Half of my model is apparently underground, so i have used the HB Custom Ground Component with a surface to act as the ground plane. However it doesnt seem to be working.
Thank you
As the component says:
Room faces that are coplanar with the ground surface or have a center below it
will get a Ground boundary condition.
So I would guess that your underground Faces must have their centers below your ground surface.
Note that you can also use the HB Properties by Guide Surface to get a little more control over how a surface like a ground sets the boundary condition of individual faces.