Air pressure differences are a key factor in the behavior of building ventilation and air leakages through the building envelope. Field measurements of the air pressure differences over the building envelope were conducted in 24 Finnish municipal service buildings. The measured buildings were mainly schools and daycare centers, of which half were new buildings and half recently retrofitted. All buildings had mechanical ventilation. The measurements were conducted during 2016–2018. The total number of measurement points was 100, and the duration of individual time series varied. According to the results, the mean air pressure difference was within the range of national recommendations (small underpressure indoors) in 81–89% of measurement points, but some cases experienced either strong underpressure or overpressure conditions. In some cases, the air pressure difference showed a clear stepwise constant behavior, while other cases showed larger temporal variation. The conditions varied between different operating situations and the time of year. The study also supports the current recommendation that air pressure difference measurements should be done as continuous measurements of at least one week duration.