The structural stability and properties of single silicon interstitials in their neutral state are investigated via ab initio methods in 3C- and 4H-SiC. By carefully checking the convergence with Brillouin Zone (BZ) sampling and supercell size we show that the split interstitial along <110> direction and tetrahedrally coordinated structure have similar formation energies in the cubic polytype. We discuss possible artifacts coming from the well known Density Functional Theory (DFT) underestimation of the band gap, which is particularly relevant for 3C-SiC. For 4H-SiC, the most energetically favorable silicon interstitial is found to be the split interstitial configuration ISisp<110> but situated in the hexagonal layer. The defect formation energies in 4H-SiC are in general larger than those in 3C-SiC, implying that the insertion of silicon interstitial introduces a large lattice distortion to the local coordination environments and affect even the second- or third-nearest neighbors. We also present an extensive comparison between well converged plane waves calculations and SIESTA [1,2] calculations based on localised orbitals basis sets.