Hi @kim1,
Looking at your parameters, the ambient resolution (ar) is very low, which could lead to inappropriate blending of cached irradiance values. This is only an issue if you have shading geometry or a ground surface, and I’m not sure if you do. If that is the case, a smaller ambient accuracy (aa) value would also be helpful. I also think you need a larger ambient division (ad) setting. With a value as low as 512, there may be some chance that your sampling misses the sky patch containing the sun, although I don’t think that’s your issue.
No, it implies the opposite. When the sky is clear, there will be less irradiance on the east side of the building late in the day. When the sky is cloudy, the irradiance on the east side will be higher, more similar to the other facades. Again, you could produce a rendering of the sky to verify this.
Were you expecting the sky to be darker when it is overcast than when it is clear? Take a look at this set of photos of the sky, for example. On a clear day, the sky is not very bright.