A user posted a question about a common problem—in his rendering he was getting white speckled lines around the edges of walls and the ceiling.
After examining his model, we discovered he had walls made of a single face, rather than thick walls, with dimension. Single face walls can work but they often leak light through the corners where two walls meet.
If you have light leaking through, then stretch out the walls to make them thick, or add additional faces behind them.
If the geometry is rectangular and perpendicular, there is usually not a problem. But if any of it is at an angle, then light can slip through.
Here is another rendering of the model after adding faces behind the walls and above the ceiling.