Extremely unpredictable directional+skylight lightmap results depending on room mesh approach


I am trying to make simple 1-room Archviz project, but despite my experience in couple of Unreal projects this time I have failed completely just on the beggining.
I want to light the room using only the light coming from window, with natural feeling, thus I am just setting up directional light + skylight.

I have tested 6 approaches to create basic room with a hole as a window, but I am struggling with quite random results:

  • #1: single mesh Room that is created as just 5 rectangles then a hole is cut in the wall (no wall thickness)
  • #2: single mesh Room that has 2-layered walls with some thickness
  • #3: separate floor + ceiling + walls mesh, where walls a little bit higher than the room height, to avoid lightmap errors on edges
  • #4, #5, #6 - same as #1, #2, #3 but additional black material model is added to surround the original mesh to restrict light hitting the outside of faces

Looks like this time I have got quite decent results in #2 and #2+surrounding approaches, and I could continue with that approach, but unfortunatelly it came up that the results are random, and when I have created the actual (measured) room with this approach, everything got super-dark and failed like #1 or #3. Do you have any ideas why? Does it dependens on “direction” of the faces itself? Why just all the models cannot be lightened similary as they coexists in exact same conditions (!)? I totally do not get the reason why one of the room goes normally and then neighbor room goes totally darsk despite the directional light…

All meshes done the same way, and lightmap resolution overriden to 512.

I was supposing it’s an error related to SketchUp FBX cale import.
I have created multiple similar boxes (with some thickness) and exported at scales: milimater, centimeter, meter, then adjusted UDK import ratio to proper one from 0,1-100.

All the models works fine, thus I still do not know what is wrong with my original model.