Question about how wall, roof, and floor thickness affects internal zone dimensions in DesignBuilder

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Hi everyone,

I'm working on a model in DesignBuilder and I've defined a block with dimensions 8 m × 6 m × 3 m. I'm trying to understand how material thicknesses for walls, roofs, and floors affect the actual internal zone dimensions used for thermal simulation.

I noticed that when I assign a wall thickness (e.g., 30 cm), it seems to be applied inward, reducing the usable interior space. I would like to confirm the following:

Does this same behavior apply to roof and floor thicknesses? For example, if I assign a 30 cm thick floor or roof, does that also reduce the 3 m internal height?

Is there any way to define wall/roof/floor thicknesses so they apply outward, preserving the internal zone dimensions?

Are there any official manuals, documentation, or forum discussions that explain this behavior in detail?

My goal is to make sure that the simulated thermal zone represents the actual conditioned internal volume (e.g., exactly 3 m of usable interior height).

Thanks in advance for any clarification or references!