The thermal conductivity of single crystal silicon can be reduced by the introduction of boundaries at the nanoscale. We present the measured thermal conductivity of single crystal silicon nanobeams patterned with a single row of holes at room temperature: the hole diameter and the spacing vary from 100nm to 250nm and from 200 nm to 800nm, respectively. A steady-state four-probe joule heating measurement technique is used to extract the thermal conductivity of the porous silicon nanobeams across a range of pore geometries. The reduction in thermal conductivity owing to the hole boundaries is up to a factor of two.