black windows?

note: i’m not too sure where to put this so plz move if i chose the wrong thread

in yafray and indigo, three of the four windows in my scene are always black. i dont know why!!!

the two middle ones are linked objects (reversed so both door handles are on the inside) and the two outer ones are linked objects.

the blend file:

Assuming they all share the same material. I would guess Normals are pointing the wrong way.

ok thanks it worked but now the one that was see-through is black. i removed the link between the two middle ones and recalculated the normals to be outside on both. i was expecting this to fix the problem and didn’t have the sligtest worry so i just did a quick render BUT THAT ONE WINDOW WAS STILL BLACK!1!! yes the normals on all the windows are outside and no there is no link between the two middle ones.

here’s the blend:
i just dont understand. it just doesn’t work with indigo