Hydrophobization reduces the water absorption by facade materials and is often suggested as an optional measure to reduce the risk of moisture problems in internally insulated facades. Studies regarding hydrophobization have mainly focused on moisture performance and whether the treatment is safe to apply to historic building facades. Little focus has been on how the treatment affects heat loss due to the walls becoming drier. This study investigated the effect of hydrophobization on heat loss for solid masonry walls. The study comprises: field measurements from a large-scale experimental set-up containing solid masonry test walls with and without internal insulation and hydrophobization, and a simulation study. Both studies showed a generally positive effect of the hydrophobic treatment on heat loss, although simulations show a low dependency on the type of insulation system compared with field measurements.