The variation of the total moisture content is expressed through the sum of the transport of vapor in moist air and liquid water. See Theory for Moisture Transport in Porous Media for details.

The liquid saturation, sl, which describes the amount of liquid water within the pores, is defined from the specification of a moisture storage function , which specifies the total moisture content (left hand side of the equation below) as a function of the relative humidity :

where εp (dimensionless) is the porosity.

When there is no variation of the total pressure and under specific temperature conditions, the Building Material node may be used instead. See Theory for Moisture Transport in Building Materials for details.

This section has fields and values that are inputs to expressions that define material properties in the Liquid Water, Moist Air, and Porous Matrix subnodes.

The default Temperature Τ and Absolute pressure pA are User defined. When additional physics interfaces are added to the model, the temperature and absolute pressure variables defined by these physics interfaces can also be selected from the list. For example, if a Heat Transfer in Moist Porous Media interface is added, you can select Temperature (ht) from the list. If a Laminar Flow interface is added, you can select Absolute pressure (spf) from the list.

If the node was added automatically after selecting the Heat and Moisture Transport predefined multiphysics interface, the temperature of the Heat and Moisture multiphysics node is used by default and the input field is not editable. To edit the Temperature field, click Make All Model Inputs Editable ().

Select a coordinate system from the Coordinate system list for the interpretation of directions in anisotropic material properties. The default is the Global coordinate system, and the list contains any additional coordinate system (except boundary coordinate systems) added under the Definitions node.

The Hygroscopic Porous Medium node and its subnodes inherit these coordinate system settings. In particular, the Moist air velocity field (in Moist Air (Hygroscopic Porous Medium) subnode), the Permeability (in Porous Matrix (Hygroscopic Porous Medium) subnode), and the Capillary liquid flux density (in Hygroscopic Porous Medium node) should be set according to the coordinate system selected in this section.

See Coordinate Systems in the COMSOL Multiphysics Reference Manual for more details.

The Moisture storage function should be set to characterize the relationship between the amount of accumulated water and the relative humidity in the material. The default Moisture storage function is taken From material. For User defined, enter another value or expression.

When a Porous Material node is defined on the domains where the Hygroscopic Porous Medium feature is active, the Moisture storage function and the Moisture diffusivity can be set in the Homogenized Properties section of the Porous Material node.

Physics Tab with interface as Moisture Transport in Air, Moisture Transport in Building Materials, or Moisture Transport in Porous Media selected: