"For the practitioner the subject "moisture transport in building materials" mainly evokes vapor diffusion, dew point and the Glaser method described in German standard DIN 4108."..."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.".." The heat and moisture transport processes in buildings are usually strongly coupled. This is particularly evident in the effect of moisture on the heat insulation of building components. Fig. 1 shows the increase in heat conductivity of three different building materials as dependent on their moisture content [1]. While the heat conductivity of mineral materials, such as the cellular concrete shown here, increases linearly with moisture content, the heat conductivity of polystyrene foam shows a slightly progressive increase. Surprisingly, it takes only a very low moisture content to increase the heat conductivity of mineral wool markedly. This is due to the pronounced moisture redistribution by vapor diffusion in the mineral wool when a temperature gradient is applied across the sample. "..."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."..."In building materials with coarse pores, such as brick, the capillary water region is larger than the hygroscopic region, so that pressure plate measurements are indispensable for detailed analyses. In materials with fine pores, such as concrete, the sorption moisture content at 93% RH is already so high that the sorption isotherm can be extrapolated into the capillary water region and up to free saturation without loss of accuracy. Wood and processed wood materials, too, already absorb large quantities of moisture in the hygroscopic region, so that in these cases extrapolation up to free saturation is usually sufficient as well. In non-hygroscopic materials, for example glass, metal or some plastic foams, no water accumulates unless the temperature falls below the dew point. Under ambient conditions below 100% RH they dry out completely."..."In porous building materials the predominant moisture transport mechanisms are vapor diffusion, surface diffusion and capillary conduction. In materials which do not have a rigid pore structure, for example plastics, so-called solution diffusion occurs because water molecules squeeze between the polymeric macromolecules. Experience shows that this kind of diffusion can best be described by the laws governing vapor diffusion, with a diffusion resistance factor which is now dependent on ambient humidity (in contrast to normal vapor diffusion where the diffusion resistance is independent of humidity)."..."The driving force for surface diffusion is therefore relative humidity and not vapor pressure. Thus under the boundary conditions assumed here, vapor diffusion and surface diffusion go in opposite directions. "..."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."..."4. Hygrothermal Material Properties The results of a computer simulation are only as good as the employed material parameters. Since the notorious lack of reliable material data has been a longstanding obstacle for the acceptance of modern calculation methods, it will be discussed in the following which parameters are actually needed for different kinds of investigations. In general, the following material properties are necessary for the non-steady computation of the temperature fields: * Bulk density rho of the dry material * specific heat capacity c * thermal conductivity lambda If the effect of moisture in the material on the U-factor is to be quantified, the thermal conductivity must be entered as a function of moisture content, cf. Fig. 1. Relevant data can be found in [1] or in WUFI's material database. On the other hand, if the investigation concerns mainly the hygric behavior of the component, then it is sufficient to use the design value lambda_R which already allows for the practical moisture content of the respective material. That is, in most cases all necessary thermal properties can be found in German standard DIN 4108-4 or in the respective certificates of approval. The hygric properties that need to be known for all (i.e. also for non-hygroscopic) materials are: * Water vapor diffusion resistance factor µ (µ-value) * Porosity epsilon (as a measure of the maximum possible water content wmax) 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 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."..."Therefore the average values for the exterior and interior heat and vapor transfer cofficients listed in table 1 are sufficient for most applications. The heat transfer coefficients contain a component which describes long-wave radiation exchange. This is only valid, however, as long as the convective and the radiative heat flows go in the same direction. When the surface temperature of a highly insulated component falls below the ambient air temperature by night due to radiative cooling, this is no longer the case. Computer simulations of nighttime condensation on such components therefore require a suitable correction of the surface transfer coefficients or compensation by empirically adapting the long-wave emissivity, based on comparison with experimental results."..."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."..."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. Liquid transport (through surface diffusion and capillary conduction, both due to a gradient of relative humidity) shows only a relatively minor temperature dependence. Vapor diffusion, on the other hand, is strongly affected by the temperature field, since the saturation vapor pressure increases exponentially with temperature."...

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