Ventilation or airing refers to the replacement of air by the exchange of air between the exterior and interiors in buildings. The most common reason for ventilating rooms is to remove unwanted substances from the indoor air . The air exchange through openings can take place naturally through thermal convection and wind pressure or through mechanical ventilation . Various standards and guidelines can or must be used to design the minimum air exchange that is appropriate for the application . The type and efficiency of the ventilation, especially when outside temperatures are above average, determine the comfort of the room users and the energy consumption of heating and air conditioning systems .
Reasons for ventilation
The task of ventilation is to ensure the desired indoor air conditions. Depending on the requirements and the use of the structure, one or more of the following functions must be provided:
- Outside air supply: e.g. B. to increase the oxygen content of the indoor air
- Air pollution control: z. B. to discharge pollutants in smoking rooms
- Removal of thermal loads
- Removal of moisture loads
- Protection pressure maintenance: z. B. Clean room with positive pressure or safety laboratory with negative pressure
- Combustion air supply: e.g. B. Rooms with stoves , chimneys or gas appliances that rely on a combustion air supply from the installation room
The prerequisite for aerobic life is elemental oxygen (O 2 ). In return for the uptake of oxygen, living things produce carbon dioxide (CO 2 ). In this context, people perceive an increased CO 2 concentration to be more unpleasant than the corresponding decrease in O 2 . The technical rules for workplaces that apply in Germany say: "Experience has shown that an increased CO 2 concentration has a negative influence on attentiveness." In this document (ASR A3. 6.) called a CO 2 concentration of 1000 ppm . This limit value was already proposed by Max von Pettenkofer and is called the "Pettenkofer number" after him.
The CO 2 concentration can be measured using measurements. Measuring devices can regulate mechanical ventilation systems accordingly or remind the user of the need for manual ventilation via a display. An alternative offers a free available CO 2 - App IFA. This promises to calculate the CO 2 concentration in rooms. According to the IFA, the app is based on the results of a study on CO 2 measurement in schools and can also be used in the office. The description of the app also refers to the limit values specified in ASR A3.6.
The building structure can be damaged by water entering via the air. In addition, mold can damage the health of room users. As a rule, there is a risk of mold formation if the local air humidity is 80% or more over a longer period of time. Exceptions are rooms with special uses such as steam baths or swimming pools. Another problem is insufficient humidity in the air, which can also impair the comfort and health of the room users. Therefore, the humidity in conventional buildings used by humans should be kept within narrow limits.
|use||relative air humidity [%]||Temp. [° C]|
The body of an adult who is not physically active releases 35 to 40 g of water into the room air via the breath and skin every hour, i.e. just under a liter per day. When drying 4.5 kg of spun laundry, 1 to 1.5 liters of water can be assumed to give off moisture. A warm (shower) bath produces around 0.5 to 1 liter of water vapor per person and day. About the same amount is added when cooking and baking as well as through the evaporation of indoor plants. In a four-person household, a total of around 8 to 15 liters of water are released into the room air every day.
Since cold air can absorb less moisture than warm air, the moisture content of cold outside air is lower than that of inside air (even if the relative humidity should be the same). The colder the outside air, the drier it is and can absorb correspondingly large amounts of moisture if it meets the warm indoor air during ventilation. The so-called hx diagram shows this relationship graphically. It is used to determine the degree of saturation of the air with moisture and shows the planner what water content the air can absorb at what temperature.
Environmental problems with ventilation
Higher outside than inside temperature
If the outside temperature is higher than the inside temperature, the ventilation can lead to an increase in humidity in the building. In any case, this is the case when the humidity of the outside air is so high that condensation forms on contact with the cooler surfaces of the interior.
This unwanted moisture transport occurs:
- all year round in southern countries, especially in hot and humid tropical climates
- year-round when ventilating cellars
- when unheated buildings heat up more slowly than their surroundings in spring
- when churches , bunkers and other buildings with very massive construction are colder inside than the surroundings (especially in spring and summer)
- in all other buildings if the outside temperatures rise sharply during the day during the warm season.
The resulting condensate is particularly noticeable in uninsulated basement rooms, the temperatures of which are usually between 8 and 15 ° C all year round and therefore often below the temperature of the outside air during the day.
If cold basement rooms are ventilated during the day in summer, considerable amounts of condensation water can condense on the cold surfaces and often contribute more to the moisture penetration of the basement walls than rising soil moisture. Only ventilation in winter can reliably dehumidify basement rooms.
If adapted manual ventilation cannot be guaranteed, we recommend installing special ventilation devices (usually as duct fans) which measure the temperature and humidity in the outside and inside and only ventilate when the outside temperature is sufficiently cold.
Contaminated outside air
Various discussions prepared by the media have brought the pollutant load back into the public consciousness in high traffic areas. Various studies show that in a busy environment the outside air is often excessively polluted by fine dust, nitrogen dioxide and benzene, among other things. In the case of a room ventilation system in accordance with DIN 1946-6 or DIN 18 017-3 or free ventilation via windows, unfiltered outside air causes a higher level of pollutants. As long as politicians do not take effective steps against pollution from car exhaust fumes, ventilation with free post-flow near main roads and highways must be roofed over. The values from the more than 500 measuring stations of the Federal Environment Agency can be helpful for assessing the pollution at the project site.
If the wind is not the driving force behind natural ventilation, it is based on natural convection and is also known as free ventilation . The air exchange takes place through openings (leaks, i.e. joints, in the building envelope, open windows or special ventilation openings such as outside air passage elements and roof attachments) in the building. The drive for free ventilation is provided by pressure differences as a result of the wind flowing around the building and differences in density of the air as a result of temperature differences.
Window ventilation is understood to be the exchange of air caused by opening windows. A direct external reference contributes to the well-being in residential buildings and other lounges, as does the influence of the user on the supply of fresh air through openable windows.
To avoid energy loss, it must be ensured that the air exchange rate is easy to control. With the joint ventilation that used to be common through leaky windows and doors and other smaller openings in the building envelope, this was hardly possible. If the windows are tight, ventilation by repeatedly opening the windows at different times requires a lot of attention. In winter, discomfort caused by cold air flowing in can often not be avoided. Depending on the spatial situation, environmental influences - such as traffic noise , wind or air pollution - can become uncomfortably noticeable if the ventilation is done by opening the window.
In the cold season of the year, the necessity or success of ventilation measures can be read off the occurrence of condensate on the edge of the window pane, particularly with single-glazed and older thermal insulation windows. On hot summer days, the daytime air change should be limited to what is hygienically necessary to keep the rooms cool. The entry of street noise can be reduced through box windows , a double facade or soundproofed ventilation openings.
As intensive ventilation of the rooms as possible over a shorter period of time - so-called burst ventilation - is often recommended as a very energy-saving and healthy way of ventilating buildings without mechanical ventilation systems with high temperature differences between the mean room air temperature and the ambient temperature.
The local ventilation effectiveness with windows or doors depends on the position of the sash in the opening, i.e. the flow pattern of the openings. The flow pattern in the room and thus the global ventilation effectiveness can be further improved by cross ventilation , i.e. opening two opposite openings.
The advantages of boost ventilation:
- With the window fully open, an air exchange rate that is around 90% greater than that of a tilted window can be achieved.
- During the air exchange, the room air cools down noticeably. Air movement, temperature changes and the incoming fresh air make it easy to see when the boost ventilation can be stopped.
- The ventilation is stopped immediately after the brief exchange of air. The energy exchange is low. The temperature difference between the fresh air and the indoor air can be compensated for in a short time by the air conditioning or heating system.
- The limited ventilation time avoids any noticeable cooling of the window reveal and the areas surrounding the room.
- Due to the stronger air movement, the air in otherwise less intensively ventilated corners of the room as well as behind curtains and furniture is (partially) exchanged.
- If there is a lot of moisture in the bathroom and kitchen, or when drying clothes, moisture from the room air is temporarily stored in walls, ceilings and furniture. In order to dissipate this moisture, repeated burst ventilation is required every half an hour.
- The regularly performed burst ventilation requires a lot of manual effort. Alternatively, boost ventilation can be implemented as required. However, this requires attention in order to perceive the increased humidity in the room.
The formation of mold can be reliably prevented by frequent, brief ventilation. The more often the windows are opened briefly, the easier it is to dehumidify the walls and keep them dry. Structures that are particularly critical in terms of building physics (e.g. interior insulation or extended roofs with insufficient ventilation of the insulation or inadequate vapor barrier) can thus be protected from permanent moisture penetration during the cold season.
Gap ventilation and adjustable ventilation openings
Another possibility to minimize energy losses during manual ventilation is to use gap ventilation adapted to requirements or to use simple adjustable ventilation devices. Here it is important to open the window sash or ventilation device just enough that a hygienic exchange of air is guaranteed and at the same time enough moisture is conveyed out of the building to prevent the rooms from becoming humid.
Gap ventilation can also dry out soaked building fabric over a longer period of time due to an accident or rising ground moisture. Since the moisture contained in solid components such as walls and ceilings only comes to the surface slowly, manual intermittent ventilation would otherwise be required at all times of the day.
It can be problematic to recognize the respective need and to adjust the opening width of the ventilation openings accordingly. Adjustable locking mechanisms help to lock the only slightly tilted window sash, but are still not very common in Germany. In order to remove short-term amounts of moisture (e.g. when bathing, showering, cooking and drying clothes), it is advisable to carry out intermittent ventilation in addition to the gap ventilation . The air throughput of the gap ventilation can then be set to the hygienically desirable minimum.
Since the air throughput increases due to the differences in air pressure with falling outside temperatures, the width of the ventilation openings should be varied according to the season. The wind speed also has a major influence on the air throughput. Ventilation devices are available for installation in window frames or in the outer wall, which automatically reduce the ventilation opening when the wind pressure increases. If regular cleaning and maintenance of the mechanism can be ensured, these devices are preferable to simple night ventilation.
The classic night ventilation consists in limiting the opening width of the window sash to the required size. In order to avoid the entry of rainwater, the tilted position of the window is usually chosen. With a small opening width or with little exposed windows (facing away from the prevailing wind direction or under roof overhangs or the like), the sash can, however, also be opened and fixed in the usual way.
The simplest measure is often to clamp a wooden wedge or cardboard between the sash and the frame in order to hold the window sash in the desired position and thus to be able to control the air flow. Alternatively, an elastic band can be attached between the window sash and frame, which presses the window sash against a block pushed into the rebate at a suitable point. A less finely adjustable locking of the sash is achieved by screwing a screw from the outside into the edge of the window sash, but not completely countersinking it; an angled perforated strip is screwed to the window frame; the window sash can then be fixed in various positions by sliding one of the openings of the perforated strip over the head of the screw.
A variety of ventilation devices are available that are attached to the window frame or sash and allow the amount of air to be regulated. Many of them can also be installed later. A simple solution is e.g. B. is to drill ventilation openings in the window frame, which can be closed with a flap or a slider. If the window is equipped with rubber seals, these can be partially or completely removed in order to achieve permanent night ventilation. The air throughput then depends on the width of the gap between the window sash and frame and cannot be further regulated. If the sealing profiles can be pulled out of the retaining groove undamaged, they can also be reinserted if necessary when the outside temperature is cold.
The short range of the night ventilation is a disadvantage, especially if no draft is created. When there is no wind, slowly rotating air rolls form in the room, the intensity of which is determined by the difference between the inside and outside air. Areas outside of the air rolls are hardly ventilated. Mold is more likely to develop behind curtains and furniture standing close to the outside wall.
Shaft ventilation (without fan)
From the late 19th to the late 20th century, multi-storey buildings were often equipped with one or more exhaust air shafts per unit of use, especially in large cities with high population densities, which were usually routed over the roof. As the air throughput increases with falling outside temperatures due to the increasing differences in air pressure, it is necessary to regulate the opening width in order to limit the ventilation heat loss and to achieve an adequate exchange of air. Winter supply air preheating can be achieved with the help of a double facade and heat exchangers in the supply air flow. The latter also include geothermal heat exchangers .
Shaft ventilation was often used in larger apartment buildings and apartment blocks in the past . There are three variants to be distinguished:
- In the case of Berlin ventilation , the supply air was routed through the neighboring rooms through door and window joints or specially provided openings. The air throughput then depends in particular on the tightness of the windows and any leaks that may be present in the building envelope. This type of ventilation is no longer permitted according to the current provisions of the EnEV .
- With Dortmund Ventilation , the supply air is routed from the facade through a transverse duct for each apartment via a separate duct into the respective hallway . Due to the pre-heating of the air in the hallway, the inflow of fresh air is less noticeable in the surrounding rooms. Appropriate regulation of the draft requires the attention of users.
- With Cologne ventilation , the supply air is sucked in through its own duct, similar to Dortmund ventilation , but is then directed directly into the room to be ventilated, usually windowless. This can lead to drafts.
The German DIN 1946-6 provides detailed specifications for the design of apartment ventilation shafts. They are generally to be guided vertically, but may run diagonally once, at an angle to the horizontal of at least 60 °. They must also be easy to clean and at least 140 cm² in size. Precise information is also given for guidance over the roof. Another German standard for shaft ventilation was DIN 18017-1, which was withdrawn in December 2010.
The joint ventilation of a room is caused by the fact that air penetrates the room through leaks in the building envelope. A prerequisite for this ventilation is a pressure difference between inside and outside, which is caused on the one hand by temperature differences and on the other hand by wind between the windward and lee side of the building.
Roof top ventilation
Roof top ventilation is understood to be the free ventilation that occurs through attachments, short shafts or similar ventilation openings in the roof of buildings. As with duct ventilation, the drive is the thermal lift, which results from the temperature difference between outside and inside.
In the case of mechanical or mechanical ventilation, the air delivery is driven by a flow machine. In the specialist literature, a distinction is made between air conditioning devices and air conditioning systems. As a rule, ready-to-plug-in room heat exchangers and room material exchangers are defined as devices. Ventilation systems, on the other hand, include all ventilation, partial air conditioning and air conditioning systems.
In order to relieve the users of the regular control or implementation of manual ventilation, mechanical ventilation systems with independent controls are also increasingly being provided in residential buildings. With increased awareness of the energy requirements of buildings, more attention is also paid to heat losses due to manual ventilation. The air exchange inherent in older buildings is no longer automatically guaranteed due to the current building standards.
In commercial buildings, ventilation or air conditioning systems are usually part of the standard equipment. In addition to energy savings and the simple control of the room climate through precise control of the ventilation function, the ventilation system often also serves to prevent the indoor air from being polluted. As required by the EneV 2014, the plants usually have a heat exchanger , a heat recovery guarantee of at least 80% of the exhaust air.
Decentralized ventilation units
If demand-controlled, manual ventilation cannot be guaranteed by the user, the installation of small ventilation devices with reversing operation and regenerative heat exchangers in the outer wall is recommended. In addition to a wall opening with a diameter of approx. 160 mm, all that is required is a power supply for the devices. With the help of a core drill, circular openings can be made in the building's outer wall, into which the ventilation devices can be pushed. These electrical ventilation devices change direction every few minutes in order to return the heat stored in the heat exchanger when the air is blown out back into the room.
The heat recovery achieves such a high level of efficiency that the overall efficiency is hardly inferior to that of full-fledged ventilation systems with significantly expensive heat exchangers. This solution is particularly useful when the windows should remain closed for reasons of soundproofing.
For an efficient and controlled exchange of air, the devices should be installed in pairs and connected in such a way that one fan is switched to supply air mode when the neighboring fan is in exhaust air mode. If the door gaps allow sufficient air exchange between the rooms, the ventilation devices assigned to one another can also be located in different rooms.
Demand-based living space ventilation
An indicator of inadequate ventilation is the permanent presence of condensation water. This occurs when there is a large difference between the indoor and outdoor temperatures at thermal bridges such as B. the lower edge of window panes or on the sealing joint between the glass and the frame of windows. In insulated residential buildings in which there are no other dominant thermal bridges, the edge areas of insulating glass panes are often the coldest areas in the room, where the humidity is first reflected. An exception are windows, which have a significantly lower coefficient of thermal conductivity than the surrounding outer wall. In this case, the condensation will settle on other components with a greedy temperature than the indoor air, such as B. the coldest surfaces of the walls, without the user receiving an indication of increased humidity levels from condensation on the windows. The condensate then typically arises first on window reveals or less ventilated areas such as B. in the corners of a room or behind furniture that is without a gap on the outside wall. If the walls are not able to absorb the condensate immediately and to lead it away by capillary action, there is a risk of mold formation.
In the kitchen and in damp rooms, the formation of condensation can often not be avoided. However, the water droplets should have evaporated a few hours after cooking, bathing or drying laundry. If this is not the case, more frequent ventilation is required to prevent long-term structural damage and the formation of mold.
To reduce the air humidity, both short, intensive and constant ventilation are suitable if the following conditions are met:
- The dew point of the outside air is lower than the dew point of the inside air. This is usually the case when it is colder outside than inside. In summer, the conditions can be reversed in very warm, humid weather. The windows and especially the ventilation openings to the basement should then be kept closed so that the warm, humid outside air does not lead to condensation on cold interior surfaces.
- The ventilation must be controlled. In the case of short, intensive ventilation, the windows should be closed again after a few minutes, as the dehumidification effect is drastically reduced once the room air has been completely exchanged. If ventilation is continued, the surfaces of the walls and floors cool, which prevents the walls from drying out further. With constant ventilation, the air exchange must be reduced so much that in cold outside temperatures and windy weather there is a minimal, barely noticeable draft in the immediate vicinity of the ventilation openings.
If the entire interior air is exchanged once by briefly opening the window wide, the moisture content of the room air in a 90 m² apartment is reduced by around one to two liters in winter. A comparison with the above-mentioned values for the moisture load in the air shows that in an airtight new building without mechanical ventilation it would be necessary to carry out a complete air exchange 8 to 15 times a day (if the windows otherwise remain closed). On the other hand, if the outside temperature is low, it may be sufficient to ventilate 4 to 7 times a day for a few minutes, as the cold air can absorb more moisture.
Since problems often arise in practice due to user behavior with regard to manual ventilation, the following constructive tips can help:
- Vapor retarders and other water-impermeable building materials are dispensed with in the wall structure in order to enable condensing moisture to be dissipated to the outside by capillary and sorptive effects . With the appropriate wall structure, traditional mineral and natural building materials in particular are able to absorb larger amounts of moisture by sorptive means and divert it to the outside by capillary action. By increasing the moisture content of the outer wall, the thermal insulation value of the wall decreases . The loss of insulation is minimized if the building materials can drain the condensate to the outside relatively quickly. For the systematic formation of condensation in the outer wall, see also: Interior insulation .
- A ventilation system with heat recovery will be installed. In a properly planned system, the electrical energy required to drive the fans is designed as required, so that this variant can be an economically and technically sensible alternative despite higher investment costs.
The recommendation for action should be that, at least during the cold season, care should be taken to ventilate the room sufficiently frequently for a short time. In the transition period, the windows should be left in the tilted position several times a day for a longer period of time when the user is present. If the rooms are heated at the same time, the higher heat loss due to the temporary "permanent ventilation" must be accepted. Heat losses can be limited by opening the window sash just a crack instead of using the full tilt position of the window.
Structural framework conditions
Leaky building envelope in old buildings
Before increasing importance was attached to a tight building envelope from the end of the 20th century, there was always a certain exchange of air via joints in windows and doors, chimneys and leaks in the entire building structure (see: Joint ventilation ). If the air flow at one point became so great that the users felt a cold draft, the gap that caused it was usually sealed. The air exchange was thus regulated to an effective minimum. A complete sealing of the building, as is common today in low-energy houses, would have hindered the flow of combustion air for the operation of the wood and coal stoves and kitchen stoves that were common in the past. At the same time, the negative pressure of the chimney draft in winter ensured a constant flow of exhaust air from the living rooms, even when the stoves were not in operation.
If the dehumidification of the indoor air via leaks and chimney drafts in the kitchen, bathroom and laundry drying room was not sufficient, condensation occurred on cold areas of the outer walls. With single-glazed windows, the pane of glass is always the coldest surface in the room, so that the humidity is reflected there and runs down the glass. Until the middle of the 20th century, window sills were usually provided with a channel under the drip edge of the window sash to collect the condensate. In the recessed center of the collecting channel, a tube was often let in to direct the condensate outside or into a collecting container.
The room air was reliably dehumidified by regularly removing the condensate in the collecting channels in single-glazed windows. As a result, no more condensation occurred on other areas of the room and the outer walls remained dry. The formation of mold on the cement joints and the frame wood of the windows could be prevented by occasional wiping.
Modernization of the windows
The installation of modern, double-glazed and thermally insulated windows in uninsulated old buildings thus contributes to the moisture content of the building structure in two ways :
- Since the air humidity no longer condenses on the window, it is reflected on the entire outer wall surface if there is insufficient ventilation. The thermal insulation value of the outer wall decreases with increasing moisture content. The lower the insulation value, the lower the temperature of the inner wall surface, which promotes condensation again. This self-reinforcing effect can lead to moisture penetration of the outer walls (and possibly roof surfaces) in the case of physically unfavorable wall structures, which often only dry out completely in the course of the following summer.
- Modern windows usually have sealing profiles and, thanks to precise manufacturing and triple rebates, are still significantly more airtight than traditionally manufactured windows, even after the sealing lips have been removed. In the past, the joints between the window sash and the window frame resulted in a constant exchange of air, through which a certain proportion of the resulting air humidity was removed at all times. Since the amount of cold outside air entering the building was dependent on the wind pressure and therefore not precisely calculable, importance is now placed on a tight building envelope, so that the necessary air exchange must be ensured by manual ventilation or a ventilation system.
Heat loss through ventilation
With free ventilation, for example through window or door openings, the outside air temperatures are lower than the inside air temperatures. To evaluate these losses, it must be taken into account that air has approximately the same specific heat capacity as building components made of brick or concrete - 1.0 kJ / (kg · K) - but due to the significantly lower mass of air compared to that of walls or the ceiling, the actual heat capacity of the room air is negligible. The following applies: If the room air is exchanged once within a short period of time, only a small amount of the thermal energy stored in the air is lost. The thermal energy of the components, on the other hand, is almost retained as the cold outside air supplied is quickly warmed up by them. With constant ventilation with high air exchange rates, on the other hand, the massive components are cooled down by the fresh air and energy is lost. If the room user compensates for these energy losses through his heating behavior, this leads to rising heating costs.
Energy savings can be achieved through needs-based heating behavior in combination with ventilation appropriate to the use. This can be achieved, for example, through presence or activity-influenced control of the heating system and intelligent behavior of the users in manual ventilation.
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