The short answer: at least 12 feet . . . but 14 feet is even better. Generally speaking, the higher you can go the better. The space above the cabinets accumulates rising heat, which minimizes its mixing with cold air. This improves air conditioning efficiency. Obviously, it also provides space for overhead cable trays and good lighting.
The only drawback to a very high ceiling is the cost of an inert gas fire suppression, if that is what you're using. With sprinklers, it doesn't matter. The amount of gas needed is based on the volume of the room. Thus, high ceilings add to its cost. So there is a tradeoff.
With gas fire suppression, I probably wouldn't go over 14 feet. We have done data centers with as little as nine foot ceilings. But if it's this low, it's best to duct the return air back to the air conditioners from each hot aisle. It is also probable that you will have a fairly low raised floor (assuming you're using under-floor air distribution).
If you can't get at least an 18 inch floor height, you're going to significantly limit how much wattage you can put in a cabinet. If the data center is small as well, you'll also have rather uneven air flow. Engineering is a business of tradeoffs, and you should have a careful evaluation, including CFD (Computerized Fluid Dynamics) modeling, before deciding on floor and ceiling heights in a restricted space.
ABOUT THE AUTHOR: Robert McFarlane is a pioneer in the field of building cabling design. He has been asked to speak at countless seminars on building infrastructure for electronic communications, evolving technologies and the requirements of trading floor and data center design. Mr. McFarlane served for twelve years as President of Interport Financial, Inc., a firm specializing in designs for financial trading floors and critical data centers.