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.
V18, V19, V20, V21, 2015 Pro. Platinum
Deluxe V20, V21, 2015
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB