Clothing insulation

Clothing insulation is the thermal insulation provided by clothing.
Even if the main role of clothing is to protect from the cold, protective clothing also exists to protect from heat, such as for metallurgical workers or firemen. As regards thermal comfort, only the first case is considered.
Mechanisms of insulation.
There are three kinds of heat transfer: conduction (exchange of heat through contact), convection (movement of fluids), and radiation.
Air has a low thermal conductivity but is very mobile. There are thus two elements that are important in protecting from the cold:
Another important factor is humidity. Water is a better conductor of heat than air, thus if clothes are damp — because of sweat, rain, or immersion — water replaces some or all of the air between the fibres of the clothing, causing thermal loss through conduction and/or evaporation.
Thermal insulation is thus optimal with three layers of clothing:
The three layers of air between the skin and the exterior layer also play an insulating role. If the clothing is squeezed tight (as by the straps of a backpack), insulation will be poorer in those places.
Units and measurement.
Clothing insulation may be expressed in clo units. The clo has the same dimensions as the R value used to describe insulation used in residential and commercial construction.
This is the amount of insulation that allows a person at rest to maintain thermal equilibrium in an environment at 21°C (70°F) in a normally ventilated room (0.1 m/s air movement).
There are a number of ways to determine clothing insulation provided by clothes, but the most accurate according to ASHRAE Fundamentals are measurements on heated manikins and on active subjects. Equations may then be used to calculate the thermal insulation.
Because clothing insulation cannot be measured for most routine engineering applications, tables of measured values for various clothing ensembles can be used.
According to ASHRAE-55 2010 standard, there are three methods for estimating clothing insulation using the tables provided.
Another unit that is used is the "tog":
The name comes from the word "togs", British slang for clothes.
Further considerations and examples.
Other factors that influence the clothing insulation are posture and activity. Sitting or lying change the thermal insulation due to the compression of air layers in the clothing, but at the same time - depending on the materials that are made of - chairs and bedding can provide considerable insulation. While it is possible to determine the increase of insulation provided by chairs, sleeping or resting situations are more difficult to evaluate unless the individual is completely immobile.
Body motion decreases the insulation of a clothing ensemble by pumping air through clothing openings and/or causing air motion within the clothing. This effect varies considerably depending on the nature of the motion and of the clothing. Accurate estimates of clothing insulation for an active person are therefore not available, unless measurements are made for the specific condition (e.g., with a walking manikin).
A rough estimate of the clothing insulation for an active person is:
Icl, active = Icl ×(0.6+0.4/M) 1.2 met < M < 2.0 met
where M is the metabolic rate in met units and Icl is the insulation without activity. For metabolic rates less than or equal to 1.2 met, no adjustment is recommended.
Clothing insulation is correlated with outdoor air temperature, indoor operative temperatures, relative humidity and also by the presence of a dress code in the environment in question.
Recent studies have developed dynamic predictive clothing insulation models that allow more precise thermal comfort calculation, energy simulation, HVAC sizing and building operation than previous practice. As a matter of fact, usually simplifications are used (0.5 clo in the summer, 1.0 in the winter). This may lead to systems that are incorrectly sized and/or operated. A model that is able to predict how building occupants change their clothing would greatly improve HVAC system operation.
As mentioned, clothing adaptation has an important role in achieving thermal comfort and is probably the most effective adjustment that occupants can make to adapt themselves in a thermal environment. Moreover, clothing variability may also depend on factors unrelated to thermal conditions, such as for a dress code or social influences, style preferences that may differ due to gender or work position. According to ASHRAE-55 standard, only if individuals are freely making adjustments in clothing to suit their thermal preferences, it is acceptable to use a single representative average value for everyone.
Some basic insulation values can be considered as examples of typical conditions
Temperature of thermal equilibrium.
The ambient temperature at which someone's body will be at thermal equilibrium depends on the rate of heat generation per unit area "P" and the thermal insulance of the clothing "R". The empirical formula is:
or, if "R" is taken to be the number of clos and "P" the number of watts per square metre,