Requirements for height above a cleanroom
In my experience, I have seen a minimum height of 24″ above the total height of the cleanroom. This in fact should be more if roof access is necessary. If the room height is 20′ tall, it is acceptable to build a room that is 18′ tall. This said, you want to make sure that you are not building a large room just to maximize space. A lot of operational, mechanical and design costs are associated with the size of the cleanroom. Roof access is always a plus to ease FFU installation and maintenance and to ensure unobstructed air flow to the FFU.