What is the minimum wall thickness to prevent light bleeding in Unreal Engine? Google’s AI summary says at least 30 CM is required and 50 CM is the gold standard:
20 cm is risky for exterior shells in Lumen-driven projects. While it is physically “thick,” it sits on the knife-edge of Unreal’s default occlusion thresholds.
1. Light Leaking (The “Sponge” Effect)
Lumen uses Mesh Distance Fields (MDF) for software ray tracing. At 20 cm, if your Distance Field Resolution Scale is at the default (1.0), the resulting MDF volume may be thin enough that high-intensity light (Directional Light at 100,000 lux) “samples” through the mesh. This results in splotchy light leaks in interior corners.
2. Surface Cache Precision
Lumen creates “Cards” to represent surface lighting. If a wall is 20 cm, the front and back cards are very close. High-frequency light (like sunlight hitting the exterior) can bleed into the interior surface cache if the Shadow Bias or Lumen Scene precision isn’t perfectly tuned.
3. Modular Seam Artifacts
If you use modular 20 cm wall sections, any tiny misalignment (even 0.01 units) or vertex normal mismatch will be magnified. Light will “jet” through the seams because 20 cm doesn’t provide enough “shadow depth” to naturally occlude the light before it reaches the interior.
As always, AI gives a condensed summary of rumors and bad answers
It really depends on what you’re constructing. In one of the examples there, the mention of 100,000 lux for the sun is totally NUTS, unless you’re trying to do some sort of super realistic lighting scenario.
Best thing is to have a totally un-tweaked setup ( default sun, fog, atmosphere etc ), and try what you can with the meshes. Start with an example level, don’t drag them into an empty map.
It’s definitely true that one sided, or very thin meshes will not help, but apart from that most meshes will be fine.
If you’re trying to create a map where you walk into a very dark room in the middle of a sunny environment, you might encounter problems, otherwise it should be fine.
There are about a dozen different features in Unreal that will all leak light in various ways and not all of them can be fixed by using thicker walls and there is also no standard safe minimum thickness to prevent light leaking for all surfaces or features.
For example:
Anything relying on the lighting volume (such as translucency) for example will leak lighting through any wall that is under 2 meters thick with default settings.
Skylights will beam light in through every direction through all walls/ceiling no matter how thick it is unless you’re using Lumen or Lightmass with skylight shadowing enabled. For lightmass it must also either be stationary or static.
Raytraced cast shadows do not need any thickness at all because they are effectively pixel-perfect
There’s a very long list of these, Unreal is a very complex engine with a lot of lighting features and it is not in any way a good idea to find one thickness to rule them all; Because there isn’t one. Instead, you should block out your scene geometry and test it, tackling the problems as they come up.
If you want an actual ‘official’ number, the Lumen documentation states that when using Software Raytracing your walls should be no thinner than 10cm. I don’t think this is a helpful number though and I would not take it very seriously for the reasons I listed above.
