Why does baked light leak through walls in Unity?

Light leaks usually come from thin geometry, low lightmap resolution, or mesh chart padding too small. Add interior thickness to walls, raise lightmap resolution for affected meshes, and increase Lightmap Padding to 4 px or higher in the lightmap settings.

How does lightmap padding affect leakage?

Padding controls the buffer between charts in the lightmap atlas. Too little padding lets neighboring charts bleed into each other when bilinear sampling. Setting padding to 4 or 6 pixels eliminates most adjacent-chart bleed.

Should I use Light Layers to prevent specific lights bleeding?

Light Layers (URP/HDRP) restrict which renderers a light affects. For dynamic lights leaking past geometry, this is exact. For baked lights, fix the bake source (geometry, padding, resolution) rather than masking after the fact.

Fix: Unity Baked Light Leaks Through Walls

Quick answer: Light leaks come from thin geometry, low lightmap resolution, or chart padding too small. Add wall thickness, raise the affected meshes’ Scale In Lightmap, set Lightmap Padding to 4–6 px, and rebake.

Here is how to fix Unity baked lightmaps where the bright outdoor sun bleeds into your interior corners. Indoor rooms feel washed out at corners that should be dark. The cause is the lightmap atlas: charts are packed close together, and bilinear sampling reaches across chart boundaries when the wall geometry is too thin or the padding too small.

The Symptom

Walls separating bright and dark regions show a halo of bright color seeping into the dark side. The leak is most visible at corners and along seams. Toggling lightmap visualization confirms the leak is in the baked data, not screen-space lighting.

What Causes This

Thin walls. A 0.05 m wall has lightmap charts on each face right next to each other. Atlas packing places them adjacent; texture filtering bleeds.

Low chart resolution. If the wall’s Scale In Lightmap is 0.1, the charts are tiny. Each pixel covers a large world area; bleed errors are huge.

Padding too small. Default 2 px padding is borderline. Anti-aliased filtering during sampling pulls in adjacent values.

Outdoor light unbounded. Sun light with very high intensity amplifies any bleed; even a 1% leak shows visibly.

The Fix

Step 1: Thicken walls. The cleanest fix. Set wall meshes to at least 10–20 cm thick. Light samples on each face have physically separated positions; bleed becomes geometrically smaller.

Step 2: Raise Scale In Lightmap on affected meshes.

// MeshRenderer Inspector -> Lightmapping
Scale In Lightmap = 2.0   // was 1.0

Doubles texel density on this mesh, halving the world area each pixel covers, sharply reducing leak distance.

Step 3: Increase Lightmap Padding. Open Window → Rendering → Lighting → Lightmapping Settings. Find Lightmap Padding and set to 4 or 6 pixels. Each chart now has more buffer space; sampling cannot pull from neighbors.

Step 4: Use baked Sky Box vs separate sun light. If your scene has both a Skybox bake contribution and a directional sun, ensure the Sun’s Indirect Multiplier is balanced — values above 1 amplify leaks dramatically.

Step 5: Rebake and verify. Generate Lighting. Walk the camera to the previously leaky corner. The dark area should now stay dark.

Light Probes For Dynamic Objects

Even with perfect baked lightmaps, dynamic objects (NPCs, player) sample Light Probes, not lightmaps. If a probe sits in the bright zone but the dynamic object stands in the dark zone, the object lights bright. Place probes carefully on both sides of walls, especially at doorways.

Per-Light Light Layers (URP/HDRP)

For real-time lights that bleed, Light Layers offer surgical control:

// On the Light: Light Layer = OutdoorOnly (custom layer 1)
// On indoor renderers: Rendering Layer Mask excludes OutdoorOnly

The light cannot affect renderers outside its layer regardless of geometry.

“Thicker walls. Higher chart resolution. More padding. Three knobs that close every leak.”

Related Issues

For baking stalls, see Lightmap Bake Stuck. For GI seams, see Baked GI Seams.

Thick walls. High Scale. 4-6px padding. Rebake. Corners dark again.