Material study for outdoor thermal comfort- Model and results queries

Hi LBT Forum,

I have a question regarding the modeling of exterior zones for a materials study using different custom construction sets. I see that the ground needs to be input into RoomSolid component as a closed polysurface. So modeled two polysurfaces, one 0.1m deep and another 1 m deep, both assigned the same soil construction set. I see different results in UTCI(attached below), not only between two polysurfaces of different depths, but also vary with runs. The second time I ran the study for the 0.1 m deep ground, I got different ranges of UTCI. (77.2 KB)

So, the modeled thickness makes a difference and not just the thickness of the material input into the OpaqueMaterial component.
My questions are:
What is the right depth of the polysurface to model?
How can you make comparisons between the impact of different materials when there are differences in results every time you run a simulation?

I would be grateful to your insights!

Thank you,