This presentation highlights why energy-efficient cleanrooms should be addressed in the early planning phase and why estimates or SOP specifications are not a suitable basis for precise design. Cleanrooms have significantly higher energy demands compared to unclassified areas, making data-driven planning essential. Central to this is the correct definition of the air cleanliness class according to ISO 14644 and EU GMP, the identification of relevant contamination sources, and the calculation of the actual supply air volume flow rate instead of general air exchange rates. Studies show that approximately 80% of contamination originates from personnel, making process analysis, airflow management, and ventilation effectiveness crucial factors. Experiments demonstrate that optimized air distribution and properly positioned exhaust air inlets can significantly improve contamination removal effectiveness. The presentation concludes with a call to consistently utilize existing tools such as data-driven volume flow calculation, flow efficiency assessment, and demand-controlled filtration to save energy and simultaneously make cleanrooms safer and more efficient.