Energy use for hydroponic tulip forcing in multiple layer systems is determined by transpiration. Reduced transpiration when Relative Humidity (RH) is high (RH> 95%) results in topple. Previous research of Wageningen University & Research (WUR) showed transpiration (RH < 80%) to be important especially during the second third part of forcing: the topple sensitive phase. However, during this phase a daily exposure to a RV > 95% of 5 – 10 hours appeared to be tolerated. This is confirmed by the present WUR research: up to 6 hours without ventilation does not increase the incidence of topple. While more days with good ventilation during the topple sensitive phase reduces the amount of topple. However, cultivars differ remarkably in sensitivity to limited transpiration. Some are hardly sensitive, some are sensitive for a short time only (6 days), some for a much longer period (10 days). Maintaining a RV < 80% only during the topple sensitive phase will reduce energy use strongly. For application of these findings, more research is necessary. This research was funded by Min. LNV.