Hi Frameframe,
The problem is to do with the fact that the 'turn plaster' option does not have a physical effect in 3d, only in 2d. To get around this, with your particular type of detail, you may have to be a bit tricky, but it is possible.
I have assumed that the lower face of the wall is outside. If it isn't on yours, you will have to swap a few of the settings.
Firstly, edit your composite and split the core into two identical layers, separated by a hidden line. Edit the thickness of each layer so the total is the same as the original but the outer thin layer is the same thickness as the render. This will allow you to turn the outer skin as a cavity closer instead.
Set the cavity closer settings for the window as shown below. You will need to set the thickness to a small figure (1mm) so a line will close it.
Use the reveal settings to bump the render layer out a bit as you require.
Also make sure that the reference line is on the outside face of the wall.
You should find, after all of this, that the 3d view should be correct also.
