@Tontow: Subtractive brushes cannot be used for this type of effect. Geometry/BSP brushes are meant to be used primarily for blocking out your level.
@Paradoc: Just a note about your destructible to get a better result with the mesh itself. I noticed that you can see through parts of your Destructible when it’s destroyed. A key thing to remember when you build your mesh is to closet he geometry. For instance, if you make your walls and you don’t have a face on the bottom it will give this hollow effect you’re seeing.
If you have any questions feel free to ask! 
Tim