Hi Sam,
Interior windows are very easy to make in HB and I feel that your confusion is my fault as I have not put up a tutorial video on interior windows yet. I have attached an example file that shows you how to create interior windows, change their construction, and even include interior/exterior blinds on the windows. It should be noted that, by default HB is going to add different default constructions to interior and exterior windows (interior will usually be single pane and exterior will usually be double pane). As you see in the file, you can change these as you like.
The attached GH file also shows you that I have worked functionality of interior window transmissvity into the thermal maps that I made for my thesis.
Let us know if you have any other questions,
-Chris
InteriorWindowsExample.gh (582 KB)