Unreal 4 Lighting Academy....or something like that ;)

[Tural]

@rosegoldslugs

Volumetric Lightmaps are per-pixel sampling of probes of various sizes, based on the density and number of static meshes. It only provides indirect lighting and sky occlusion(if you bake the Skylight). So you won’t get any contact shadows, baked AO, or multi-bounce light that you’d otherwise get in surface lightmaps with Lightmass. It’s a good tradeoff for large meshes that would require multiple parts and/or large lightmaps, or like Tilmann said, foliage that shouldn’t be baked, but provided that you have enough sample density, the indirect lighting is almost indistinguishable between surface and volumetric lightmap objects.

Thanks you for your answer. So I think the problem in my test scene was, there was not enough density of samples. And another thing I noticed is, lets say I have dark room and wall with 30cm width. The furniture which I placed in front of wall was bright. Looks like it is illuminated from outdoor when I used volumetric lighting method for it. But when I moved it to the center of the room, it illuminated correctly. So from my understanding, the samples which placed in outdoor next to my wall leaked into room and made my objects lit wrong. So what is the efficient way to solve this problem? Do not want to make wall larger. Is the only way is to increase the sample density other than switching to surface method or am I miss something here?