maintain a certain temperature and water level. This tests thermal resistance and wetting. The test plate is a porous metal plate with a water supply device.

If the test plate discharges no water, it is the same as in the static thermostatic plate method. We can measure the thermal resistance (dry resistance). The measurement process and principle are the same.

To determine wet rent, first test a porous plate.

Cover it with a vapour-permeable but water-impermeable film and seal the edges. Then, place a sample on the film. Supply water to the plate until the film surface evaporates. The vapour can only escape through the test specimen to the environment.

Next, maintain a constant temperature on the test plate for a time. Measure the heat lost in the evaporation of the vapour. Also, measure the test specimen’s vapour pressure difference. You can convert it from the temperature and relative humidity.

To find thermal resistance and moisture rent, we must also find and subtract the value of the empty plate.

The evaporation hot plate method and an evaporative sweating device are not like the ordinary plate method. It cancels the semi-closed climate chamber. And it will replace it with a chamber that holds the test device. It will have a micro-environment. A deflector port is set 50mm above the test plate. It lets ambient air flow at 1m/s. This creates a uniform flow over the hot plate’s surface. The team measures the temperature and humidity 15mm from the test specimen’s surface. This allows for better control of the test environment’s temperature and humidity. It also ensures the specimen’s environment is stable and at the right levels. This eliminates any gradient in the air around the specimen. It also makes the measured temperature and humidity closer to the specimen’s surface. Therefore, the method from the principle of improved detection accuracy, stability and comparability.

ASTM F1868 defines two terms.

The intrinsic thermal resistance/humidity rent of the test material is Rctf /Retf. The total thermal resistance and humidity rent of the specimen and air layer at the test time is Rct /Ret. In ISO, GB, and EN standards, Rct /Ret is the thermal resistance/humidity rent of the tested material. The total thermal resistance/total humidity rent of the specimen at the time of testing is Rct /Ret. In ISO, GB, EN, and other standards, Rct/Ret means the thermal resistance/wet rent of the tested material. The test did not name or symbolize the total thermal resistance/wet rent of the specimen and the air layer. In this paper, we indicate this using Rt and Re, which you should note in use.

Also, the test plate, heat protection ring, and base plate must have separate temperature controls, no matter the method. If the hot plate’s thermostat has low accuracy (e.g., 0.5 ℃), the three boards will cool at different rates. This causes their heating moments to differ. It causes heat transfer between them. They heat and cool in cycles. So, the test plate’s heat will not reflect the specimen’s actual heat dissipation. This distorts the test data. Early flat plate testing instruments had this issue. With advances in electronics, thermostatic control is now very accurate. It can even control temperature to an ‘infinite’ level. This has improved the problem. However, labs should check the instruments used. In particular, they should be wary of older, less precise thermostatic controls.

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