The out-of-plane losses experienced in conventional photonic crystals waveguides are an essential limitation for theirs future applications. This article presents a theoretical study of this losses for one-dimensional photonic crystals waveguides. Our numerical results are obtained using a new and powerful algorithm based on an extension of the rigorous coupled wave analysis. The origins of the losses are highlighted qualitatively in terms of effective medium theory and quantitatively in terms of mode mismatch. From these insights, a generic approach based on an adiabatic mode conversion is proposed and validated numerically for cavities with large quality factors and large peak transmissions.