Lumen causing light leaking through thick walls

Development Rendering
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Hello,

We have been having issues with light leaking caused by the directional light when Lumen is enabled (UE 5.4.4). The walls are already 50cm thick and we have tried thickening them further but to no avail. We are also using double-sided materials. So far, the only thing that has worked was enabling the Volumetric Translucent Shadows option for the floor, ceiling and wall meshes which entirely solves the problem (even though our material is completely opaque). But, that seems like a hacky fix rather than the right solution. Anyone has any other suggestions besides the usual “thicken the walls”, “blocking volumes” or “reduce shadow slope bias”.

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Thank you!

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Root cause identified. It seems that the normals for the mesh faces were incorrectly exported leading to wrong geometry normals and thus, Lumen miscalculating the lighting. In Blender, we had to flip the normals for the affected faces and that fixed the issue.