How do humidity and relative humidity differ?
Lexicon> Letter L> Humidity
Definition: the content of water vapor in the air, for example in a room
Alternative term: humidity
Categories: Basic Concepts, Physical Basics
Formula symbol: f (absolute humidity), φ (relative humidity)
Unit: g / m3 (absolute humidity),% (relative humidity)
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: May 20, 2010; last change: 01/09/2021
URL: https://www.energie-lexikon.info/luftfeuchte.htmlHumidity is usually not visible - it's about water vapor in the air, not about fog droplets.
The humidity (or Humidity) tells you how much water vapor is in the air, e.g. B. is a living space. This plays an important role in human wellbeing, and it has to do with energetic aspects in a number of ways. This article focuses on the importance of humidity in living spaces.
Note that humidity is usually not visible because the water vapor is finely divided as a gas. Visible mist droplets can only appear when more water is supplied than the air can absorb.
Air humidity, which is also related to moisture in building components, is an important issue in building physics. Some aspects of this are covered in this article.
Absolute and relative humidity
The absolute humidity is the content of water vapor, usually given in units of grams per cubic meter (g / m3). This value can vary between zero and a maximum value, which strongly depends on the air temperature (see Figure 1). For example, a maximum of 9.4 g / m 2 can be used at 10 ° C3 can be achieved, at 20 ° C already 17.3 g / m3. When the maximum humidity is reached and more water vapor is added, some of the water condenses, which can lead to the formation of fine mist droplets or a film of water on the walls of the room (see also the last section). Such condensation also occurs when the air is cooled down without adding (or removing) water vapor and as a result the maximum humidity falls below the original humidity. The latter happens z. For example, when air rises in the atmosphere and cools down in the process, creating fog or clouds.In many ways it is relative Humidity more relevant than the absolute. It depends not only on the water content of the air, but also on its temperature.
The relative Air humidity indicates the percentage to which the absolute air humidity exhausts the maximum value. When it is low, the air can easily absorb more moisture. In such dry air, moist objects can be dried quickly. On the other hand, as soon as 100% humidity is reached, condensation will occur and objects in the room (especially cold ones) can become damp.
If the air z. If, for example, heat is supplied to a radiator without water vapor being added or removed, the absolute humidity remains unchanged, while the relative humidity drops (see Figure 2), since the air could absorb more water vapor at a higher temperature.
For human well-being (see below) is primarily the relative Humidity relevant, not absolute.
If there is an object in a room that is cooler than the room air (e.g. an outside wall in a house that is not thermally insulated), condensation can occur earlier because the air is cooled locally, so that the relative humidity is 100% locally. can reach. In other words, the dew point fell below there. The dew point is the temperature to which the air would have to be cooled for condensation to start. It depends directly on the absolute humidity.
As an example, consider a room with an air temperature of 20 ° C and a relative humidity of 60%. According to Figure 1, the air at 20 ° C can contain a maximum of 17.3 g / m3Absorb water vapor; at 60% humidity it is 0.60 · 17.3 g / m 23 = 10.4 g / m3. This would correspond to 100% humidity at 11.6 ° C, which can be seen in Figure 1. So the dew point is 11.6 ° C. This can also be seen in Figure 3.
The article on the dew point contains further details, also in connection with thermally insulated wall constructions
One speaks of humid weather when the dew point is above 16 ° C. For example, this corresponds to a relative humidity of more than 79% at 20 ° C, or more than 59% at 25 ° C. If additional moisture is released into the air indoors, you naturally feel a sultriness earlier.
The term is more difficult than the dew point Wet temperature to understand the z. B. is common in meteorology and in cooling technology. To do this, imagine that you would, for example, bring a certain volume of the air to be examined into contact with a water surface (e.g. a moistened sponge or with sprayed droplets) initially at the same temperature, so that water can evaporate until the air is saturated with water vapor. This would cause the temperature to drop because the evaporation consumes heat (→ heat of evaporation). The final temperature, which is set under the assumption that no heat is exchanged with the environment, is called the humid temperature (or Wet bulb temperature) designated.
If the air was already very humid at the beginning, only a little water can evaporate and the humid temperature is only slightly below the original temperature. In the case of very dry air, on the other hand, the humid temperature is correspondingly lower - similar to the dew point.
The wet temperature can also be determined by measurement, in particular with an Assmann aspiration psychrometer. From this, together with the temperature, z. B. calculate the relative and absolute humidity. Conversely, of course, the wet temperature can be calculated from the other variables.
The wet temperature is of practical importance, for example when using evaporative cooling in cooling towers. Here, the humid temperature of the air used sets the lower limit for the achievable cooling temperature; therefore they are also called Cooling limit temperature designated. This can be used to estimate how the effectiveness of a cooling tower depends on the humidity.
Measurement of humidity
Since the humidity is different from z. B. If the room temperature is difficult to judge by perception, a measurement is necessary in case of doubt. There are different types of Hygrometers, both with analog display via a pointer and with digital display. Inexpensive devices unfortunately often turn out to be not very reliable; If in doubt, it is advisable to compare the display of several devices. It should also be noted that it often takes a few minutes for a device to correctly display the humidity.
Humidity in heated apartments
There are various influences on air humidity in living spaces:
- Water vapor is constantly being released into the room air, e.g. B. on the breath and skin of people, from plants and when cooking and showering. This alone would lead to a steadily increasing humidity.
- On the other hand, air humidity is transported out of the house through the exchange of air with the environment (ventilation), usually through window ventilation.
- Some building materials (e.g. untreated wood, lime, plaster or brick) and other objects (e.g. books, upholstered furniture, curtains, carpets) can store a relatively large amount of moisture, i. H. Absorb water vapor when the air humidity is high and release it again later when the air humidity is lower. This reduces the fluctuations in humidity.
- In new buildings that have not yet fully dried out and after major renovations, considerable amounts of moisture can escape from the building materials. This increases the humidity and often requires increased ventilation (e.g. over a heating period).
Problems with low humidity
If the outside air is cold in winter, its absolute humidity cannot be very high, as the air can then absorb little water vapor. If this air comes into the house and is heated there (e.g. by the heating system), this does not change the absolute humidity; however, the relative Humidity, since the air could absorb more moisture at a higher temperature. In this sense, heating - regardless of the method used - dries out the air without removing its water vapor. This effect is particularly strong on dry and cold winter days and with strong air exchange; the relative humidity can then become so low (below 30%) that the living comfort is impaired:
- Air that is too dry dries out the mucous membranes in particular, but also the skin. This increases the risk of respiratory infections, dry skin and burning eyes. Infections are also promoted by the fact that viruses and bacteria stay in the air longer in dry air. (If the air is more humid, they are enveloped by a film of water, which increases their mass and thus the rate of descent.)
- Dry dust is also blown up more easily, which can be very stressful, especially for people with a dust allergy.
- In very dry air, electrostatic charges can easily build up, e.g. B. when walking over a carpet. This leads to annoying small electric shocks z. B. when touching water taps and can damage electronic devices.
Various measures can be taken to increase the humidity in winter:
- Various types of humidifiers can be used. Unfortunately, under certain circumstances these can consume a lot of electrical energy, emit pollutants when contaminated and cause annoying noises. In addition, there is usually a certain amount of maintenance required for refilling, descaling and cleaning. But there are devices that work quite satisfactorily, even without too much power consumption.
- Natural humidifiers are plants that need a lot of water, so they have to be watered accordingly.
- You can intentionally leave the bathroom door open after showering, use the steam extractor sparingly when cooking, etc.
- An unnecessarily high air exchange rate z. B. through excessive ventilation (constantly tilted windows) or through leaky windows, doors, chimneys, etc. should be avoided.
- There are ventilation systems that not only enable heat recovery, but can also partially feed the moisture in the exhaust air back into the fresh supply air. This is particularly recommended when a large living space is used by a few people, so that little moisture is released into the air in the room.
Since the subjective assessment of the air humidity (for example via “dry heating air”) can be completely wrong, measures for air humidification should only be used if a measurement actually confirms that the air humidity is too low over a longer period of time. Only if significant electrostatic charges occur in the house can one safely assume that the air is too dry, even without measurement. Unnecessary moistening should be avoided - not only because of the effort involved, but also because of the disadvantages of excessively high humidity, as explained below.
Problems with high humidity
With little air exchange and a high supply of moisture, a very high relative humidity can develop in the apartment, which can be particularly problematic. In inadequately thermally insulated houses, in particular, condensation of water can occur in cold spots on the inside of the outer walls (see also the last section). Improperly installed interior insulation can also be problematic, as air can get to the cooler rear side. This favors the infestation with molds, which release very harmful substances into the room air. These can cause allergies and they also favor asthma, rhinitis and atopic eczema.
A very high humidity is also unpleasant, especially at higher temperatures, because the cooling of the body through evaporation of sweat then becomes ineffective; the body has to produce more sweat (profuse sweating) and is still insufficiently cooled.
In order to avoid moisture damage and health risks, the relative humidity should never exceed 60%, at least in houses without thermal insulation of the outer walls. If there are strong thermal bridges, there can be moisture problems well below 60% humidity.
Mites also multiply more intensely at high humidity levels (over 60%) and then release more allergenic substances that are problematic for those who are allergic to house dust. They often also live in mattresses, in which the average humidity can be significantly higher than in the room air if the beds are insufficiently ventilated (and possibly sealed with a thick bedspread).
The simplest and most effective measure against excessive humidity in winter is adequate ventilation, ideally using a ventilation system with heat recovery. Even systems with moisture recovery should hardly lead to excessively high humidity, then the moisture recovery usually automatically becomes less effective.
Excessive humidity in rooms is often attributed to the supposedly too tight windows (after the installation of modern windows). What is overlooked here is that the ventilation is completely inadequate as a result of leaks alone, as long as there are no extreme leaks. However, there are cases in which specifically introduced air inlet channels z. B. in window frames are necessary for the correct functioning of a pure exhaust air ventilation system; If such types of leaks are eliminated, the ventilation system becomes largely ineffective and the window ventilation would have to be operated more intensely.Dehumidifiers can be useful in certain situations. However, they are usually not the best solution for living spaces.
There are devices for dehumidifying (Room dehumidifier), which can be useful in certain situations: for example, to dry out rooms after construction work or in rooms where laundry is dried. There are special room air dryers for drying clothes. The energy expenditure for the operation of such a device is considerable, but can be justified if this avoids massive heat losses through ventilation. In normal operation of living spaces, however, room dehumidifiers are unnecessary, as the ventilation, which is necessary anyway, keeps the humidity sufficiently low.
If there is already a mold infestation, the affected areas of the wall must be properly renovated. After that, the recurrence of the problem must be reliably prevented, for example by improving the ventilation (but this can only be done reliably with an automatic ventilation system) or by insulating the house on the outside walls. Simple methods such as mold-inhibiting coatings are often ineffective and could possibly give off additional harmful substances.
If the humidity is uncomfortably high in summer (in muggy weather), ventilation naturally does not help. Air dehumidification is then only possible via an air conditioning system. Here, the air that is drawn in is initially strongly cooled so that part of the water vapor condenses out and can be discharged as waste water. The air may then be warmed up again a little. Such processes cause a not inconsiderable amount of primary energy.
The article on building ventilation provides more details on how to reduce humidity through air exchange.
A calculation example
The following sample calculation should help to understand quantitatively the drying out of living spaces due to ventilation. We assume that a single-family house is fed 200 cubic meters of fresh air per hour with the help of the fans of a ventilation system.How effectively can you dry out the house by ventilating it in damp weather?
If the weather outside is humid with 90% humidity at 5 ° C, this corresponds to a water vapor content of only 4.36 g / m3 (Grams per cubic meter). If the indoor air discharged from the ventilation system has 50% humidity at 20 ° C, this corresponds to 8.62 g / m3. So per cubic meter (8.62 - 4.36) g = 4.26 g of water are discharged net. At 200 m3/ h this corresponds to approx. 0.85 liters of water per hour or 20.4 liters in 24 hours. In even colder or dry weather it would be a little more - in extreme cases (completely dry outside air) 41.4 liters per day. In addition, the amount of water withdrawn naturally increases when the humidity in the rooms is even higher.
This shows first of all that when the outside temperature is z. For example, 5 ° C does not really depend on the relative humidity - either way the cold air cannot contain too much water vapor. Ventilation then leads to the dehydration of the living space, even if the weather is very humid. Of course, this only applies to heated rooms; In the case of an unheated basement room, it can be difficult to achieve a drying effect at all through ventilation.
On the other hand, you can see that only a few grams of water can be removed per cubic meter of air. So if in a household z. B. 15 liters of water are evaporated per day, you need quite large amounts of air. These are of course necessary for hygienic reasons, in particular to reduce the CO2- Do not allow the concentration to rise too much. These can only be reliably achieved with a suitably dimensioned ventilation system; Just with window ventilation and / or leaks in the building envelope, you can hardly reliably supply the house with more than 100 cubic meters of fresh air per hour - and if you do, then with considerable heat loss. For example, the 200 m transport3/ h in the above example 24 kWh of heat per day outside, which corresponds to approx. two and a half liters of heating oil. A ventilation system with heat recovery could greatly reduce this energy loss.
Moisture equalization over walls
It is often argued that the removal of moisture through the house walls is an essential factor for the indoor climate; it is therefore important that a wall can “breathe”. However, this is incorrect: Even with relatively permeable building materials, the air permeability is so low that the associated transport of water vapor is negligibly small. Major leaks in windows and doors, roof structures, chimneys, etc., and especially ventilation, are more significant.
However, a really important aspect can be the retention of moisture on and under the surface of a wall. Particularly porous materials such as plaster of paris, but also wooden walls can store significant amounts of moisture. Although this cannot result in a permanent increase or decrease in the air humidity, it can certainly lead to a significant reduction in fluctuations in the air humidity Moisture buffering. This can be advantageous in bathrooms, for example.
As described above, damp walls can be the result of excessively high humidity in the room - especially if the exterior walls get cold in winter due to inadequate thermal insulation. The problem becomes acute when a wall falls below the dew point: water then constantly condenses on the wall and it inevitably becomes damp.
In basement rooms, however, damp walls can also have other causes and then result in high humidity. If an outer seal (i.e. the seal between the outer wall and the soil) is defective, water can penetrate the wall from the outside, causing it to become damp inside. Other possibilities are rising damp from the foundation and waterlogging as a result of defective sewer pipes. The wall then evaporates water inside (in the basement), so that the air humidity increases there - with poor ventilation until the dew point is reached on the wall, so further drying of the wall through the air is hardly possible.
In order to make decisions about renovation measures, it is clearly essential to know whether a damp wall is the cause or the consequence of excessive humidity. In order to find out, one should observe whether the wall remains moist even if the air humidity is lowered through sensible ventilation or through the use of a dehumidifier so that the wall temperature is always above the dew point. If so, the moisture does not come from the air, but z. B. due to poor sealing of the wall or foundation, or defective pipelines. Then only remedying the cause will solve the problem. It is true that the humidity can also be kept low by constant ventilation or constant operation of a dehumidifier, but this does not solve the problem, but at most prevents consequential damage, especially caused by mold.
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See also: ventilation system, latent heat, humidifiers, dehumidifiers, window ventilation, air conditioning, air exchange rate, mold in living spaces
as well as other articles in the categories basic concepts, physical fundamentals
Question: Why can a basement room get more humid in summer if it is ventilated - even in sunny weather?
Correct answers: (b) and (c)
Question: How can the humidity of the room air change through increased heating?
Correct answers: (b) and (d)
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