The moisture conservation equation in a solid material is written as follows:
where is the moisture content (for example in [kg/m]) and vector is the massic flux of moisture (in [kg/ms]). When several materials are present in a structure, as often in laminates, it is more practical to work with moisture percentage (in [%w]). The moisture percentage is related to moisture content by the following expression:
where is the moisture coefficient in [kg/(m%w)].
Generally, the moisture flux is related to the gradient of moisture by Fick’s law, and the moisture diffusion equation can be written:
Here is the tensor of moisture conductivity coefficients. This expression is very similar to the equation of thermal diffusion:
As one works with moisture percentages, nothing equivalent to the laminate thermal capacity is defined for moisture.