Thank you so much, RobMitchell.
Here is a material setup of windows
https://picasaweb.google.com/maxwellren ... 3539497890
you have to zoom in to see.
It has two materials for the windows.
First material is normal glass material for inner faces of windows.
Second material is waterdrop covered glass material for outer faces of windows.
Very simple.
At first I used normal map instead of bump map, but I found that bump map is better because that normal texture which converted from photo is not so good.