Engineer-office work-formation and construction-biology Peter Rauch 
(This page was automatically translated from German. Humidity in the outside-wall)

The role of the moisture in the outside-wall with reference to the heat-household of a building

"Under comparable conditions, older constructions, that were produced about the turn of the century, 3-once require 2 presently until less space-heat-energy than that was produced in the last 5 to 10 years after the point of view of the unit of heat-transfer. Although miscellaneous differences (heat-bridges) pass coolness-ribs regarding the construction-construction, etc..) the elevated energy-consumption-values with conventional calculations don't let themselves explain. As well as the Unit of heat-transfer-theory, as is not also the theory about non-stationary heat-performance of buildings in the situation, that to justify only approximately up to 300 per cent of amounting energy-consumption-increases.

in 1953, non-stationary k-Werte were determined by the Eidgenössischen-Material-Prüfanstalt (EMPA) for 5 years at attempt-little houses. It took place, extremely correct temperature and energy-measurement as well as the recording of the moisture-changes.

Acquisition of the moisture-changes of several houses

It will determine that the moisture in the stonework in the summer to - and in the winter decreases. With the desiccation, the value of the heat insulation got worse by 30 per cent. [1] Neither the non-stationary calculation also as the legally stipulated unit of heat-transfer - is theory experimentally sufficiently proved.
The different dampness-salaries for the stonework with its respective energy-streams, this Heat-storage-ability just as they Temperature-code number becoming in the calculations taken not into account. In an attempt-transaction 1954/55, it was determined that the outside-stonework dry off in the winter and the moisture increases in the summer. [1]

It is assumed that a moisture-increase the heat-code number in the stonework-construction approximately by 5 to 6 percent (Max). 10 percent) can make worse. According to volume-size, where waters or air-steam-mixtures are in the wall-construction, these values could be much bigger. See to this the material-values in the table 1.

The heat and dampness-transportation-processes in buildings are strongly connected normally. This especially clearly appears on the insulation with the dampness-influence from prefabricated parts. In graphics, the increase of the heat-conductivity is shown by three different construction-materials in dependence on the water-salary. (Form 1, [2]

Veränderung der Wärmeleitfähigkeit bei Feuchte

While the heat-management mineral wall-creator, as with the here represented pore-concrete, rises linearly with the water-salary, the increase with Polystyrol is - hard-foam easily progressive. Surprising is the strong increase of the heat-conductivity of mineral-wool already with very small water-salary. [2]
The influence of the moisture on the insulation becomes also with Eichler / Arndt described, here influences with what in coast-proximity, weather-sides and hit-rain and other is named. Concrete numbers, as this has an effect, are not named. [3]

The physical states, steam, water and ice (winters) occur in a wall-construction. The phase-postponement becomes pores, capillary, through the temperature, material-structure, Salt, and Pressure decide. No constant conditions are available in the stonework. You/they constantly change qualitative and quantitative. Energy-streams are available, where additionally the wall-construction-material through the heat-conductivity of this Air, Air-steam-mixture, Water and in the winter through ice is influenced.

Table 1: selected material-values

MaterialWrite poetry [kg/m3] Specific heat-capacity c [kJ/kgK] Heat-conductivity [W/mK] Temperature-code number a [m2/s]
Water (20ºC) 998,4 4,182 0,604 -
Ice 917 1,93 2,2 1,2
Air - 1,0 - -
Sand dry 1500 0,80,33 2,74
Sand wet 1650 2,1 1,1 0,49
Bricks 1800 0,92 0,81 0,49

One can recognize that so itself with moist sand and also at stonework the specific heat-capacity increases. The more heat a material can store, all the more lazily, it responds with the Aufheizung and cooling. The dampness sand has the same heat-capacity so approximately as from wood. However, also the heat-conductivity changes, which with moist sand and increases so at stonework. Another value, this Temperature-code number or temperature-conductivity a [m2/s] should find observation here. It is the measurement for the reproductive-speed of a temperature-alteration in a body. A temperature-alteration plants all the more quickly away the bigger the heat-conduction is and the smaller the specific heat-capacity and the density is. Wet sand shows clearly more favorable values here.
How already aforementioned, different dampness are available in a wall-cross-section, like kernel-dampness, condensation, Spritzwasser and other, that changes also in its expansion or doesn't exist at times. [6] with it alters also the physical conditions for itself in the construction-material. There are sections within the wall-cross-section consequently, where a higher heat-storage and Heat-conductivity be available, the values are lower with others. Hike the frost-point into the inside of the stonework in the winter, reduces the heat-storage and the heat-conductivity for itself so increases itself. This has an effect adversely on the insulation of the outside-wall.

An energy-stream always takes place from the higher temperature to the lower. If you increase the air-temperature for itself at the outer surface of the outside-wall, so two energy-streams are available, one of interior and one from outside. The known amplitudes (temperature-curves), that overlap in the wall-cross-section somewhere and lift, come about with it. At a temperature-alteration, the pressure changes also in certain scope in the pores, cavities and capillaries. According to dew point, liquid thaws from or changes into an air-steam-mixture. The vaporization-heat of the water (0 ºC, 2500 kJ / kg) or the glaze-heat of the ice (333,4 kJ / kg) are still taken into account now that so the heat-reception and heat-depositing respectively, according to which which physical state appears, occur additionally locally. Alter the temperature again for itself in the wall-cross-section, so also the physical states change conceivably. Wintry Direkteinstrahlung and also the indirect in-radiation (only essentially more inferior just) cause an absorption of the heat-energy, that is stored over many hours and again slowly outside escapes. This was proved also in the attempt-transaction 2001 in Leipzig where a curve is represented for a day-section here.

Temperaturverlauf in einer Außenwand durch solare Erwärmung

Explanation: Score points in 1 = 7.00 o'clock until 23.00 o'clock, from 9.10 to 15.30 o'clock sunshine; Row 1 = area-temperature; Row 2 = temperature at the inside of the outside-wall; Row 3 = temperature in the wall (10 cm from outside); Row 4 = temperature at the outside-surface of the wall; Row 5 = air-temperature outside (protected)

"... It could be determined that a low temperature-increase also on days, if no sun shines, at the outer wall-surface takes place. This temperature is higher than this on that occasion in the wall-inside. In this time, a heat-stream lay additionally from outside inside before. On average a temperature-increase of 5 K occurred with 12 days. Time delayed took place a cooling, that stretched over a period of 10 to 18 hours, until the original level was reached..." [5]

Under the point of view of the higher heat-storage-ability could a moist wall-construction-material and the more inferior temperature-conductivity so additional Heat is stored for example in an outside-wall, with what also a delayed heat-depositing takes place. One is every moisture with a certain temperature Enthalpie assigned. Still, also the Teildrücke play a role here. (Comparisons to this the Mollier (h, s) - diagram of the contribution Atmospheric humidity.) At a complete dry wall, for example steel-plate, the moisture can be neglected under normal climatic conditions, if abandoned the temperature of the wall-surface and this of the surface of no fluctuations so only low quantity of condensate at the surface lasts and from-thaws. Heat-swappers work on this basis. However, the moisture is not picked up by the material. With more-layer-y wall-constructions, the variation-possibility is even bigger.

Besides the phase-postponement, also the individual dampness-conditions should be investigated practical Versuchensreihen since there will be critical and also optimal areas with certain dampness-shares, that also for the development of construction-material-systems and - constructions of interest should be.

The present legal u-Wert-Berechnung represents an ideal-condition (laboratory) under exclusion of climatic conditions. For coarse assessments for what this calculation-method was also developed and surely in the area of higher unit of heat-transfer sufficient.
Calculation of the unit of heat-transfer at an outside-wall.

[1] Energieverbrauchsanalysen von Hochbauten (Bossert/Nagel Januar 1980) aus Deutsche Bauzeitung 9/1982 S. 58-62 als pdf-Datei (308 KB)
[2] IBP - Software - WUFI - Grundlagen (2001) Fraunhofer Institut
[3] Eichler/Arndt; Bautechnischer Wärme- und Feuchteschutz, 1989
[4] Günter Meyer, Erich Schiffner; Technische Thermodynamik, Fachbuchverlag Leipzig, 1983
[5] Eingespeicherte Solarenergie bei einer Außenwand - praktische Versuchsdurchführung 2001
[6] Trocknung von feuchten Bauwerksteilen 2002

Ingenieurbuero Peter Rauch

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Engineer-office work-formation and construction-biology Peter Rauch - Leipzig -