One last point. Many effects of age on buildings are more intense near the ground than the ceiling. (Water and dirt accumulate more on or around the ground or floor.)

You can incorporate the V node (or perhaps a P for position node) to modulate the cloud bias according to height above the floor as I've done here.

cb = .1 + .7 * Bias(edge, .01) + .4 * (1-V)

This concludes what I had planned to show you.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)