@roy_fredy @ZacD a clean one sided mesh wont help you with GPU Lightmass what your seeing with those light leaks is because well like i said your mesh i only one sided i bet you dont have an outer Shell for the walls because ist an interior Scene and you were thinking you wont Need outer facing walls. just model the walls outside also and facing also outwards and you wont have those leaks anymore and DO NOT USE DOUBLE SIDED MATERIALS OR GEOMETRY with GPU lightmass yet cus this will ■■■■ your Scene up even more. 2 sided materials or meshes wont work propperly Right now with GPU Lightmass