LB Incident Radiation has non-zero values on bottom surfaces

Hi community!

This might be an easy question since I’m very new here. I have used the “LB Incident Radiation” component to study solar in urban areas but I found that there are some unreasonable results on the bottom of buildings.
Results above ground level are fine but in the bottom of buildings, isn’t it supposed to be zero (0 kWh/m2)?

I have created the ground surface, half of the buildings are solved but there are still some buildings that have radiation values from bottom surfaces.

Thank you in advance for any reply
TestArea01_solar.rar (893.5 KB)