Some more information on rendering interior scenes with appleseed:
appleseed currently supports two light transport algorithms, so which one best to choose?
- Unidirectional Path Tracing (similar to Cycles or Arnold)
Stochastic Progressive Photon Mapping (SPPM)
This depends strongly on the scene. The default method to use is Path Tracing. It renders faster and is easier to set up correctly. However, if your scene contains lots of glossy reflections or caustics then SPPM is the better option. Path tracing has especially difficulties whenever there is a strong concentration of rays within a small region in space due to refraction or reflections on specular surfaces (caustics).
Unfortunately two major factors still limit the efficiency of many render methods: light transport by multiple highly glossy or specular interactions, and scenes with complex visibility between the camera and light sources (see image).
To repeat my comment on the thread https://forum.appleseedhq.net/t/why-is-my-sppm-render-that-noisy/533/9
The scene above is difficult for pathtracer because the room is only lit through a window from outside, most of the scene illumination is indirect.
The smaller the windows, the more difficult this gets to render. The (backwards) pathtracing algorithm emits rays from the camera into the scene until it hits an object and then needs to find a path to the light source. If the light source is small and difficult to connect to from the hit-point, the chance that a ray finds it is small .