Can you show me a sample of this Environment texture? BTW, do you know how can I make the transparency texture for buildings? I think it's the better choice because at least we can see lights moving thought the building.
If you want transparencies in a model in addition, Make a different material, and you'd have to make the glass section separately. You can't just have a section of a box semi-transparent.
But I can't seem to use reflections on a transparent object, so either do that or use the diffuse alpha as the reflection map:
Tick "use inverse alpha for reflection" and either use "Global environment map" or one you have supplied.
I wouldn't make a separate cube map for say each side of the building,.. because it would use quite a lot of memory,..
The default "cube map" or environmental map,.. its a general representation,.. for trees, hills, and without buildings. So for instance, with say a downtown area where a lot of buildings around other buildings,.. the default cube map wouldn't be the best fit.
What I would do, would create a "cube map" for each general region,.. the default would be best for the rural areas,. another for a city or downtown area,.. and so forth...