Thin glass windows

I’m starting to get somewhere with rendering cars in a more or less realistic way. I’m having problems with the windows, however. The windows in the models I have, are simple planes, so they have no thickness. Nevertheless, when I give them an index of refraction and I render them, they cause huge distortions as if they’re massive blocks of glass. What kind of thickness does Blender or Yafray assume for a planar transmissive surface, and is there any way to control this? Or should I just extrude all glass surfaces so they become volumes?

A single polygon layer? Now how would you model a solid glass object?
By a surface made of a single polygon layer, no? You just filled your car with glass…
Your light enters the glass but never leaves it again until it hits another window.

OK, I was suspecting something like this. Even with the IOR set to 1.04, the distortion seen through the glass was huge. Thanks!