  Beyond a critical disorder, two-dimensional (2D) superconductors become
insulating. In this Superconductor-Insulator Transition (SIT), the nature of
the insulator is still controversial. Here, we present an extensive
experimental study on insulating Nb_{x}Si_{1-x} close to the SIT, as well as
corresponding numerical simulations of the electrical conductivity. At low
temperatures, we show that electronic transport is activated and dominated by
charging energies. The sample thickness variation results in a large spread of
activation temperatures, fine-tuned via disorder. We show numerically and
experimentally that the localization length varies exponentially with
thickness. At the lowest temperatures, overactivated behavior is observed in
the vicinity of the SIT and the increase in the activation energy can be
attributed to the superconducting gap. We derive a relation between the
increase in activation energy and the temperature below which overactivated
behavior is observed. This relation is verified by many different quasi-2D
systems.
