Why does a Directional Light illuminate surfaces inside a completely sealed Static Mesh enclosure in a stock UE5.8 First Person/Lumen project?
The enclosure consists entirely of imported 20 cm-thick closed Static Meshes with Cast Shadows enabled. Floor and ceiling use the same mesh. Directional Light Cast Shadows is enabled.
With the Directional Light disabled, the enclosure is completely black even with the Skylight enabled.
With the Directional Light enabled, the enclosure is visibly illuminated even when the Skylight is disabled and Directional Light Indirect Lighting Intensity is set to zero.
Switching between Virtual Shadow Maps and Shadow Maps makes essentially no difference.
What basic UE5.8 setting or workflow am I missing?
classic lumen leak. your walls are sealed but lumen does not ray trace your raw geometry by default, it samples a low res surface cache built around the meshes, and thin walls plus a big bright directional light mean the cached walls have gaps the light falls through. the room being closed in actual geometry does not help if the cached representation has holes.
quick confirm: enable Lumen Hardware Ray Tracing (project settings plus the post process toggle) if your GPU supports it. if the leak changes or disappears, it was the surface cache.
fixes in order: give the walls real thickness for a test, double them up or go 40cm+, thin single skin walls are the number one cause. in the post process volume under Global Illumination raise Lumen Scene Detail, and pull Lumen Scene View Distance in a bit so the cache is denser near the room. also check the wall material is not Two Sided, two sided materials do not occlude lumen traces properly and will leak light every time.
if muting the skylight fixes it instead of the directional, that is the far field path, different fix, so test one light at a time to see which one bleeds.
