I feel like a crazy person when I read this thread.
Where. Is. The. Opacity. Bar. In Sketchup Windows?
I just want to select a color and set an opacity to almost completely transparent (retail window) and the mac UI is great, but I have a PC now and I cannot find this stupid opacity bar. WTF?
I had a similar problem. A material with transparency set will only appear transparent from one side. Do “Reverse faces” on the plane that won’t go transparent. If a wall has 2 sides apply the material to both and if either remains opaque do “Reverse Faces.”