1. Here's why

Unlike the section plane/box function in Revit, ArchiCAD etc, the Clip Plane in Lumion does not 'know' whether an object is solid or not, so walls and other solid objects will appear hollow.

Example of hollow walls and floors when using the Clip Plane in Lumion:

2. Workaround

2.1: We recommend using the section plane/box functionality in Revit, ArchiCAD etc as it caps clipped objects as solid surfaces.

Example of capped walls and floors when using the Section Box tool in Revit:

2.2: Consider using the Variations function, and the Variation Control Effect to show the clipped version of the model along with the original version:

Video Tutorial: Build Mode: Add model variations

Video Tutorial: Effects: The Variation control effect