Why Does my Lighting Leak Through Walls with Lumen?

Why does my lighting leak through walls?

A commonly asked question, especially around UE4.X to UE5.0 converted projects is “why is my lighting with Lumen leaking through walls?” While there could be a handful of reasons, a common issue is the thickness of the geometry the light is being casted on.

For Lumen lighting, it’s recommended to use walls that are at least 10cm (10 units) thick (or thicker) to minimize light leaking. Because of this, meshes and geometry should be at least 10cm thick for lighting to render correctly with the mesh.

This is demonstrated below.


(10cm/10 unit wide barrier)


(20cm/20 unit wide barrier)

Additionally, thin one-sided static meshes will also cause lighting errors, as the Distance Fields generation can not represent these types of meshes.

Learn More

Read more about Lumens Technical Details on the Unreal Engine Documenation page
Learn more about Lumen with this video, “Lumen in UE5: Let there be light!”
Check out more about lighting and rendering on the Knowledge Base.

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