WUFI

I enter "0" in the box of vapor transfer coefficient to set the boundary condition of moisture impermeability.
However, it didn't work.

Could you tell me how to make the impermeable boundary ?

If you set the vapor transfer coefficient to zero for a surface, only the diffusion is influenced. If you like a moisture impermeability of the surface, set the rain water absorption factor to zero and set the sd-value to a high value. Then the surface is impermeable.
Second possibility is to insert a membrane in front of the surface and uncheck the box in the field water diffusion resistance factor dry. Then this material is calculated without any water.

Greetings,
Christian Bludau