- 1 Introduction to Thermal Insulation
- 2 Thermal Comfort
Introduction to Thermal Insulation
The thermal insulation property of a fabric is defined as its ability to resist the transmission of heat by all modes.
It is also defined as the effectiveness of fabric in maintaining the normal temperature of the human body under equilibrium conditions.
The principal function of textile fabric in a clothing assembly is to enable the body to maintain itself in an acceptable physiological state with respect to thermal balance, core and skin temperature, and perspiration or sweat dissipation for all types of environmental conditions and for all degrees of bodily activity.
Physiologically, a human being is regarded as feeling comfortable when his skin temperature is between 33 and 35°C and there is no deposition of liquid sweat on the skin.
The production and disposal of perspiration are the principal mechanisms through which the body copes with changes in the metabolic rate or ambient conditions unless the changes in either are extreme.
Clothing will alter the balance between the rate of heat production and the rate of heat disposal by creating a microclimate in the layer of air entrapped next to the skin.
Whether or not the clothing can then be described as comfortable will depend on the activity of the wearer and range of ambient conditions to be encountered.
Heat is gained by the body from the sun, by internal metabolism, by physical exercise or by involuntary contractions of skeletal muscles in shivering.
Heat loss by conduction, convection or radiation depends partly on the temperature gradient between the skin and its environment, and this gradient is modified by varying the skin temperature.
Excessive heat may be dissipated rapidly by vaporization of body water, the body is used as a source of latent heat for the purpose, and clothing systems that hinder free evaporation to any appreciable extent will thus be uncomfortable.
In cold weather, the clothing must control the rate of loss of heat from the body to the environment by keeping it to a value that can be balanced by that produced by the body’s metabolic process.
Here the wearer’s principal concern is to conserve body heat, that is, to reduce its rate of loss to the environment, part of which occurs by radiation.
In hot weather, the clothing must control the rate of absorption of external heat by the body to such a value that, within limits, balance is achieved between this, the body’s metabolic heat production, and the loss of body heat to its environment, which may largely occur by the evaporation of sensible sweat.
Under these conditions, the principal concern of the wearer of clothing is to keep cool, and one of the functions of his clothing is to reduce the absorption of solar radiation by his body.
In temperature climates, the temperature of the body is greater than the air temperature, so the body is constantly losing heat to its surroundings.
Under these conditions, one of the fundamental requirements of clothing is to control heat loss from the body.
Thermal comfort is defined as the condition of mind which expresses satisfaction with the thermal environment. Defining thermal comfort becomes more complex as environmental aspects, as well as personal factors, need to be taken into consideration.
Broadly there are two major factors affecting the thermal comfort
I) Environmental Factors
a) Air Temperature
This is the temperature of the surrounding and can be measured in degree Celsius (°C)or degrees Fahrenheit (°F).
b) Radiant Temperature
There may be a temperature difference from the body to the environment. Based on the temperature difference, flow of the heat may be from body to environment or vice versa.
The transmission of heat will be more when the radiant temperature is more. In this manner, the way of heat transmission is through radiation.
c) Air/Wind Velocity
This is the speed of air movement in the environment near the wearer. This is one of the major factors which can be sensed by the human body.
Hot or cold is felt when the temperature of the air blown is higher or lesser than that of the environment respectively. The unit of measurement of air velocity is meters/sec or km/hr.
It is a measure of water vapor present in the environment. Relative Humidity is an indication of water present in the surrounding environment in the form of vapor to the maximum amount of water vapor the surrounding air can have at that temperature.
II) Human or Personal Factors
a) Clothing Insulation
Thermal comfort is highly dependent on the thermal insulation of the clothing of the wearer. There is a common practice, we use lighter clothes in summers compared to the heavy clothes in winter.
It is nothing but making the human body comfortable in the environment by using appropriate thermal insulation of clothing.
b) Work Rate/Metabolic Heat Flow
As we work or do the movements in the form of physical activities, heat is produced in our body and our body temperature goes up.
The more physical work we do the more heat we produce. The more heat we produce, the more heat needs to be lost so we don’t overheat.
Need for Thermal Comfort
Human body tries to maintain a constant core temperature of 37.4°c. The actual value of this may vary slightly from person to person but is always in a narrow range.
The comfortable surrounding environment temperature for the human body in India is 21°c. When the temperature is more or less than this body feels discomfort.
The human body needs some external agency to maintain this temperature, and clothing is one of the most common devices to provide comfort in this regards.
The metabolic heat generated together with the heat received from external sources must be matched by the loss from the body of an equivalent amount of heat.
If the heat gain and loss are not in balance then the body temperature will either rise or fall, leading to a serious threat to life.