Subtracting Surface from Other Surface (Hole in Wall)

3Hey guys,

I have been struggling trying to figure this out.  I am attempting to cut a hole in a wall, subtracting a clear glazing unit from a curtain wall (see images 1 - 2).  The height of curtain wall is parametric, so that as the height (area) changes the window hole will stay constant.

I have cut a hole from the surface using subtract and cull (images 1-2).  As seen on the second image, the command works when the curtain wall reaches below the window.  However, in image 3, when the curtain wall is above the window, the entire curtain wall disappears.

I believe because it is the only indexed item in the cull command at that point so it is deleted.  Any suggestions on how to fix this?

Thanks in advance,

Reinhardt