Building Material
Use this node to model a building material as a porous medium containing a mixture of liquid water and vapor. The overall material properties change due to moisture transfer is accounted for through an apparent thermal conductivity change and a heat source or sink given in the divergence operator in function of the latent heat of evaporation:
(6-1)
(6-2)
with the following material properties, fields, and source:
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(ρCp)eff (SI unit: J/(m3·K)) is the effective volumetric heat capacity at constant pressure.
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keff (SI unit: W/(m·K)) is the effective thermal conductivity (a scalar or a tensor if the thermal conductivity is anisotropic).
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 Lv (SI unit: J/kg) is the latent heat of evaporation.
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δp (SI unit: s) is the vapor permeability.
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 φ (dimensionless) is the relative humidity.
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psat (SI unit: Pa) is the vapor saturation pressure.
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Q (SI unit: W/m3) is the heat source (or sink). Add one or several heat sources as separate physics features. See Heat Source node for example.
For a steady-state problem the temperature does not change with time and the first term disappears.
Model Inputs
This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups are added, the model inputs appear here.
Relative humidity
This section has an input for the definition of the relative humidity, used in the right hand side of Equation 6-2.
The default Relative humidity φ is User defined. When additional physics interfaces are added to the model, the relative humidity variables defined by these physics interfaces can also be selected from the list. For example, if a Moisture Transport interface is added, you can select Relative humidity (mt/pm1) from the list.
If the node was added automatically after selecting the predefined multiphysics interface Heat and Moisture Transport, the relative humidity of the multiphysics node Heat and Moisture is used by default and the section is not editable. To edit the Relative humidity field, click Make All Model Inputs Editable ().
Heat Conduction
This section provides two options for the definition of the effective thermal conductivity keff:
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When Equivalent thermal conductivity is selected (the default), a value for the Effective thermal conductivity keff should be specified directly. The default Effective thermal conductivity is taken From material. For User defined, select Isotropic, Diagonal, Symmetric, or Anisotropic based on the characteristics of the thermal conductivity, and enter another value or expression. For Isotropic enter a scalar which will be used to define a diagonal tensor. For the other options, enter values or expressions into the editable fields of the tensor.
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When Dry material thermal conductivity is selected, the effective thermal conductivity is defined in function of the solid matrix and moisture properties:
This definition neglects the contribution due to the volume fraction change of the moist air.
The Dry solid thermal conductivity ks (SI unit: W/(m·K)) and the Thermal conductivity supplement b (dimensionless) should be specified. The default Dry solid thermal conductivity and Thermal conductivity supplement are taken From material. For User defined, enter values or expressions into the editable fields.
The Density ρs and the Moisture storage function w are specified in the Thermodynamics, Dry Solid and Building Material Properties sections respectively.
Thermodynamics, Dry Solid
This section sets the thermodynamics properties of the dry solid.
The specific heat capacity describes the amount of heat energy required to produce a unit temperature change in a unit mass of the dry solid material.
The Density ρs and the Specific heat capacity Cp,s should be specified. The default Density and Specific heat capacity are taken From material. For User defined, enter values or expressions into the editable fields.
The effective volumetric heat capacity at constant pressure is defined to account for both solid matrix and moisture properties:
where
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ρs (SI unit: kg/m3) is the dry solid density.
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 Cp,s (SI unit: J/(kg·K)) is the dry solid specific heat capacity.
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w (SI unit: kg/m3) is the water content given by a moisture storage function.
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Cp,w (SI unit: J/(kg·K)) is the water heat capacity at constant pressure.
Building Material Properties
This section sets the properties of the building material for moisture storage and vapor diffusion.
The Moisture storage function w 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.
Two options are available for the specification of the building material properties for vapor diffusion:
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Vapor permeability (default) to define directly the vapor permeability δp. The default Vapor permeability is taken From material. For User defined, enter another value or expression.
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Vapor resistance factor μ to define the vapor permeability δp as:
where δ (SI unit: s) is the vapor permeability of still air. The default Vapor resistance factor is taken From material. For User defined, enter another value or expression.
If the node was added automatically after selecting the predefined multiphysics interface Heat and Moisture Transport, the building material properties of the multiphysics node Heat and Moisture are used by default and the inputs are not editable. To edit these fields, click Make All Model Inputs Editable () in the Model Inputs section.
The Heat Transfer in Building Materials Interface
The Heat and Moisture Transport Interfaces
Heat and Moisture
See Building Materials Database in the COMSOL Multiphysics Reference Manual for a description of materials containing the thermal and hygroscopic properties required by the Building Material node.
The Building Material node is defined on the spatial frame. The material properties should be entered in the spatial frame, and the coupling with a moving frame interface is not supported. See Handling Frames in Heat Transfer for more details.
When Surface-to-surface radiation is activated, the Opacity subnode is automatically added to the entire selection, with Opaque option selected. The domain selection can’t be edited. To set some part of the domain as transparent, add a new Opacity subnode from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu.
For a detailed overview of the functionality available in each product, visit http://www.comsol.com/products/specifications/
Location in User Interface
Context menus
 Heat Transfer in Building Materials>Building Material
More locations are available if the Heat transfer in porous media check box is selected under the Physical Model section. For example:
Heat Transfer in Solids>Building Material
Ribbon
Physics Tab with interface as Heat Transfer, Heat Transfer in Solids, Heat Transfer in Fluids, Heat Transfer in Porous Media, Heat Transfer in Building Materials, Bioheat Transfer, Heat Transfer with Surface-to-Surface Radiation or Heat Transfer with Radiation in Participating Media selected:
Domains>interface>Building Material