A good indoor climate: How important is indoor air humidity?
Introduction
Indoor climate is comfortable when humidity and temperature are perceived by people as being pleasant. In modern houses, this is ensured by the ventilation of living space and in large buildings by ventilation systems known as HVAC systems. Unfortunately, air conditioning too often ignores humidity and its influence on the indoor climate and level of comfort. This often comes with serious health consequences.
The situation is different in trade and industry. If the product quality or manufacturing processes are crucially dependent on the indoor air humidity, the supply air is constantly monitored with sensors or humidity measuring devices. If the indoor air humidity is not right, it is adjusted via the ventilation system or a decentralized humidification solution. The same applies to the storage of moisture-sensitive goods, or to valuables and art artefacts in museums, archives and even vintage car garages. After all, production downtimes, mould, spores, cracks in old paintings or rusted classic cars mean financial losses.
The air we breathe is a mixture of two components. Component one is dry air, consisting of gaseous elements and compounds such as nitrogen (78.1%), oxygen (20.93%), noble gases (0.94%) and carbon dioxide (0.03%). Component two is water vapour, the proportion of which depends on the air temperature and the amount of moisture available at the respective location.
Recommended indoor air humidity
Figure 2: The comfort level chart shows when people feel comfortable. It allows the temperature and the corresponding relative humidity of the indoor air to be derived.
To date, there are no mandatory regulations in Europe for maintaining the correct indoor air humidity. This is why it has not received enough attention in ventilation and air conditioning technology. There are only recommendations and reference values that have found their way into national standards and regulations based on many years of experience or medical reports. However, efforts are currently being made in associations and standardisation bodies to turn these recommendations into facts. After all, the right air humidity plays at least as important a role in a good indoor climate as the right temperature. In fact, this applies all year round in the home, in offices or warehouses, in production facilities or in trade. Consequently, the ideal temperature range for comfort air conditioning is between 20 and 27 °C. At the same time, the relative humidity of the indoor air should be between 30 and 65 %. By way of explanation: “Relative humidity” means the ratio of the actual water vapour content in the air to the maximum amount of vapour that can be absorbed at a given air temperature. For people, 40 % is considered to be the minimum value. Relative humidity in rooms is perceived as particularly pleasant at 55%.
Room temperature and relative humidity go hand-in-hand
The connection between temperature and relative humidity is established by the so-called “humidex”. This index provides information about the temperature “felt” by people. For example: At a room temperature of 26 °C and a relative humidity of 70 percent, people actually feel a temperature of 33 °C and find it very humid. However, if the relative humidity measured with a hygrometer or the new testo 400 universal IAQ instrument is only 40 percent at exactly the same temperature, then the room feels like 28 °C. That is almost comfortable. The perceived difference in this example is 5 °C. This is why the room temperature should not be cooled down unnecessarily in summer with an air conditioner or an HVAC unit if the indoor air humidity is not taken into account at the same time. This will just waste energy unnecessarily and not increase comfort levels.
Incidentally, air conditioning experts agree that people indoors do not actually feel any temperatures at all. It is the combination of these temperatures with the indoor air humidity that they feel. Experts refer to this as the perceived enthalpy.
Natural air exchange is a thing of the past
Previous building materials such as clay, but also leaking windows or door gaps provided for a natural air exchange. To some extent, this was the solution itself to any problems with indoor air humidity. These days, energy-efficient yet leak-proof construction methods do not allow this. Without the right system technology and humidity control, the result is dry indoor air. This is especially the case in the freezing cold of winter. This is when radiators or underfloor heating systems heat up the interior temperature. If the relative humidity is then not measured and the room air is not humidified as required, the relative room air humidity will drop well below the 40 percent mark. This has a negative impact on the mucous membranes and means an increased risk of infection. Furthermore, moisture-sensitive furnishings such as wooden furniture or parquet floors will suffer – in fact, they can split. Something else that can happen – especially in damp rooms such as bathrooms, or from cooking or plants and aquariums – is too much moisture in the room. In winter, this condenses on cold surfaces such as windows, tiled floors or exterior walls. This causes unwanted mould and results in structural damage – which in turn has a negative impact on health, the fabric of the building or the furniture. Thus, achieving an optimum indoor climate always depends on correctly regulated indoor air humidity.
How does indoor air humidity become “right”?
What is already taken for granted in the manufacture and storage of products is also to be expected for people and their health in the future: In addition to room temperature, the right indoor air humidity will take on greater significance. But how does indoor air humidity become “right”? The answer is simple. Residential ventilation systems already offer integrated moisture recovery. Moreover, large HVAC systems can extract moisture from the exhaust air and return it to the supply air. In winter, this means that dry air becomes more humid. In summer, a cooling effect is produced by evaporating return moisture in the supply air flow. This type of solution is energy-efficient, but must always be operated in a hygienically clean manner. If this moisture recovery is not sufficient, there are various options for humidifying the air, as shown in Figure 2.
Regardless of whether the air is humidified adiabatically or with steam: in these cases, energy is always invested. This is either electrically or in the form of fossil fuels. This not only costs money, but also means additional CO2 emissions. Therefore, if the available humidity is recovered first, then the additionally required air humidification systems are dimensioned sensibly and, on top of this, the relative humidity is cleverly combined with the room temperature, this not only creates a comfortable indoor climate, but also saves energy and money. For these reasons, it is definitely worth addressing the issue of indoor air humidity during the planning stage, and more intensively than has been the case to date.
Figure 3: If indoor air needs to be humidified, there are various solutions that are either installed in the ventilation unit, or which introduce moisture as steam directly into a room.
Are you looking for a reliable way to measure indoor air humidity and moisture?
testo 400 universal IAQ instrument: Professional high precision
Apart from humidity, there are usually other climate parameters to be determined. The testo 400 covers all applications in the climate & ventilation sector, featuring impressive Smart-Touch operation, integrated measurement wizard for standard-compliant measurements (e.g. PMV/PPD), measurement point management, professional evaluation and documentation, and reporting directly by e-mail.
testo 440 multifunction measuring instrument: convenient and intuitive
The testo 440 is an all-rounder that offers numerous other measurement parameters in addition to humidity. Its impressive features include intuitive operation and clear display, wireless Bluetooth® probes for more convenience, internal data storage and USB interface for data export.
testo 625 thermohygrometer:
practical and compact
For measuring points that are difficult to access it has an exchangeable probe for measuring humidity and temperature.
testo 610 humidity measuring instrument: handy and easy to use
With integrated probes for measuring humidity and temperature.
testo 605i thermohygrometer:
small, but smart
Ideal for quick spot measurements of humidity and temperature, this app-based Smart Probe also offers the option of evaluation and documentation via smartphone or tablet if required.