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Most existing material design studies assume that radiative cooling (RC) windows should be ideal radiators with high emissivity (ε λ ∼ 1) in wide infrared band (IR, 2.5–20 μm). However, whether this assumption is universal has not been tested. Different from previous studies based on experiments and steady-state theoretical calculation models, a transient building heat transfer model with facade windows verified by experiments is used to explore the optimal design of IR emissivity of RC windows. Results show that the RC effect of windows is not always improved as traditionally expected with the increase of emissivity ( ε ‾ NAW) in the infrared " non-atmospheric window (NAW) " band. In fact, the adjustment trend of ε ‾ NAW for the RC effect of windows is reversed. Smaller ε ‾ NAW can weaken the heat flow into the room and prolong the time that windows become " coolers ". A wider range of window-to-wall ratios are also studied, and it is found that the design of selective high emissivity ( ε ‾ NAW ∼0) is more conducive to the RC of windows. It is proved that the current default design of the RC window emissivity in IR band is non-universal, and new laws found provide an enlightening guide for the material development of RC windows. • A reversal trend of RC regulation of window emissivity outside the AW is revealed. • Smaller window emissivity outside the AW extends the time windows become coolers. • Designs with selective high emissivity may be more favorable for the RC of windows. • The default "broadband, high emissivity" of RC windows in IR band is not universal. [ABSTRACT FROM AUTHOR]