Computer simulation of building airtightness measurements shows the effect of changes inpressure distribution across the building envelope due to wind force and temperature differenceon measurement accuracy. The wide range of leakage distributions, wind directions and velocitiesconsidered give information on the boundaries of these uncertainties. For wind velocities on siteof v(site) ~= 3 m/s, the additional uncertainty in the flow rate at 50 Pa (Q50) found is comparable tothe uncertainty due to standard pressure gauges or operator (about 3%). The additionaluncertainty for on site wind velocities of no more than V(site) ~= 4,7 m/s is in the range of 7%. Thisis comparable to overall uncertainty in calm conditions. Unfavorable building location, leakagedistribution and unlucky choice of external pressure taps can lead to significantly largeruncertainties in the measured flow rates of 10% for on site wind velocities of 3 m/s up to 40% foron site wind velocities of approx. 4.7 m/s, though.