To avoid light leaks:
Build all surfaces with real-world thickness (walls, floor, ceiling)
Set proper photon shooting and final gathering for your scene.
If your model is built with thickness, then you may need to increase photon shooting and final gather.
Easiest way to do that is to choose a higher quality render settings. If rendering on "Low" try using "Medium", and so on.
If you prefer to keep "Low" setting - try the "01.SunSkyLow" render setting under the Sun-Sky render settings. The final gather and photon shooting is set much higher in those settings, as they presume a fewer number of lights (it assumes 2 lights in the scene, aka the Sun and the Sky).
Or you can create your own render setting:
v2 Pro users can right click any render setting and choose to edit the setting within SketchUp.
Hobby users can open the settings in a program such as Notepad++ to edit them
NOTE: save a copy of the file, don't overwrite your default render settings.
How to Modify Twilight Render Setting
