Hi Tom,
Welcome to the Sweet Home forum!
Do you know I did exactly the same thing, with exactly the same shower door model, for some render tests recently. They say great minds think alike.
I already knew that light can't pass through glass objects (see below) in Sweet Home (yet!) so instead I started out with an Alpha blended model but have since found Alpha blending doesn't seem to be supported in SunFlow (the Java render engine used by Sweet Home)
I was hoping to create a partially condensation covered mirror with just a partial reflection for this photo shoot scene I created. (I'll be posting a larger version in the gallery soon)
At the moment Sweet Home 3D doesn't render Caustics. The glass transparency you see, in your render, is kind of an illusion produced by Sweet Home, think of it more like a hack to provide a form of transparency. You can see lights and objects on the other side of the glass and the effect light has on those objects but no light can pass through the Sweet Home glass model.
Even with the correct glass parameters in the .mtl file (without the sweethome3d_window_pane line in the .obj file) when loaded in to Sweet Home, it would show as a transparent glass object in the quick (preview window) view but appear as a sold black object in the final render.
I am hoping that one day we will have Caustics (plus maybe also Alpha channel blending and Bump mapping) but it will slow down render speeds.
Hope this helps,
Looking forward to seeing your model in the gallery section.
db4tech
Edit:
For the type of effects you would like to create, at the moment it might be best to design your model in Sweet Home, then export it as an obj, file (found under the 3D View menu)
Import the obj file into a program like Blender, add lights and glass objects then render in blender. Poetry
Edit 2: To add a correction.
