Airtightness of buildings is necessary to obtain healthy, sustainable and energy efficient buildings. Measuring the airtightness of a building has become more common lately, much due to the higher energy use in leaky buildings. The airtightness of a building can for example be measured in order to attain a certification, or on demand from a developer. In some studies, there have been large seasonal variations in airtightness. In most cases, the buildings are more leaky in wintertime, but there are also some investigations that show the opposite. In the current project, the aim is to investigate how, and if, the airtightness varies over the year. The airtightness is measured, using a blower door, approximately eight times a year, in three different buildings. The air leakages in the buildings are also detected and the air velocities at a number of leakages are measured. Two of the buildings are one-family, two story, wooden frame houses and one is a multi-family, concrete building (where one apartment is measured). In Swedish wooden houses, the air barrier is often a polyethylene foil, which is also the case in these two buildings. The measurements have been analyzed with respect to indoor/outdoor temperature and indoor/outdoor relative humidity. The trend in the measurements is that the airtightness is lower (more leaky envelope) when the indoor air is drier (low relative humidity). Consequently, the air leakage is largest during the winter measurements. The decrease in airtightness from summer to winter is in the order of 8-10%.