Okay maybe we’re getting somewhere now. There are three heat transfer mechanisms: convection, conduction and radiation. You are asking to see just the radiation portion, correct?
In that case what you want is the following formula:
Qrs = a * e * sb * A * (Tsrf^4 - Tenv^4)
Qrs = Radiant energy exchange of a single surface with a surface in it's environment [Wh]
a = absorptivity of your surface [unitless]
e = emissivity of the environment surface [unitless]
sb = Stefan-Boltzman constant [5.67 * 1e-8 W/m2/K^4]
Tsrf = Temperature of your surface [Kelvin]
Tenv = Temperature of environment viewed by your surface [Kelvin]
A = Surface area [m2]
For a single surface the total radiant exchange will be the sum of the radiant exchange for all surfaces it can “view” multiplied by that view factor.
Qrt = sum(f * Qrs)
Qrt = Total radiant exchange for surface with entire environment
f = view factor for that surface with the corresponding surface it's calculating it's radiant exchange with.
Pretty much everything in these equations you can retrieve from your existing HB model, except the view factor determination which can get kind of tricky, but there is a view factor method in HB to get those. You can also make a simplifying assumption if you have a simple environment. For example if you have a building in the middle of an empty site, you can assume that half the surface is exposed to the sky, and half the surface is exposed to the ground. So your view factor would be 0.5, 0.5 for each, and temperature of environment would be ground surface temperature, and the long-wave temperature of the sky.
Alternatively, there may be some built in method to retrieve radiation energy from the exterior surface, I just can’t think of one right now.