To reduce energy consumption and carbon dioxide emissions in existing houses in heating-dominated climates, there is a drive to reduce ventilation heat loss by tightening the building envelope. Energy efficient domestic retrofits, which neglect ventilation requirements or assume without inquiry that adequate ventilation rates have been met, have the potential to impact negatively on the health and well-being of occupants by creating unhealthy indoor environments. The natural ventilation rates of UK dwellings created by building envelope air leakage is commonly estimated by applying a rule-of-thumb to the air flow required to create a pressure differential across a building envelope of 50 Pa using a fan pressurisation technique. To analyse the appropriateness of the rule, a tracer gas concentration decay technique was used on four Victorian solid wall houses to ascertain their natural ventilation rates created by building envelope air leakage. Results indicate that applying the rule overestimated the natural ventilation rates of the four houses tested. These findings have the potential to impact on retrofit ventilation strategies for existing houses.