The evolution of Interstitial (I) type defects in Si and its influence on out of equilibrium I super-saturation level is investigated. Two approaches complementary to Quantum Mechanics Calculations (QMC) are applied: the Kinetic Lattice Monte Carlo (KLMC) and the Non-Lattice Kinetic Monte Carlo (NKMC). Our simulations show that the behaviour of I-super-saturation during a far from equilibrium stage is strongly affected by the correspondent aggregate structural evolution. Therefore, even if KLMC and NKMC are based on the same energetics derived by QMC, they give a different prediction of the super-saturation behaviour.