Leen,
I would think that the approach would differ depending on whether the
curtains were right up against the window or if they were in a doorway
between two zones. I would also guess that there would be some
differences in approach depending on what simulation tool you are using.
In TRNSYS, curtains up against windows can be handled as a window
shading device. For curtains separating two zones, you could model the
opening as an adjacent window (using the virtual /open window
properties), which would then allow you to define a shading device and
pass solar radiation from one zone to the other. You can also define
coupling airflow depending on whether the curtain is open or closed.
best,

David BRADLEY