Radiation in conjugate heat transfer

Hello all,

I have the CFD module and am using conjugate heat transfer. This means that I don't have surface to surface radiation but I do however have surface to ambient radiation.

I am modelling external natural convection in 2D using a rectangle of air with open boundaries. I am trying to add surface to ambient radiation on the boundary between the air and the solid part. COMSOL wont let me do this however. I don't know whether it is because it thinks that the air is opaque or because it wont let heat flux's be defined at boundaries between materials.

Would anyone be able to advise me on how to define this surface to ambient radiation flux at such a boundary.

Thank you all very much,
Josh