From air temperature to room temperature
The air temperature (also called room air temperature) is clearly defined and easy to measure: with the help of an air thermometer. An expansion thermometer, for example, indicates the room air temperature because the substance in the tube (e.g. alcohol or gallium) expands when heated. So to get solid results, the location is crucial: ideally, the thermometer is located indoors about one metre above the floor (body height) and is not affected by direct heat from the sun or from devices such as a PC.
However, the informative value of the measured air temperature is limited. Therefore, the so-called room temperature includes more factors that can influence the temperature in a room. The basis is the local air temperature, but in addition, the measurement will also include the thermal radiation of appliances and heaters as well as the radiation temperature of the surrounding surfaces (wall, ceiling, window).
The windows are a major influencing factor here - because most heat enters the respective room through the windows or is lost accordingly. The penetrating radiant heat, for example from the sun's rays, increases the air temperature only indirectly: instead of heating the air in the room, the sun's rays heat the surrounding surfaces or other bodies they hit. If it is very cold outside and the sun is not shining, hardly any heat radiation penetrates from there. If a person sits near a window, it feels cold.
By the way: humans also produce radiant heat. Even at rest, 70 to 90 watts of body heat (basal metabolic rate) are emitted. During office work between 110 and 130 watts, during brisk cycling 400 to 500 watts. While the human core body temperature is a constant 37 degrees, the optimal skin temperature is 27 to 32 degrees Celsius.
The room temperature and the feel-good factor: comfort
So conclusions about the well-being of people inside the room can hardly be drawn from the air temperature alone. The room temperature, on the other hand, also includes the radiation temperature of the surrounding surfaces, which have a decisive influence on the extent to which the ambient air in a room is perceived as comfortable. Here, the heat radiation properties of the surfaces surrounding the room (floor, wall, ceiling, window), the air velocity and the relative humidity are of particular importance.
In addition, purely subjective factors also influence perception and the feeling of comfort. These include physical activity, clothing, age and the state of health of the person present in a room.
In order to optimise the well-being of all people in a room, technologies for air-conditioning rooms are frequently used today. However, these merely generate a cold or warm air flow that is designed to influence the air temperature. "But it is much more pleasant to control the room temperature over a wide area," says Alexander Buff, founder and managing director of the Fraunhofer spin-off interpanel GmbH.
The company develops and produces heated-cooling ceiling systems that are used for both heating and cooling. Buff continues: "Compared to a classic heating system, which focuses on heating the air temperature, our climate ceiling heats and cools the room surfaces. This way of regulating the radiant temperature is more effective, more pleasant and also draught-free."
Other experts even go one step further. Karl-Heinz Weinisch developed the further training course "Master of Indoor Climate" and is himself an expert for indoor climate - a variable that includes room temperature, humidity, acoustics, light and air quality. He says that the radiation properties of the surfaces must already be taken into account during construction. "Otherwise, builders get a lot of things wrong, which is associated with discomfort and high ongoing costs for heating and indoor climate optimisation once construction is complete."
(Image: Pexels / Lisa Fotios)
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