It doesn’t seem like it’s caused by lightmap resolution, because (as you can see from the shadow on the floor) the lightmap resolution is much higher than the bleeding. Also the bleeding has kind of a round falloff.
The walls are interpenetrating with the ceiling and floor, so I’m certain there’s no gaps in between. Here’s my lightmap UVs, in case it helps:
You are much better off using individual wall meshes as you can then give better quality lightmaps to them which will solve your problem. Your lightmap resolution may be high but its covering all of your walls with a single lightmap and then you get bleeding on the UV.
It looks like it’s somehow bouncing around inside the mesh and then bouncing through the backfaces on the other side, even though the wall mesh is now closed and perfectly flush with the floor and walls. Is there anything I can do to prevent this besides making the walls all cartoonishly thick?
You can’t have a surface extending beyond the visible area, in that last situation, the pixels of the lightmap won’t line up with where the wall is, and so you’ll either have shadow pixels extend inside or lit pixels extend outside.
Your surface can’t extend beyond the visible area, the part that’s not visible will receive shadows, so you have a floor with a wall on top of it, but the floor goes underneath the wall. When you build the lighting pixels that are lit will extend underneath the wall and might be visible on the other side, or you might get a shadow underneath the wall which can cause dark pixels to appear on the floor since the mesh can’t line up perfectly with the lightmap.
It’s not worth the trouble to try and line up meshes with lightmap pixels, since you can’t do that with meshes that aren’t rectangular.
From what I can see, your first room mesh is correct, your second example is not. I can’t see how the light would bleed over there unless the floor or ceiling go over the space between walls, which it looks like it doesn’t.
Well, the walls don’t need to be capped, but it wouldn’t be bad to have wall/ceiling surfaces facing out like the exterior of the building, but I wouldn’t think that would have an effect on interior lighting.