A comprehensive investigation of InAs epitaxy on silicon using template-assisted selective epitaxy is presented. The variation in axial growth rate of InAs nanowires inside oxide nanotube templates is studied as function of nanotube diameter (20-140 nm), growth time (0-30 min), growth temperature (520-580 °C), V/III ratio (40-160), nanotube spacing (300-2000 nm), and substrate crystal orientation. It is found that the effective V/III ratio is reduced at least by a factor of two within the nanotube templates compared to the outside, detectable by changes in the growth facet morphology. The reduced V/III ratio originates from the different transport mechanisms for the As and In precursor species; As and In species are both transported by Knudsen diffusion in the vapor, but an additional contribution of In surface diffusion reduces the V/III ratio. The results reveal the interplay of growth parameters, crystal facets and template geometry and thus are generally applicable for nanoscale selective epitaxy.