If you want an air gap I a complex profile then don't put any fill for it - just leave a gap.
But if you want that gap at the top of your profile as it seems you do then it won't work unless you have something over it to maybe represent your window frame position.
Or maybe I misinterpret what you want - maybe post a screen shot - 3D view with a marquee through your window sill would be perfect.
You can make a material transparent (as with glass) but this causes other problem in that you will be able to see through your entire profile - not just the transparent bit.
See this related post.
http://archicad-talk.graphisoft.com/viewtopic.php?p=228955
But if you can wait for 18 then your problems will be solved.
That will have true transparent materials in profiles.
Barry.
One of the forum moderators.
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