“Human thermal comfort is ultimately dependent upon six
simple parameters that can be measured and assessed for any individual in any
particular environment. This allows us to design buildings within which all
occupants will be thermally comfortable”. This essay will attempt to determine
whether or not this statement is true, it will indicate the factors that
influence human thermal comfort and how these can be measured and controlled in
order to achieve a reasonable comfort level within a building. This essay will also
prove why despite of all modern advanced technology, techniques and vaste knowledge,
the human thermal environment cannot be designed to satisfy 100% of its users
due to various aspects which cannot be predicted.
BS EN ISO 7730 defines thermal comfort as ‘…that condition
of mind which expresses satisfaction with the thermal environment.’, ie the
condition when someone is not feeling either too hot or too cold.
The parameters in which human thermal comfort can be
measured are split into six main factors: four environmental and two personal. These
factors may be independent of each other, but together contribute to a person’s thermal
The four environmental factors are air temperature, radiant
temperature, air velocity and humidity. The air temperature is a measure of how
hot or cold the air is (temperature describes the energy of motion of the gases
that make up air). The radiant temperature is the heat emitted from an object,
such as a radiator or even from another person. The air velocity is the rate of
air movement, the faster the air is moving, the greater the exchange of heat between
the person and the air, and the humidity which is the level of moisture in the
air, specifically defined as “the ratio between the actual amount of
water vapour in the air and the maximum amount of water vapour that the air can
hold at that air temperature, expressed as a percentage” .
The primary two personal factors that influence human
thermal comfort are clothing insulation (clothes prevent exchanging heat with
the surrounding air and surfaces as well as affecting the loss of heat through
the evaporation of sweat) and metabolic heat, which is the heat produced
through physical activity.
The environmental factors, which are based around the
temperature of the air, the moisture content in that air, the rate at which it
travels and the source of its heat, can provide us with the capacity to create
more precise indicators of what is needed from a structure. Even though in many
cases it is relatively simple to determine the thermal comfort level only through
experiencing the setting, more exact results can be obtained by measuring these
four parameters with adequate instruments.
The temperature of the air is measured with a dry-bulb
thermometer freely exposed to the air and for this reason it is also known as
dry-bulb temperature. Temperature is usually measured in degrees Celsius (°C),
kelvins (K), or degrees Fahrenheit (°F). UK regulations suggest a minimum
temperature of 16 degrees Celsius, or 13 degrees Celsius if work involves
severe physical effort.
A hygrometer is a device used for measuring the humidity of
air. Perhaps the most important of these environmental elements is the humidity
due to its relation to the other factors . A high humidity level influences
one’s impression of temperature, making it feel a lot warmer. High levels of
humidity can also lead to breathing difficulties and even anxiety and an acute
loss of concentration. Very low humidity levels can create discomfort,
respiratory problems, and aggravate allergies in some individuals. The recommended
level of indoor humidity is in the range of 30-60% in air conditioned buildings.
A wide variety of devices are commercially available for
measuring air velocities. These include hot wire anemometers for low air
velocities, rotating and swinging vane anemometers and variable area
flowmeters. The air velocity is measured in m/s.
There are different ways to estimate the mean radiant
temperature, either applying its definition and using equations to calculate
it, or measuring it with particular thermometers or sensors such as black-globe
thermometer, two-sphere radiometer and the constant-air-temperature sensor.
Mean radiant heat is measure in degrees Celsius (°C).
With advances on modern technology and construction, it is
becoming easier to control these levels through differing mechanisms and
through clever design. Controls for thermal comfort are sensors and
controllers, such as thermostats, humidistats, mechanized blinds, mechanized
windows or ventilation openings, and any other building automation systems that
keep occupied spaces within comfortable temperature and humidity values.
These parameters can all be measured on an individual basis,
but it is only when all six are combined that we can judge an individual’s
thermal comfort. As opposed to enviromental factors, the personal factors
cannot be predicted or measured with instruments therefore impossible to
control. For this reason various individuals located in the same space will
feel different levels of thermal comfort. In addition to the two major personal
factors, clothing and metabolic heat, many other secundar personal aspects can
affect the overall thermal comfort.
For example, height, weight, age, sex and differing personal
fitness levels can have an effect on how individuals judge thermal comfort. Another
factors are wellbeing and sicknesses, as example the common cold or flu which
affects the ability to maintain a body temperature of 37°C at the core.
Moreover, individuals experience these sensations a bit
differently based on their physiology or psychological parameters, such as
Even if all the people from the same building would be wearing
same type of clothing, for example uniform, and doing exactly the same physical
activity, each individual’s thermal comfort level would be different therefore not all
occupants will be thermally comfortable.
These are however, impossible to predict and just as
impossible to control, it is for this reason they are referred to as the
personal factors so the building regulations expectancy is to design spaces
thermally comfortable for at least 80% of the occupiers.
Another aspect that makes the personal factors very
difficult to predict and is becoming more prevalent in modern construction is
the need for multi-purpose buildings. Multi-propose buildings have become
attractive as they offer a much larger target of use and much higher revenue in
most cases. In modern construction for example, many cinemas are now being
built with the tools needed to hold professional meetings. This makes it much
more difficult to determine the personal factors as the building may be used
for differing functions meaning that the work rate and clothing of its users
also vary. Someone eating popcorn in comfortable clothes may experience a
differing level of thermal comfort than a professional in a suit having to talk
loudly for prolonged periods of time. After all, the more we insulate ourselves
through clothing and increase our work rate the more we naturally produce heat
in our bodies which could be crucial in thermal comfort.
Given all the information provided, we can now go back and
re-evaluate the original statement that human thermal comfort is ultimately
dependent upon six simple parameters that can be measured and assessed for any
individual in any particular environment, which allows us to design buildings
within which all occupants will be thermally comfortable. From the evidence it
is clear that measuring the human thermal comfort level is a simple process, in
most cases all that’s required is to ask the individuals using the area or just
experience it for yourself. Simply asking whether the air feels hot or cold can
quickly establish what changes must be made. If people are complaining about
the heat, then perhaps it’s time to turn down the radiator and open a window,
if you can.
But if every individual in the same area is experiencing
differing levels of human thermal comfort, then any change will surely affect
someone else’s thermal comfort in a negative way. This may be because some had
breakfast and some didn’t, some walked to the location and some didn’t or maybe
just because they arrived at differing times. The environmental factors far
outweigh the personal factors and if all of those factors are maintained and
monitored then the human thermal comfort levels should remain at a high
standard. So in conclusion, yes, human thermal comfort is ultimately dependent
on six parameters, four basic, assessable, easily measurable factors along with
two impossible to assess, impossible to manage factors. This allows us to
design buildings within which most occupants will be thermally comfortable.