Airtightness of timber frame buildings not having a plastic film vapour barrier Good airtightness of a building can be achieved by the incorporation of an inner sealing layer for the exterior walls and roofs in the form of a plastic film, which also serves as a vapour barrier. However, if it is not wished to use plastic film as an inner sealing layer, then airtightness must be effected through the use of other materials or in some other way. This project has been concerned with investigation of a number of alternatives. It has been found that the arrangement most commonly used in Sweden today comprises polymer-based fibre sheets (which permit diffusion) and gypsum board, as alternatives to plastic film on the inside of the structure. The work of the project has shown that, provided that the materials are airtight in their own right, it is possible to achieve as good airtightness with alternative materials as can be achieved with plastic film. This has been demonstrated in the laboratory and in three of five houses in the field investigation. However, the airtightness performance of the finished building can be very poor unless care is taken both in the design and in construction, and this was also revealed in the field work.