What is permanent ventilation?

We are required to have minimum lower fixed ventilation and minimum fixed upper ventilation.
The area calculation is found in Section 7 of the standard. The section has a lot to say! Briefly: The amount of ventilation required varies dependent on the number and gas rating of the appliances.
The minimum required area is 4,000mm². It’s termed as “cross ventilation”, in other words opposite each other. Location can be no more than 150mm from the floor and no more than 150mm from the roof.
A window or a roof hatch which can be opened or closed cannot be part of the calculation. If the vehicle has a drop down roof (eg a pop-top caravan) the ventilation must be operational with the roof down.