What is thermal comfort?
Thermal comfort is defined in British Standard BS EN ISO
7730 as:
‘that condition of mind which expresses satisfaction with
the thermal environment.’
So the term ‘thermal comfort’ describes a person’s psychological
state of mind and is usually referred to in terms of whether someone is feeling
too hot or too cold.
Thermal comfort is very difficult to define because you
need to take into account a range of environmental and personal factors when
deciding what will make people feel comfortable. These factors make up what is
known as the ‘human thermal environment’.
Thermal comfort is not measured by air temperature, but by
the number of employees complaining of thermal discomfort.
Thermal comfort
results from a combination of environmental factors and personal factors,
Environmental
factors,
Air temperature,
The temperature of
the air that a person is in contact with, measured by the dry bulb temperature
(DBT).
Air velocity.
The velocity of
the air that a person is in contact with (measured in m/s). The faster the air
is moving, the greater the exchange of heat between the person and the air (for
example, draughts generally make us feel colder).
Radiant
temperature.
The temperature of
a persons surroundings (including surfaces, heat generating equipment, the sun
and the sky). This is generally expressed as mean radiant temperature (MRT, a
weighted average of the temperature of the surfaces surrounding a person, which
can be approximated by globe thermometer) and any strong mono-directional radiation
such as radiation from the sun.
Relative humidity
(RH).
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 higher the relative humidity, the more difficult it is to lose heat through
the evaporation of sweat.
Personal factors:
Clothing.
Clothes insulate a
person from exchanging heat with the surrounding air and surfaces as well as
affecting the loss of heat through the evaporation of sweat. Clothing can be
directly controlled by a person (ie they can take off or put on a jacket)
whereas environmental factors may be beyond their control.
Metabolic heat.
The heat we
produce through physical activity. A stationary person will tend to feel cooler
than a person that is exercising.
Well being
generally and sickness, such as the common cold or flu which affect our ability
to maintain body temperature, 37C at the core.
Other contributing
factors can include; access to food and drink, acclimatisation (this can be
more difficult where there is a high outdoor-indoor temperature gradient) and
state of health. In addition, thermal comfort will be affected by whether a
thermal environment is uniform or not. For example, draughts and heaters can
create a scorched face / frozen back effect and hot feet / cold head and hands
effect.
Controlling thermal
comfort
Thermal comfort
can be controlled or adjusted by a number of different measures:
Environmental
monitoring and control (automated or user-controlled systems,
active systems
such as heating and cooling and passive systems such as shading). NB User-controlled
systems require that users are properly trained.
Adapting or
changing clothing.
Businesses can allow people to wear different
clothing depending on conditions. They can also provide things like cloak rooms
or lockers so that people can change clothes or take off and put down coats.
The golden rule is layering, generally 3 layers, and use zips and buttons to
regulate temperature.
Allowing flexible
working hours, or changing start and finish times.
Adjusting tasks.
For example,
allowing breaks or reducing the length of time people are exposed to particular
conditions.
Providing
information telling people what sort of conditions to expect so that they can
dress and behave appropriately.
Providing or
allowing personal equipment such as desk fans.
Separating people
from sources of discomfort.
For example
putting heat generating equipment such as ICT equipment in separate rooms,
insulating pipes, preventing draughts and so on. NB draughts can be caused by
high local surface temperature differences even in a space where there is no
air infiltration – for example a cold down-draught near a window.
Providing
protective clothing (PPE Personal Protective Equipment). This should be a last
resort option.
Sources:
Thermal comfort:
http://www.designingbuildings.co.uk/wiki/Thermal_comfort_in_buildings
http://www.hse.gov.uk/temperature/thermal/explained.htm