The shielding performance of planar conductive nanoscreens against transient sources are studied in detail by means of an approximate semi-analytical formulation based on a Cagniard-De Hoop approach. In the presence of a pulsed electricline source, such a formulation allows for easily deriving the transient fields as a single convolution integral. Several definitions of time-domain shielding effectiveness, recently introduced in the literature, can thus be discussed and compared with the conventional time-harmonic shielding effectiveness. Comparisons with results obtained numerically through an exact canonical double inverse Fourier transform are also provided, showing the accuracy of the proposed semi-analytical formulation for screens with nanometric thickness.