.. png textures were made semi transparent... + ..transparency was specified in the .mtl file ...

I should add that finding the right balance between png transparency and mtl transparency is tricky. Especially with several transparent layers (curtains, window pane, lamp) - and even more so for evening rendering. Some experimenting is needed for the right effect to work for you (and I suspect that a different graphics card could produce different effects).

I added semi-transparent curtains to the model in the Sourceforge ticket (second post) (sorry, didn't have vertical blinds in stock [:)] ).

Good luck - and consider sharing, I guess vertical blinds are quite common many places.

ok