If there should be a "ventilation gap" between the 2 parts, change it to a 3 Component wall and add the ventilation.
If you just move 1 of the part for make it look like a gap and later use the TC database tool to find the area, TC will think it is 2 walls and you will get nearly double area
( or at least TC database tool did earlier )
So all the component's should touch each other, the ventilation i dont give brush and use glass/ glass as a material, then it will look like there is an emty space there in wireframe and render.
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