Building Material
Use this node to model moisture transfer in a building material through vapor diffusion and capillary moisture flows. The moisture content variation is expressed through the transfer of relative humidity
(7-1)
(7-2)
with the following material properties, fields, and source:
•
ξ (SI unit: kg/m3) is the moisture storage capacity.
•
δp (SI unit: s) is the vapor permeability.
•
  (dimensionless) is the relative humidity.
•
psat (SI unit: Pa) is the vapor saturation pressure.
•
T (SI unit: K) is the temperature.
•
Dw (SI unit: m2/s) is the moisture diffusivity.
•
G (SI unit: kg/(m3⋅s)) is the moisture source (or sink). See Moisture Source node.
For a steady-state problem, the relative humidity does not change with time and the first term disappears.
The transfer equation above, specifically designed for building materials under various assumptions, should be used carefully for some other type of porous media.
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.
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 Building Materials 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 ().
Building Material
This section sets the material properties for moisture diffusivity, moisture storage, and vapor diffusion.
The default Moisture diffusivity Dw is taken From material. For User defined, set a value to characterize the liquid transport in function of the moisture content.
The default Moisture storage function w is taken From material. For User defined, set a value to characterize the relationship between the amount of accumulated water and the relative humidity in the material.
Two options are available for the specification of the material properties for vapor diffusion:
•
Vapor permeability (default) to define the vapor permeability δp directly. The default is taken From material. For User defined, set a value.
•
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, set a value.
The Moisture Transport 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.
For a detailed overview of the functionality available in each product, visit http://www.comsol.com/products/specifications/
Location in User Interface
Context menus
 Moisture Transport in Air>Building Material
Moisture Transport in Building Materials>Building Material
Ribbon
Physics Tab with interface as Moisture Transport in Air or Moisture Transport in Building Materials selected:
Domains>Building Material