We design and fabricate a high silica nanobeam cavity that supports both transverse electric (TE) and transverse magnetic modes in the 1.55  m wavelength range. The values obtained for both modes exceed and are the highest reported values for photonic crystal (PhC) nanocavities made of silica. We also investigate the optimum conditions for coupling with the cavity in a side-coupled configuration. We achieve a coupling efficiency of 87% with the TE mode while maintaining a loaded of more than . We also found that the presence of a coupled waveguide reduces the intrinsic of the cavity, depending on the gap distance. This provides useful quantitative information for establishing an efficient scheme for coupling with low-index PhC nanocavities.