The airtightness of the building envelope is a very important factor in determining the energy use of a building, and thus its environmental impact and running costs. Good airtightness is also important for the quality of a building's internal environment and for its moisture resistance. Unfortunately, it is all too seldom that the airtightness of building envelopes is followed up and verified. In Sweden, such measurements are mainly made when the building envelope in its entirety has been completed, but it is technically difficult to correct any defects at this stage. Corrective actions may require the removal of materials that have been erected which, at a late and sensitive stage of the construction process, is expensive in terms of materials wastage, labour and completion delays. The parties involved (developers, consultants and contractors) need other methods for describing and evaluating airtightness: methods that can make it easier to monitor airtightness of more building projects, and which can be carried out at an early stage of the construction process. Finding air leaks at an early stage means that they can be dealt with in time, at a reasonable cost, and that the building will be sufficiently airtight when it is completed.