Sorry for the late reply, @n.dewaard ,
You can see the formula for sky temperature in the ladybug-core documentation:
You can see that we also use a default of 1 for the source emissivity there but, for the SolarCal model, we tend to use a default body emissivity of 0.95:
I think, with the 0.95 emissivity assumption, you will still find that you get sky temperatures between -50 and 20 C. But, yes, you will get some weird results if you set the emissivity to something low. The chances of that happening are pretty slim, though. I guess it you wrapped yourself in aluminum foil, you might be able to lower the emissivity and get some odd results.