Details:Physics - Versionsgeschichte

Tes: /* Mechanical Stress due to Alternating Hygric Loads */

2009-07-08T08:30:05Z

Mechanical Stress due to Alternating Hygric Loads

← Nächstältere Version		Version vom 8. Juli 2009, 10:30 Uhr
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	====Mechanical Stress due to Alternating Hygric Loads====		====Mechanical Stress due to Alternating Hygric Loads====

			[[Bild:PhysicalBackgroungFig2.jpg\|thumb\|280px\|right]]
	One example for the strongly non-steady character of the coupled thermal and hygric effects is the mechanical strain due to '''thermal and hygric expansion''' and the resulting internal stress in the renderings on exterior thermal insulation composite systems (ETICS). During the course of the day the surface of these renderings is exposed to thermal and hygric variations of up to 40 K and 80% RH [3]. '''Fig. 2''' compares the thermal and hygric expansion of such exterior renderings. The shown values represent averages over laboratory measurements performed on more than 20 specimens of different manufacturers. Compared with practical conditions to which a building is exposed this means a thermal expansion of 0.4 mm/m and an even larger hygric expansion of 0.7 mm/m (if the rendering could expand freely). Both processes often partially cancel each other out, however, since increased temperature dries the rendering, so that the thermal expansion is compensated by hygric shrinking after some delay. Since the rendering surfaces usually can not move freely, any expansion results in internal stress and - if the material strength is exceeded - in cracks or in separation from the rendering base due to shear failure.		One example for the strongly non-steady character of the coupled thermal and hygric effects is the mechanical strain due to '''thermal and hygric expansion''' and the resulting internal stress in the renderings on exterior thermal insulation composite systems (ETICS). During the course of the day the surface of these renderings is exposed to thermal and hygric variations of up to 40 K and 80% RH [3]. '''Fig. 2''' compares the thermal and hygric expansion of such exterior renderings. The shown values represent averages over laboratory measurements performed on more than 20 specimens of different manufacturers. Compared with practical conditions to which a building is exposed this means a thermal expansion of 0.4 mm/m and an even larger hygric expansion of 0.7 mm/m (if the rendering could expand freely). Both processes often partially cancel each other out, however, since increased temperature dries the rendering, so that the thermal expansion is compensated by hygric shrinking after some delay. Since the rendering surfaces usually can not move freely, any expansion results in internal stress and - if the material strength is exceeded - in cracks or in separation from the rendering base due to shear failure.
	~~[[Bild:PhysicalBackgroungFig2.jpg\|thumb\|280px\|right]]~~
	Other examples for damages caused by alternating hygrothermal strain are '''salt and frost damages'''. Below a certain relative humidity which is characteristic for a specific type of salt, salts are crystallizing out of the solution. Conversely, when this crystallization point is exceeded, they absorb ambient water vapor and dissolve again. These processes also occur in the pore spaces of building materials if these contain salts. There, however, crystallization is impeded by the narrow pore structures and considerable crystallization pressures may result, so that frequent humidity cycles can embrittle the material. Depending on the solubility of the salt, this process may occur at the surface, or several millimeters or centimeters below, and may thus lead to crumbling at the surface or to shaling due to destabilization of deeper layers [4]. Frost can cause similar types of damage, with the ice crystals acting the same way as the salt crystals. Here the decisive factors are frequent temperature variations around the freezing point and sufficient moisture in the frost zone [5].		Other examples for damages caused by alternating hygrothermal strain are '''salt and frost damages'''. Below a certain relative humidity which is characteristic for a specific type of salt, salts are crystallizing out of the solution. Conversely, when this crystallization point is exceeded, they absorb ambient water vapor and dissolve again. These processes also occur in the pore spaces of building materials if these contain salts. There, however, crystallization is impeded by the narrow pore structures and considerable crystallization pressures may result, so that frequent humidity cycles can embrittle the material. Depending on the solubility of the salt, this process may occur at the surface, or several millimeters or centimeters below, and may thus lead to crumbling at the surface or to shaling due to destabilization of deeper layers [4]. Frost can cause similar types of damage, with the ice crystals acting the same way as the salt crystals. Here the decisive factors are frequent temperature variations around the freezing point and sufficient moisture in the frost zone [5].

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	===Moisture Storage and Transport Mechanisms===		===Moisture Storage and Transport Mechanisms===

Tes: /* Moisture Transport in Building Materials */

2009-07-08T08:29:36Z

Moisture Transport in Building Materials

Änderungen zeigen

Tes: /* Moisture Transport in Building Materials */

2009-07-07T08:39:05Z

Moisture Transport in Building Materials

← Nächstältere Version		Version vom 7. Juli 2009, 10:39 Uhr
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	One example for the strongly non-steady character of the coupled thermal and hygric effects is the mechanical strain due to '''thermal and hygric expansion''' and the resulting internal stress in the renderings on exterior thermal insulation composite systems (ETICS). During the course of the day the surface of these renderings is exposed to thermal and hygric variations of up to 40 K and 80% RH [3]. '''Fig. 2''' compares the thermal and hygric expansion of such exterior renderings. The shown values represent averages over laboratory measurements performed on more than 20 specimens of different manufacturers. Compared with practical conditions to which a building is exposed this means a thermal expansion of 0.4 mm/m and an even larger hygric expansion of 0.7 mm/m (if the rendering could expand freely). Both processes often partially cancel each other out, however, since increased temperature dries the rendering, so that the thermal expansion is compensated by hygric shrinking after some delay. Since the rendering surfaces usually can not move freely, any expansion results in internal stress and - if the material strength is exceeded - in cracks or in separation from the rendering base due to shear failure.		One example for the strongly non-steady character of the coupled thermal and hygric effects is the mechanical strain due to '''thermal and hygric expansion''' and the resulting internal stress in the renderings on exterior thermal insulation composite systems (ETICS). During the course of the day the surface of these renderings is exposed to thermal and hygric variations of up to 40 K and 80% RH [3]. '''Fig. 2''' compares the thermal and hygric expansion of such exterior renderings. The shown values represent averages over laboratory measurements performed on more than 20 specimens of different manufacturers. Compared with practical conditions to which a building is exposed this means a thermal expansion of 0.4 mm/m and an even larger hygric expansion of 0.7 mm/m (if the rendering could expand freely). Both processes often partially cancel each other out, however, since increased temperature dries the rendering, so that the thermal expansion is compensated by hygric shrinking after some delay. Since the rendering surfaces usually can not move freely, any expansion results in internal stress and - if the material strength is exceeded - in cracks or in separation from the rendering base due to shear failure.
	[[Bild:~~PhysicalBackroungFig2~~.jpg\|thumb\|280px\|right]]		[[Bild:PhysicalBackgroungFig2.jpg\|thumb\|280px\|right]]
	Other examples for damages caused by alternating hygrothermal strain are '''salt and frost damages'''. Below a certain relative humidity which is characteristic for a specific type of salt, salts are crystallizing out of the solution. Conversely, when this crystallization point is exceeded, they absorb ambient water vapor and dissolve again. These processes also occur in the pore spaces of building materials if these contain salts. There, however, crystallization is impeded by the narrow pore structures and considerable crystallization pressures may result, so that frequent humidity cycles can embrittle the material. Depending on the solubility of the salt, this process may occur at the surface, or several millimeters or centimeters below, and may thus lead to crumbling at the surface or to shaling due to destabilization of deeper layers [4]. Frost can cause similar types of damage, with the ice crystals acting the same way as the salt crystals. Here the decisive factors are frequent temperature variations around the freezing point and sufficient moisture in the frost zone [5].		Other examples for damages caused by alternating hygrothermal strain are '''salt and frost damages'''. Below a certain relative humidity which is characteristic for a specific type of salt, salts are crystallizing out of the solution. Conversely, when this crystallization point is exceeded, they absorb ambient water vapor and dissolve again. These processes also occur in the pore spaces of building materials if these contain salts. There, however, crystallization is impeded by the narrow pore structures and considerable crystallization pressures may result, so that frequent humidity cycles can embrittle the material. Depending on the solubility of the salt, this process may occur at the surface, or several millimeters or centimeters below, and may thus lead to crumbling at the surface or to shaling due to destabilization of deeper layers [4]. Frost can cause similar types of damage, with the ice crystals acting the same way as the salt crystals. Here the decisive factors are frequent temperature variations around the freezing point and sufficient moisture in the frost zone [5].

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	====Moisture Storage====		====Moisture Storage====

	[[Bild:~~PhysicalBackroundFig3~~.jpg\|thumb\|275px\|right]]		[[Bild:PhysicalBackgroundFig3.jpg\|thumb\|275px\|right]]
	There are '''hygroscopic''' and '''non-hygroscopic''' building materials. If a material is hygroscopic, then an initially dry sample will absorb moisture from the air until it reaches its '''equilibrium moisture''' corresponding to the ambient conditions. Since water vapor absorption mainly depends on the ambient '''relative humidity''' whereas the ambient temperature has less influence, the hygroscopic moisture storage is described by means of material-specific sorption curves.		There are '''hygroscopic''' and '''non-hygroscopic''' building materials. If a material is hygroscopic, then an initially dry sample will absorb moisture from the air until it reaches its '''equilibrium moisture''' corresponding to the ambient conditions. Since water vapor absorption mainly depends on the ambient '''relative humidity''' whereas the ambient temperature has less influence, the hygroscopic moisture storage is described by means of material-specific sorption curves.

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	Other transport phenomena, for example seepage flow through gravitation in non-saturated pore spaces or migration of water molecules due to electric fields or osmotic pressures, cannot yet be computed in a satisfactory way. However, since they only play a major part in exceptional cases, they are not further considered here. Convection effects, for example moist interior air permeating building components because of pressure differentials between the interior and the exterior side, are ignored as well. Since airtightness is an essential property of a building wall, air convection is in practice only found in unplanned cases of defective parts or inappropriate building components. It is therefore difficult to quantify beforehand and could also only be realistically determined by three-dimensional fluid dynamical simulation programs.		Other transport phenomena, for example seepage flow through gravitation in non-saturated pore spaces or migration of water molecules due to electric fields or osmotic pressures, cannot yet be computed in a satisfactory way. However, since they only play a major part in exceptional cases, they are not further considered here. Convection effects, for example moist interior air permeating building components because of pressure differentials between the interior and the exterior side, are ignored as well. Since airtightness is an essential property of a building wall, air convection is in practice only found in unplanned cases of defective parts or inappropriate building components. It is therefore difficult to quantify beforehand and could also only be realistically determined by three-dimensional fluid dynamical simulation programs.

	[[Bild:~~PhysicalBackroundFig4~~.jpg\|thumb\|475px\|right]]		[[Bild:PhysicalBackgroundFig4.jpg\|thumb\|475px\|right]]

	The combined effect of the abovementioned predominant moisture transport mechanisms is illustrated in '''Fig. 4''', using a cylindrical capillary in a building material as an example. On both sides of the capillary typical boundary conditions are assumed as they are usually encountered in practice, i.e. the vapor pressure is greater on the interior side and the relative humidity is greater on the exterior side. If the material is sufficiently dry or not hygroscopic, then the '''water vapour''' will diffuse from inside to outside, following the direction from higher to lower '''vapor pressure'''.		The combined effect of the abovementioned predominant moisture transport mechanisms is illustrated in '''Fig. 4''', using a cylindrical capillary in a building material as an example. On both sides of the capillary typical boundary conditions are assumed as they are usually encountered in practice, i.e. the vapor pressure is greater on the interior side and the relative humidity is greater on the exterior side. If the material is sufficiently dry or not hygroscopic, then the '''water vapour''' will diffuse from inside to outside, following the direction from higher to lower '''vapor pressure'''.
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	The µ-values for a large number of building materials can also be found in DIN 4108-4. The porosity can be determined from bulk density and true density or from the maximum water saturation. It only applies if the material can take up water or water vapor in its pore spaces. The material data mentioned so far, however, only allow simulations without sorption or liquid transport effects, i.e. a kind of non-steady Glaser calculation.		The µ-values for a large number of building materials can also be found in DIN 4108-4. The porosity can be determined from bulk density and true density or from the maximum water saturation. It only applies if the material can take up water or water vapor in its pore spaces. The material data mentioned so far, however, only allow simulations without sorption or liquid transport effects, i.e. a kind of non-steady Glaser calculation.

	[[Bild:~~PhysicalBackroundFig5~~.jpg\|thumb\|300px\|right]]		[[Bild:PhysicalBackgroundFig5.jpg\|thumb\|300px\|right]]

	If the behavior of hygroscopic, capillary active materials is to be simulated correctly, the moisture storage function shown in Fig. 3 and the moisture-dependent liquid transport coefficients shown in '''Fig. 5''' are also needed. For these coefficients the process-specific differentiation shown in the figure has proved advantageous. The reason is that the capillary absorption of water by mineral materials in contact with water is a much faster process than the capillary redistribution or the drying process after the water supply has been cut off.		If the behavior of hygroscopic, capillary active materials is to be simulated correctly, the moisture storage function shown in Fig. 3 and the moisture-dependent liquid transport coefficients shown in '''Fig. 5''' are also needed. For these coefficients the process-specific differentiation shown in the figure has proved advantageous. The reason is that the capillary absorption of water by mineral materials in contact with water is a much faster process than the capillary redistribution or the drying process after the water supply has been cut off.
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	====Exterior Climate====		====Exterior Climate====

	[[Bild:~~PhysicalBackroundFig6~~.jpg\|thumb\|280px\|right]]		[[Bild:PhysicalBackgroundFig6.jpg\|thumb\|280px\|right]]
	The exterior climate conditions acting on the component are '''air temperature''', '''relative humidity''', '''solar radiation''' and '''precipitation'''. The radiation and precipitation loads depend on the inclination and orientation of the component and must accordingly be computed for the specific component. In addition, wind speed and direction as well as the exposition of the building to wind flow and local wind flow patterns need to be known for determining driving rain. If long-wave emission is to be allowed for, too, data on ground and air counterradiation are also needed.		The exterior climate conditions acting on the component are '''air temperature''', '''relative humidity''', '''solar radiation''' and '''precipitation'''. The radiation and precipitation loads depend on the inclination and orientation of the component and must accordingly be computed for the specific component. In addition, wind speed and direction as well as the exposition of the building to wind flow and local wind flow patterns need to be known for determining driving rain. If long-wave emission is to be allowed for, too, data on ground and air counterradiation are also needed.

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	====Interior Climate====		====Interior Climate====

	[[Bild:~~PhysicalBackroundFig7~~.jpg\|thumb\|280px\|right]]		[[Bild:PhysicalBackgroundFig7.jpg\|thumb\|280px\|right]]
	The interior climate conditions specified in DIN 4108-3 for the dew period (20°C, 50% RH) and for the evaporation period (12°C, 70% RH) have proved suitable for the general assessment of diffusion processes in building components, but they do not represent realistic boundary conditions for a simulation spanning a year.		The interior climate conditions specified in DIN 4108-3 for the dew period (20°C, 50% RH) and for the evaporation period (12°C, 70% RH) have proved suitable for the general assessment of diffusion processes in building components, but they do not represent realistic boundary conditions for a simulation spanning a year.

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	[[Bild:~~PhysicalBackroundFig8~~.jpg\|thumb\|280px\|right]]		[[Bild:PhysicalBackgroundFig8.jpg\|thumb\|280px\|right]]
	The left-hand sides of both equations consist of the storage terms. '''Heat storage''' comprises the heat capacity of the dry material and the heat capacity of the moisture present in the material. '''Moisture storage''' is described by the derivative of the moisture storage function mentioned above.		The left-hand sides of both equations consist of the storage terms. '''Heat storage''' comprises the heat capacity of the dry material and the heat capacity of the moisture present in the material. '''Moisture storage''' is described by the derivative of the moisture storage function mentioned above.
	On the righ-hand side of the equations we find the transport terms. '''Heat transport''' is the sum of moisture-dependent thermal conductivity and vapor enthalpy flow. This heat transport by vapor enthalpy flow is due to water evaporating in one place and thereby absorbing latent heat from this place, and then diffusing to a different place, condensing there and releasing latent heat. This kind of heat transport is often called latent heat effect.		On the righ-hand side of the equations we find the transport terms. '''Heat transport''' is the sum of moisture-dependent thermal conductivity and vapor enthalpy flow. This heat transport by vapor enthalpy flow is due to water evaporating in one place and thereby absorbing latent heat from this place, and then diffusing to a different place, condensing there and releasing latent heat. This kind of heat transport is often called latent heat effect.

Tes: /* Computer Simulation with the WUFI Model */

2009-07-07T08:25:45Z

Computer Simulation with the WUFI Model

← Nächstältere Version		Version vom 7. Juli 2009, 10:25 Uhr
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	Only if unexpected moisture damage occurs nevertheless, or if the designed building element does not pass the standard Glaser assessment, the search for alternative assessment methods begins. Since condensation by vapor transport in winter (which is what Glaser investigates) is only one of a large number of possible moisture loads on a building element, a positive assessment according to DIN 4108 may imply moisture safety which does not really exist. Possible problems with other hygric processes, such as indoor air convection, precipitation or rising damp, are usually not considered. The same goes for construction moisture which, in view of today's deadline pressure, causes an increasing number of damage cases. In order to allow for these effects too, one has to pass on from Glaser's simple '''steady-state assessment method''' to the '''realistic simulation''' of hygric processes in building elements. To this end, new non-steady simulation methods have been developed and experimentally validated in recent years, whose reliability is winning them more and more acceptance among practitioners. This fact is also recognized in the new draft for DIN 4108-3 which now admits these methods.		Only if unexpected moisture damage occurs nevertheless, or if the designed building element does not pass the standard Glaser assessment, the search for alternative assessment methods begins. Since condensation by vapor transport in winter (which is what Glaser investigates) is only one of a large number of possible moisture loads on a building element, a positive assessment according to DIN 4108 may imply moisture safety which does not really exist. Possible problems with other hygric processes, such as indoor air convection, precipitation or rising damp, are usually not considered. The same goes for construction moisture which, in view of today's deadline pressure, causes an increasing number of damage cases. In order to allow for these effects too, one has to pass on from Glaser's simple '''steady-state assessment method''' to the '''realistic simulation''' of hygric processes in building elements. To this end, new non-steady simulation methods have been developed and experimentally validated in recent years, whose reliability is winning them more and more acceptance among practitioners. This fact is also recognized in the new draft for DIN 4108-3 which now admits these methods.
	In the following we will demonstrate the effects of increased moisture levels and of alternating hygric stresses, and we will describe the basic physics of hygric processes in building elements. Subsequently, we will analyse the necessary climate and material data and discuss the accuracy of the calculation, using the non-steady simulation model WUFI as an example which has meanwhile found widespread use.		In the following we will demonstrate the effects of increased moisture levels and of alternating hygric stresses, and we will describe the basic physics of hygric processes in building elements. Subsequently, we will analyse the necessary climate and material data and discuss the accuracy of the calculation, using the non-steady simulation model WUFI as an example which has meanwhile found widespread use.


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	The above examples demonstrate the effect of alternating hygrothermal strains on the durability and the fitness for use of building components and building materials. Therefore it is not only desirable but absolutely essential to determine precisely the thermal and hygric processes occurring in these. Assessments using the traditional methods which are exclusively based on vapor diffusion (such as the Glaser method) can achieve this in exceptional cases only. However, computer simulations of non-steady thermal and hygric processes, including all essential transport mechanisms under natural climate conditions have meanwhile been developed to maturity and have proven successful in practice, so that their systematic application in many areas of building construction appears useful. The discussion which follows is to be understood in this context.		The above examples demonstrate the effect of alternating hygrothermal strains on the durability and the fitness for use of building components and building materials. Therefore it is not only desirable but absolutely essential to determine precisely the thermal and hygric processes occurring in these. Assessments using the traditional methods which are exclusively based on vapor diffusion (such as the Glaser method) can achieve this in exceptional cases only. However, computer simulations of non-steady thermal and hygric processes, including all essential transport mechanisms under natural climate conditions have meanwhile been developed to maturity and have proven successful in practice, so that their systematic application in many areas of building construction appears useful. The discussion which follows is to be understood in this context.


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	As the example shows clearly, the transport directions of vapor diffusion and liquid transport may often be opposed to each other. Vapor diffusion usually occurs from warm to cold, whereas liquid transport goes from moist to dry, mostly independent of temperature. This phenomenon, presumably known to each practitioner who has ever observed how condensed moisture in winter can be drawn off by mineral materials, must be correctly included in a simulation model, in accordance with the above analysis. This means that different driving forces must be employed for vapor diffusion and liquid transport. The choice of temperature and relative humidity as driving potentials offers particular advantages. Vapor pressure, as the driving force for diffusion, is uniquely determined by the two quantities. The two potentials are continuous across the building component, i.e. there are no discontinuities at material interfaces, as would be the case with water content. In addition, the hygrothermal material properties and boundary conditions discussed in the following can easily be defined in terms of these quantities.		As the example shows clearly, the transport directions of vapor diffusion and liquid transport may often be opposed to each other. Vapor diffusion usually occurs from warm to cold, whereas liquid transport goes from moist to dry, mostly independent of temperature. This phenomenon, presumably known to each practitioner who has ever observed how condensed moisture in winter can be drawn off by mineral materials, must be correctly included in a simulation model, in accordance with the above analysis. This means that different driving forces must be employed for vapor diffusion and liquid transport. The choice of temperature and relative humidity as driving potentials offers particular advantages. Vapor pressure, as the driving force for diffusion, is uniquely determined by the two quantities. The two potentials are continuous across the building component, i.e. there are no discontinuities at material interfaces, as would be the case with water content. In addition, the hygrothermal material properties and boundary conditions discussed in the following can easily be defined in terms of these quantities.


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	Since all properties of building materials are subject to variations due to the manufacturing process or different standards of workmanship, it is useful to perform variations of the parameters in the calculations within certain intervals - in analogy to the span of parameters given in DIN 4108-4 - and to document the effect of these variations on the calculation results. If the effect of a material parameter turns out to be negligible in the application case at hand, then it need not be determined exactly. On the other hand, if the effect of the parameter is decisive for the interpretation of the results, then standard or literature data may be inadequate and precise measurements of the parameter may be required.		Since all properties of building materials are subject to variations due to the manufacturing process or different standards of workmanship, it is useful to perform variations of the parameters in the calculations within certain intervals - in analogy to the span of parameters given in DIN 4108-4 - and to document the effect of these variations on the calculation results. If the effect of a material parameter turns out to be negligible in the application case at hand, then it need not be determined exactly. On the other hand, if the effect of the parameter is decisive for the interpretation of the results, then standard or literature data may be inadequate and precise measurements of the parameter may be required.


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	The effects of solar radiation and precipitation on the component are best described by heat and moisture sources. An '''energy absorption factor''' which depends on the surface color allows for the fact that only part of the short-wave radiation incident on the component is converted into heat. This absorption factor is ca. 0.4 for bright surfaces, such as white exterior renderings, and between 0.6 and 0.8 for dark surfaces, such as painted wood, clinker, roofing tiles and bituminous sheeting.		The effects of solar radiation and precipitation on the component are best described by heat and moisture sources. An '''energy absorption factor''' which depends on the surface color allows for the fact that only part of the short-wave radiation incident on the component is converted into heat. This absorption factor is ca. 0.4 for bright surfaces, such as white exterior renderings, and between 0.6 and 0.8 for dark surfaces, such as painted wood, clinker, roofing tiles and bituminous sheeting.
	An absorption factor may also be employed for driving rain, since only part of the incident rain water stays at the surface and can be absorbed. The rest splashes off on hitting the facade or runs off due to gravity. Experience shows that 0.7 usually is an appropriate value for the '''rain water absorption factor''' of vertical surfaces.		An absorption factor may also be employed for driving rain, since only part of the incident rain water stays at the surface and can be absorbed. The rest splashes off on hitting the facade or runs off due to gravity. Experience shows that 0.7 usually is an appropriate value for the '''rain water absorption factor''' of vertical surfaces.


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	===The Calculation Model===		===The Calculation Model===
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	The differential equations are discretised by means of an implicit finite volume method and are iteratively solved according to the scheme described by the flow chart shown in '''Fig. 8'''. The accuracy of the numerical solution depends on the mesh widths of the numerical grid, the size of the time steps and the choice of the convergence criteria. Usually the numerical solution is sufficiently accurate, so that the effect of numerical parameters can be ignored in comparison with the effects of the physical parameters like material and climate data. After the calculation the result should be critically assessed in order to exclude user errors or severe convergence errors. Convergence errors are indicated by WUFI, and their effect is assessed by a comparison of the sum of the moisture flows with the water accumulated in the component. False input or unrealistic material data can only be controlled by plausibility checks.		The differential equations are discretised by means of an implicit finite volume method and are iteratively solved according to the scheme described by the flow chart shown in '''Fig. 8'''. The accuracy of the numerical solution depends on the mesh widths of the numerical grid, the size of the time steps and the choice of the convergence criteria. Usually the numerical solution is sufficiently accurate, so that the effect of numerical parameters can be ignored in comparison with the effects of the physical parameters like material and climate data. After the calculation the result should be critically assessed in order to exclude user errors or severe convergence errors. Convergence errors are indicated by WUFI, and their effect is assessed by a comparison of the sum of the moisture flows with the water accumulated in the component. False input or unrealistic material data can only be controlled by plausibility checks.


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	===Practical Conclusions===		===Practical Conclusions===
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	===Literature===		===Literature===

Tes: /* Literature */

2009-07-07T08:24:27Z

Literature

← Nächstältere Version		Version vom 7. Juli 2009, 10:24 Uhr
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	In recent years, these advantages of hygrothermal simulations have created strong demand for calculative investigations, especially in the context of renovation measures for old buildings since standard solutions are often not applicable here. So far, however, there are no general guidelines for appropriate contract specifications or for proper application of the novel methods. For this reason the WTA (Wissenschaftlich-Technische Arbeitsgemeinschaft für Bauwerkserhaltung und Denkmalpflege) established a WTA working group in 1997 whose task is the formulation of such guidelines. The Task Group "Moisture Calculation" established in 2000 within the European Technical Committee TC89 (Thermal Performance of Buildings and Building Components) is working on similar objectives. Thus the prerequisites for standardized application of hygrothermal simulation methods in civil engineering and architecture are being devised both on a national and on an international level.		In recent years, these advantages of hygrothermal simulations have created strong demand for calculative investigations, especially in the context of renovation measures for old buildings since standard solutions are often not applicable here. So far, however, there are no general guidelines for appropriate contract specifications or for proper application of the novel methods. For this reason the WTA (Wissenschaftlich-Technische Arbeitsgemeinschaft für Bauwerkserhaltung und Denkmalpflege) established a WTA working group in 1997 whose task is the formulation of such guidelines. The Task Group "Moisture Calculation" established in 2000 within the European Technical Committee TC89 (Thermal Performance of Buildings and Building Components) is working on similar objectives. Thus the prerequisites for standardized application of hygrothermal simulation methods in civil engineering and architecture are being devised both on a national and on an international level.


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	===Literature===		===Literature===

Tes: /* The Calculation Model */

2009-07-07T08:13:11Z

The Calculation Model

← Nächstältere Version		Version vom 7. Juli 2009, 10:13 Uhr
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	<math> \frac {\partial H}{\partial \vartheta} \frac {\partial \vartheta}{\partial t} = </math>		<math> \frac {\partial H}{\partial \vartheta} \frac {\partial \vartheta}{\partial t} = </math>
	<math> \frac {\partial}{\partial x}</math>		<math> \frac {\partial}{\partial x}</math>
	<math> \biggl( \lambda \frac {\partial \vartheta}{\partial x}\biggr)~~</math>~~		<math> \biggl( \lambda \frac {\partial \vartheta}{\partial x}\biggr)
	~~<math>~~ + h_v \frac {\partial}{\partial x} \biggl( \frac {\delta}{\mu}\frac{\partial p}{\partial x}\biggr)</math> '''Heat transport'''		+ h_v \frac {\partial}{\partial x} \biggl( \frac {\delta}{\mu}\frac{\partial p}{\partial x}\biggr)</math> '''Heat transport'''

	<math> \rho_w \frac {\partial u}{\partial \varphi} \frac {\partial \varphi}{\partial t} = \frac {\partial}{\partial x} \biggl( \rho_wD_w \frac {\partial u}{\partial \varphi} \frac{\partial \varphi}{\partial x} \biggr)		<math> \rho_w \frac {\partial u}{\partial \varphi} \frac {\partial \varphi}{\partial t} = \frac {\partial}{\partial x} \biggl( \rho_wD_w \frac {\partial u}{\partial \varphi} \frac{\partial \varphi}{\partial x} \biggr)

Tes: /* The Calculation Model */

2009-07-07T08:01:04Z

The Calculation Model

← Nächstältere Version		Version vom 7. Juli 2009, 10:01 Uhr
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	A number of hygrothermal simulation models which provide reliable results has been developed in different countries [10]. The following description discusses the model which forms the basis for the PC program WUFI ('''W'''ärme- '''U'''nd '''F'''euchtetransport '''I'''nstationär) [2]. In this model the non-steady heat ~~amd~~ moisture transport processes in building components are described by the following coupled differential equations:		A number of hygrothermal simulation models which provide reliable results has been developed in different countries [10]. The following description discusses the model which forms the basis for the PC program WUFI ('''W'''ärme- '''U'''nd '''F'''euchtetransport '''I'''nstationär) [2]. In this model the non-steady heat and moisture transport processes in building components are described by the following coupled differential equations:


Zeile 203:		Zeile 203:

	The differential equations are discretised by means of an implicit finite volume method and are iteratively solved according to the scheme described by the flow chart shown in '''Fig. 8'''. The accuracy of the numerical solution depends on the mesh widths of the numerical grid, the size of the time steps and the choice of the convergence criteria. Usually the numerical solution is sufficiently accurate, so that the effect of numerical parameters can be ignored in comparison with the effects of the physical parameters like material and climate data. After the calculation the result should be critically assessed in order to exclude user errors or severe convergence errors. Convergence errors are indicated by WUFI, and their effect is assessed by a comparison of the sum of the moisture flows with the water accumulated in the component. False input or unrealistic material data can only be controlled by plausibility checks.		The differential equations are discretised by means of an implicit finite volume method and are iteratively solved according to the scheme described by the flow chart shown in '''Fig. 8'''. The accuracy of the numerical solution depends on the mesh widths of the numerical grid, the size of the time steps and the choice of the convergence criteria. Usually the numerical solution is sufficiently accurate, so that the effect of numerical parameters can be ignored in comparison with the effects of the physical parameters like material and climate data. After the calculation the result should be critically assessed in order to exclude user errors or severe convergence errors. Convergence errors are indicated by WUFI, and their effect is assessed by a comparison of the sum of the moisture flows with the water accumulated in the component. False input or unrealistic material data can only be controlled by plausibility checks.


	===Practical Conclusions===		===Practical Conclusions===

Tes: /* Surface Transfer Conditions */

2009-07-07T07:57:58Z

Surface Transfer Conditions

← Nächstältere Version		Version vom 7. Juli 2009, 09:57 Uhr
Zeile 140:		Zeile 140:
	!Component !! !! !!Heat transfer !! !! !!Water Vapor Transfer		!Component !! !! !!Heat transfer !! !! !!Water Vapor Transfer
	\|-		\|-
	!Surface !! !! !!α [W/m²K] !! !! !!β<sub>p</sub> [kg/m²sPa]		!style="text-align:left" \|Surface !! !! !!α [W/m²K] !! !! !!β<sub>p</sub> [kg/m²sPa]
	\|-		\|-
	\|exterior \|\| \|\| \|\|style="text-align:center" \|17 \|\| \|\| \|\|style="text-align:center" \|75 <math>\cdot</math> 10<sup>-9</sup>		\|exterior \|\| \|\| \|\|style="text-align:center" \|17 \|\| \|\| \|\|style="text-align:center" \|75 <math>\cdot</math> 10<sup>-9</sup>

Tes: /* Surface Transfer Conditions */

2009-07-07T07:54:27Z

Surface Transfer Conditions

← Nächstältere Version		Version vom 7. Juli 2009, 09:54 Uhr
Zeile 142:		Zeile 142:
	!Surface !! !! !!α [W/m²K] !! !! !!β<sub>p</sub> [kg/m²sPa]		!Surface !! !! !!α [W/m²K] !! !! !!β<sub>p</sub> [kg/m²sPa]
	\|-		\|-
	\|exterior \|\| \|\| \|\|style="text-align:center" \|17 \|\| \|\| \|\|style="text-align:center" \|75 \cdot 10<sup>-9</sup>		\|exterior \|\| \|\| \|\|style="text-align:center" \|17 \|\| \|\| \|\|style="text-align:center" \|75 <math>\cdot</math> 10<sup>-9</sup>
	\|-		\|-
	\|interior \|\| \|\| \|\|style="text-align:center" \|8 \|\| \|\| \|\|style="text-align:center" \|25*10<sup>-9</sup>		\|interior \|\| \|\| \|\|style="text-align:center" \|8 \|\| \|\| \|\|style="text-align:center" \|25 <math>\cdot</math> 10<sup>-9</sup>
	\|}		\|}

Zeile 155:		Zeile 155:
	The effects of solar radiation and precipitation on the component are best described by heat and moisture sources. An '''energy absorption factor''' which depends on the surface color allows for the fact that only part of the short-wave radiation incident on the component is converted into heat. This absorption factor is ca. 0.4 for bright surfaces, such as white exterior renderings, and between 0.6 and 0.8 for dark surfaces, such as painted wood, clinker, roofing tiles and bituminous sheeting.		The effects of solar radiation and precipitation on the component are best described by heat and moisture sources. An '''energy absorption factor''' which depends on the surface color allows for the fact that only part of the short-wave radiation incident on the component is converted into heat. This absorption factor is ca. 0.4 for bright surfaces, such as white exterior renderings, and between 0.6 and 0.8 for dark surfaces, such as painted wood, clinker, roofing tiles and bituminous sheeting.
	An absorption factor may also be employed for driving rain, since only part of the incident rain water stays at the surface and can be absorbed. The rest splashes off on hitting the facade or runs off due to gravity. Experience shows that 0.7 usually is an appropriate value for the '''rain water absorption factor''' of vertical surfaces.		An absorption factor may also be employed for driving rain, since only part of the incident rain water stays at the surface and can be absorbed. The rest splashes off on hitting the facade or runs off due to gravity. Experience shows that 0.7 usually is an appropriate value for the '''rain water absorption factor''' of vertical surfaces.


	===The Calculation Model===		===The Calculation Model===

Tes: /* Computer Simulation with the WUFI Model */

2009-07-07T07:52:55Z

Computer Simulation with the WUFI Model

← Nächstältere Version		Version vom 7. Juli 2009, 09:52 Uhr
Zeile 106:		Zeile 106:

	Since all properties of building materials are subject to variations due to the manufacturing process or different standards of workmanship, it is useful to perform variations of the parameters in the calculations within certain intervals - in analogy to the span of parameters given in DIN 4108-4 - and to document the effect of these variations on the calculation results. If the effect of a material parameter turns out to be negligible in the application case at hand, then it need not be determined exactly. On the other hand, if the effect of the parameter is decisive for the interpretation of the results, then standard or literature data may be inadequate and precise measurements of the parameter may be required.		Since all properties of building materials are subject to variations due to the manufacturing process or different standards of workmanship, it is useful to perform variations of the parameters in the calculations within certain intervals - in analogy to the span of parameters given in DIN 4108-4 - and to document the effect of these variations on the calculation results. If the effect of a material parameter turns out to be negligible in the application case at hand, then it need not be determined exactly. On the other hand, if the effect of the parameter is decisive for the interpretation of the results, then standard or literature data may be inadequate and precise measurements of the parameter may be required.


	===Climate and Surface Transfer Conditions===		===Climate and Surface Transfer Conditions===
Zeile 141:		Zeile 142:
	!Surface !! !! !!α [W/m²K] !! !! !!β<sub>p</sub> [kg/m²sPa]		!Surface !! !! !!α [W/m²K] !! !! !!β<sub>p</sub> [kg/m²sPa]
	\|-		\|-
	\|exterior \|\| \|\| \|\|style="text-align:center" \|17 \|\| \|\| \|\|style="text-align:center" \|75*10<sup>-9</sup>		\|exterior \|\| \|\| \|\|style="text-align:center" \|17 \|\| \|\| \|\|style="text-align:center" \|75 \cdot 10<sup>-9</sup>
	\|-		\|-
	\|interior \|\| \|\| \|\|style="text-align:center" \|8 \|\| \|\| \|\|style="text-align:center" \|25*10<sup>-9</sup>		\|interior \|\| \|\| \|\|style="text-align:center" \|8 \|\| \|\| \|\|style="text-align:center" \|25*10<sup>-9</sup>
Zeile 154:		Zeile 155:
	The effects of solar radiation and precipitation on the component are best described by heat and moisture sources. An '''energy absorption factor''' which depends on the surface color allows for the fact that only part of the short-wave radiation incident on the component is converted into heat. This absorption factor is ca. 0.4 for bright surfaces, such as white exterior renderings, and between 0.6 and 0.8 for dark surfaces, such as painted wood, clinker, roofing tiles and bituminous sheeting.		The effects of solar radiation and precipitation on the component are best described by heat and moisture sources. An '''energy absorption factor''' which depends on the surface color allows for the fact that only part of the short-wave radiation incident on the component is converted into heat. This absorption factor is ca. 0.4 for bright surfaces, such as white exterior renderings, and between 0.6 and 0.8 for dark surfaces, such as painted wood, clinker, roofing tiles and bituminous sheeting.
	An absorption factor may also be employed for driving rain, since only part of the incident rain water stays at the surface and can be absorbed. The rest splashes off on hitting the facade or runs off due to gravity. Experience shows that 0.7 usually is an appropriate value for the '''rain water absorption factor''' of vertical surfaces.		An absorption factor may also be employed for driving rain, since only part of the incident rain water stays at the surface and can be absorbed. The rest splashes off on hitting the facade or runs off due to gravity. Experience shows that 0.7 usually is an appropriate value for the '''rain water absorption factor''' of vertical surfaces.


	===The Calculation Model===		===The Calculation Model===