Breathing wall

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The idea that a wall must be able to breathe in order to create a comfortable room climate and to avoid mold on the wall goes back to an error of Max von Pettenkofer (1818–1901) and is still popular today in various interpretations. So-called breathability is also a topic of advertising for functional textiles.

In modern building physics , the water vapor permeability of components or building materials is made dependent on whether the amount of condensation water specified in DIN 4108-2 and 3 is exceeded in the component. For this purpose, the building materials are divided into the categories "diffusion-open", "diffusion-inhibiting" and "diffusion-tight" without evaluation. Static (Glaser) and dynamic calculation methods (WUFI, COND, DELPHIN) are used to verify the amount of condensation.

Historical

The hygienist Prof. Max von Pettenkofer determined during air exchange measurements in his office in 1858 that after all the joints had supposedly been sealed, the air exchange rate decreased less than expected and explained this by a significant air exchange through the brick walls. As far as we know today, however, he had overlooked that rooms also have components other than walls and that the furnace in his test room was not sealed. In particular, the wooden beam ceilings of that time turned out to be very joint leaks when measurements were taken with and without linoleum. He demonstrated that bricks , air-lime mortar and similar porous building materials are permeable to air by means of an experiment in which he placed a small funnel on the end face of a cylindrical and laterally sealed sample piece a few centimeters in size and blew out a candle by blowing vigorously through the sample. According to Pettenkofer, the exchange of air through the walls of the room is an essential contribution to the exchange of air in the room . Wet walls, on the other hand, would hinder the exchange of air (as can also be demonstrated in the experiment). Pettenkofer was still strongly influenced by the medieval " miasm concept ".

It is true that numerous porous building materials are air-permeable in the sense of Pettenkofer. Air transport through the pore structure can only be set in motion by a difference in air pressure between the two sides of a wall. Since the air pressure in the building usually hardly differs from the outside air pressure, there is no driving force for such a transport process. The back pressure on the outer surface caused by the wind is too low to generate air exchange rates that could be significant in comparison to the other leaks. In addition, such building materials are always used in conjunction with an airtight layer, e.g. B. plasters, building boards, etc. are used so that the wall as a whole is not permeable to air. This was confirmed in 1928 through measurements under defined boundary conditions on components by Dr. Ernst Raisch proved.

In 1958, the Göttingen professor of thermodynamics Helmut Glaser was looking for a method with which he could dimension the moisture balance of the external components of cold stores. Due to their internal temperature of at least −18 ° C, there is a strong temperature and humidity gradient from outside to inside all year round. Failure to observe the diffusion of water vapor led to damage in the thermal insulation made of wood wool lightweight panels and cork on cold store walls, at the boundary layer to the inner seal (tiles, oil paints). Glaser developed the first calculation method for water vapor diffusion, which is still valid today, in order to model the diffusion flow of water vapor into the construction in such a way that a possible amount of condensation water in the component remains uncritical. Capillary water transport processes were not investigated as the aim was to keep walls dry. Capillary water transport only occurs with liquid water in the component, which would not only cause frost damage at −18 ° C. The Glaser method was included in DIN 4108-3 as a verification method in 1981.

Moisture removal

In the environmental and health discussion of the 1980s, the terms breathing wall , open-pore and moisture exchange through diffusion-open building materials were taken up again and the exchange of humidity between indoor air and outside air in the Pettenkofer sense was again interpreted as a contribution to healthy living. Some authors said that materials such as wood and brick that are permeable to water vapor (or open to diffusion) are more 'natural' than modern building materials such as glass , concrete or plastics . In addition, the moisture removal through the wall prevents excessive humidity in living spaces, even without constant ventilation. This misjudged the extremely small order of magnitude of the quantities of water vapor moved by water vapor diffusion and overlooked the fact that this moisture transport through components was treated as a potentially damaging factor in building physics.

Every building material is in a moisture equilibrium with its surroundings. Depending on its location, a moisture equilibrium and a typical water content are established in the component. A component is in a moisture equilibrium with the water vapor content of the indoor and outdoor air. This is very dry in winter (30% relative humidity) and the room air is more humid (around 30–60% relative humidity). The migration of the water vapor molecules is based on their own temperature-related movement (energy charge) and their concentration gradient inside / outside.

In a normal household are breathing, perspiration (by humans and houseplants ), cooking, etc. about 5 to 10 liters of water per day as water vapor free. Only 1–3% of this can be discharged to the outside through the walls by means of water vapor diffusion, as all building materials offer resistance to diffusion. Under certain circumstances, there is a risk of damage from frost cracking after condensation or sublimation , if moisture penetrating or penetrating the wall collects there and is not removed.

Complete vapor-tightness of the components is not required and is not required in DIN 4108. In the case of solid walls or ceilings, a winter diffusion amount of up to 1 kg of water in the component does not lead to damage if it can diffuse out again in summer. A 38 cm thick solid brick wall from the Wilhelminian era is commonly referred to as "breathing" and has z. B. 232 grams of calculated amount of condensation water per m² and dew period, while the same wall is free of condensation water from a 12 cm thick outer insulation if the water vapor diffusion resistance factor of the insulation is ≥30. With the calculation method, a wide variety of materials and constructions can be selected in accordance with standards. You just have to give up the false conclusion that components have to be permeable and water vapor diffusion is a dehumidification factor.

Air exchange

In living spaces, a calculated value between 0.3 and 0.6 per hour applies to the air change (DIN 4108-2 of 2011, Section 4.2.3 and Energy Saving Ordinance). An air exchange rate of 0.5 / h means that half the volume of air in the enclosed space is exchanged once within an hour. If there are no noticeable smells in the apartment, insufficient air exchange is often expressed in excessively high humidity. Due to the mixing of the fresh air with the existing room air, however, the room air is usually not completely renewed. The ratio of the "actual air renewal" and the air exchange rate is a characteristic parameter for the selected ventilation concept and is referred to as ventilation effectiveness . Alternatively, a ventilation system with and without heat recovery (heat exchanger) can ensure the necessary air exchange rate.

Critics of water vapor tightness due to intentionally and unintentionally attached diffusion-tight layers use the air changes in living rooms (see here ), which are too seldom carried out in reality (because of the ventilation heat losses and associated cooling of the living spaces ) as an argument not to install or allow vapor barriers.

Moisture buffering

The moisture production in homes varies greatly, as by cooking, showering, sleeping, etc. At times, high peak values of humidity with condensation water on cooler places such as thermal bridges or " ice flowers " are in poorly insulated windows by water vapor sorption in all hygroscopic , interior linings of all components (plasters Wood-based materials, fibreboard) prevented or defused. Sorption is a natural property of all mineral building materials and has no drive mechanism. The sorption quantities are also released back into the room air when the room air load with water vapor from cooking, bathing, etc. decreases again. They have to be ventilated away from the room air. There is no alternative to active ventilation. Since diffusion-inhibiting or diffusion-tight building materials are located behind the final coatings on the room side of the components, their sorption capacity is retained. In the case of interior insulation, diffusion-open, so-called capillary-active insulation materials are increasingly being used. These can suck the liquid condensation water generated in the component through their diffusion openness inside and outside their pore structure and defuse the problem again. It should be noted that condensation can occur in the zone with temperatures below zero. Damage caused by diffusion-inhibiting insulation materials in interior insulation is also not proven, but pure claims. On the contrary, the loss of condensation water can be minimized by internal diffusion-inhibiting layers, so that the calculated amount of condensation water that occurs in the wall in the uninsulated state is reduced.

Relationship to other substances

As a rule, the wall surfaces are not the only sorbent surfaces in the room. Textiles such as carpets, curtains or upholstered furniture usually have even greater sorption capacities than the wall materials and can have very large surfaces. Furnishings made from untreated wood can also contribute to sorption to a certain extent. However, there is also a moisture equilibrium in the closed system, moisture can then only be removed from the system by ventilating drier air. If this happens, the moisture peaks occurring in the living area are only of relatively short duration, so that the sorbed moisture has little time to penetrate deep into the wall before it is desorbed again. Experimental and computational studies show that under these circumstances the majority of the buffering processes only take place in the first few centimeters below the wall surface. The moisture buffering effect of the wall is not impaired if deeper parts of the wall are blocked off by a vapor barrier and the moisture is removed from the system by air exchange. A "breathable" interior planking such as B. plasterboard is sufficient to achieve the same effect. Clay plaster can absorb up to nine times more moisture than gypsum.

The airtightness, which is achieved by well-sealed new windows, prevents "automatic" air exchange, this must then be guaranteed by regular manual ventilation or a supply air / exhaust air system.

Extreme areas

Moisture buffering also requires a sufficient possibility of releasing the absorbed moisture. With regular heavy exposure to moisture, e.g. B. in the bathroom , a "breathable" wall surface may be a disadvantage if it absorbs moisture and appears dry, so that there is no longer sufficient ventilation to dry and mold growth occurs in the long term . A surface that is neither “breathable” nor absorbent (such as tiles) would be safer here, on which condensation water is clearly visible and makes the need for ventilation visible.

rain protection

Paints, protective coatings and plasters on exterior walls that are exposed to rain should allow as little water as possible to penetrate in liquid form, but on the other hand should be as permeable as possible for water vapor - that is, "breathable". The latter is important if water has penetrated the wall in another way and especially if the actually water-repellent coating forms cracks as a result of aging or different thermal expansion of the materials. The liquid water that has penetrated through the cracks could no longer dry out through a diffusion- proof coating (emulsion paints , clinker strips). The consequence would be a gradual increase in the water content up to saturation and foreseeable damage to the wall (moss and algae growth, mold growth and mold through to the inside, reduced insulation effect , increased thermal conductivity , frost cracks and flaking, further leakage). Depending on the expected long-term water absorption through the coating, a sufficiently low diffusion resistance must also be ensured. In any case, construction defects (materials of different thermal expansion meet each other) or water absorption should be prevented.

research

The Lucerne University of Applied Sciences and Arts has had the largest facade test rig in Europe since 2008 . A 2.5 meter deep test chamber with an 8 x 12 m opening enables the testing of air permeability , tightness against driving rain and resistance to wind load .

Other substances diffusing through components

All vapors can diffuse through porous building materials. Perchlorethylene , which in the past was often used as a degreasing agent in the metalworking industry and was carelessly disposed of or even evaporated while in use, gained notoriety . It can accumulate in parts of buildings, even diffuse through concrete, and accumulate in food or body fat.

literature

  • H. Künzel: Should the exterior walls of the house be breathable? In: Physics in Our Time . tape 21 , no. 6 , 1990, pp. 252-257 .
  • K. Kießl, HM Künzel: Calculation of the influence of water vapor absorption by surface materials on the moisture behavior of living spaces . In: Health Engineer . tape 111 , no. 5 , 1990, pp. 217-221 .

See also

Web links

Individual evidence

  1. ↑ DIN 4108-3: Climate-related moisture protection . Ed .: DIBT. Beuth-Verlag, Berlin 2011.
  2. ↑ Prof Max Pettenkofer: About the air exchange in residential buildings . Cottaesche Buchhandlung, Munich 1858.
  3. ↑ Dr. Ernst Raisch: Air exchange measurements on building materials and structural components . In: Health Engineer . 30. Issue. Oldenbourg, Munich and Berlin July 28, 1928.
  4. ↑ Werner Eicke-Hennig: The dew point is not a wanderer. In: www.energieinstitut-hessen.de. Energieintsitut Hessen, 2010, accessed on June 10, 2019 .
  5. ↑ a b Dr. Helmut Künzel: Critical considerations on the question of the moisture balance of external walls. In: www.energieinstitut-hessen.de. Health Engineer, 1970, accessed June 10, 2019 .
  6. ↑ The basement drying and drying of the walls as well as the causes of the moisture in the masonry.
  7. ↑ Martin Krus: Moisture transport and storage coefficients of porous mineral building materials. Theoretical basics and new measuring techniques. Doctoral thesis at the Faculty of Civil Engineering and Surveying at the University of Stuttgart, Stuttgart 1995, PDF file.
  8. ↑ Baufachinformation the Fraunhofer Institute
  9. ↑ Dipl.-Ing. Werner Eicke-Hennig: Interior insulation - The dew point mythology . In: The building energy advisor . tape 09 . Gentner Verlag, Stuttgart 2011, p. 12 .
  10. ↑ Homepage of the HS Luzern (PDF; 33 kB)
  11. ↑ Chlorinated hydrocarbons (CHC). ( Memento from November 11, 2005 in the Internet Archive ) ( MS Word ; 31 kB)
  12. ↑ TECHNICAL BASIS for the assessment of companies in which volatile halogenated hydrocarbons are used or stored ( Memento of December 13, 2014 in the Internet Archive ), Federal Ministry of Economics and Labor, 2006.
  13. ↑ Investigations into the barrier effect of interior coatings against tetrachlorethylene in dry cleaning ( Memento of the original from November 29, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , State Environment Agency North Rhine-Westphalia, PDF file @1@ 2Template: Webachiv / IABot / www.lanuv.nrw.de