Measuring Temperature: What Distinguishes Room Temperature From Air Temperature

Room temperature is the temperature a person actually perceives in a room: a combination of the air temperature a thermometer measures and the heat radiation of the surrounding surfaces such as windows, walls and radiators. That is why a room can feel cold at 21 °C air temperature when the exterior wall is noticeably cooler.

From air temperature to room temperature

Air temperature (also called room air temperature) is clearly defined and easy to measure: using an air thermometer. An expansion thermometer, for example, shows room air temperature because the substance inside the tube (e.g. alcohol or gallium) expands with heat. To get solid results, location is therefore crucial: ideally, the thermometer is indoors about one meter above the floor (body height) and isn't affected by direct heat from the sun or, for example, devices like a PC.

The meaningfulness of the measured air temperature, however, is limited. That's why the so-called room temperature includes more factors that can affect the temperature in a room. It's based on the local air temperature, but the measurement also factors in thermal radiation from devices and heating as well as the radiant temperature of the surrounding surfaces (wall, ceiling, window).

Windows are a key influencing factor here, since most heat enters or is lost from a given room through the windows. Incoming radiant heat, for example from sunlight, only indirectly raises air temperature: instead of heating the air in the room, sunlight warms surrounding surfaces or other objects it hits. If it's very cold outside and the sun isn't shining, hardly any thermal radiation enters 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 released. During office work, it's between 110 and 130 watts, and during brisk cycling, 400 to 500 watts. While human core body temperature stays constant at 37 degrees, optimal skin temperature is 27 to 32 degrees Celsius.

Room temperature and the comfort factor: coziness

Air temperature alone therefore hardly allows any conclusions about how comfortable people within a room feel. Room temperature, on the other hand, also factors in the radiant temperature of the surrounding surfaces, which significantly influences how comfortable the ambient air within a room is perceived to be. Here, the heat-radiating properties of the surfaces enclosing the room (floor, wall, ceiling, window), air speed, and relative humidity are particularly important.

Beyond that, purely subjective factors also affect perception and the sense of comfort. These include physical activity, clothing, age, and the health status of the person present in a room.

To optimize the well-being of everyone in a room, technologies for climate-controlling rooms are widely used today. However, these merely produce a cold or warm airflow aimed at affecting air temperature. "It's much more pleasant, though, to regulate room temperature across a surface," says Alexander Buff, founder and managing director of the Fraunhofer spin-off interpanel GmbH.

The company develops and produces heating-cooling ceiling systems used for both heating and cooling. Buff continues: "Compared to a classic heating system, which focuses on warming the air temperature, our climate ceiling heats and cools the room surfaces. This way of regulating radiant temperature is more effective, more pleasant, and also draft-free."

Other experts go even a step further. Karl-Heinz Weinisch developed the continuing education program "Master of Indoor Climate" and is himself an indoor climate expert — a discipline that, besides room temperature, factors in humidity, acoustics, light, and air quality. He says the radiant properties of surfaces must already be taken into account during construction. "Otherwise builders get a whole lot wrong, which after construction is completed comes with discomfort and high, ongoing costs for heating and indoor climate optimization."

Read on

• Reading temperature: limit values, sensor and measurement
• Reading relative humidity: the second factor of comfort
• The air-⁠Q product variants at a glance