Open Boundary Condition in Mathematics module

Hello,
I have built a 3D Heat-Air model of an air-permeability defect. Basically, air is flowing from the heated volume through cellulose insulation to the attic (not heated). Therefore, I see the coupling between air and heat transfer, and the influence on the temperature field.
The model works well, I am using an "open boundary condition" at the interface "insulation air" for heat. Now I intend to add moisture into the model. for that, I have added a PDE node with the moisture conservation, and I input a source term in the heat-equation.
I have 2 problems about that:
- I have not managed to write the local latent source term in Comsol: it seems that the heat source term Q of the heat equation can only be a numerical value in W/m3 (how to write nabla ...?)
- If I use the EDP node (in Mathematics module) instead of Heat Transfer for the heat equation, I am able to implement my source term, but not anymore the open boundary condition.
About the Mathematic mode and the open-boundary condition, I looked in the forum and it turned out that I am not the first to have this problem. I saw that the users solved their problem in using another module.

I would be very grateful if you could provide me some tips and advice about that.
Thanks a lot!
Clément