Air tightness is essential to building energy performance, which has been acknowledged for a long time. It plays a significant role in improving building energy efficiency by minimising the heating/cooling loss incurred during unwanted air movement through the building envelope, consequently reducing the building’s energy demand and cutting down carbon emission in the building sector. A novel nozzle pulse pressurisation technique for determining the adventitious leakage of buildings at low pressure around 4 Pa, which is regarded as a more accurate indicator than conventional steady state measurement at 50 Pa, is investigated theoretically, numerically and experimentally. The investigation is based on the ‘quasi-steady pulse’ concept which produces a pressure pulse inside the building by introducing a certain amount of air in a very short time using an air compressor, solenoid valve, nozzle and control unit. The mass flow rate from the nozzle is obtained by measuring the transient pressure in the air receiver of the compressor during a test run. Simultaneously, the pressure difference across the building envelope is measured by differential pressure transducers. The quadratic equation, which can more closely represent the flow characteristics of adventitious openings, is used to determine the characteristic of building air leakage. Due to short time operation, the technique minimizes the effects of wind and buoyancy force and has proven to be highly repeatable. The pulse pressurisation using nozzle technique is compared with that using the piston technique. The comparison indicates that the present technique is reliable for determining building leakage at low pressure. It also gives great convenience in practical applications due to being more compact and portable. Moreover, it needs only a few seconds for a test run, barely needs to penetrate the building envelope and therefore can establish the leakage of a building very quickly and efficiently.