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.
V20, V21, 2015/ 16/ 17/ 18/ 19 Pro. Platinum
Deluxe 2015/ 16/ 19
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB