Like Hansmex, I don't like a mystery either, so I downloaded your file Rob123, and noticed your wall transparency is slightly moved. If you move the wall transparency slider all the way to the left, You will get a good solid wall. You are getting the glass look because the renderer thinks of the walls as transparent. Here is the render I got...
I see your file is in what I believe is french, so if I haven't explained clearly enough, maybe someone can translate this into french for you...
BTW, Very nicely done home, it looks excellent.
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