Temperatures and relative humidities in ventilated and unventilated attics. Measurements and calculations in attics with loose fill insulation of mineral wool and cellulose fibre In well-insulated houses with a ventilated attic there is a risk for condensation and high RH during the winter. One way to lower the RH and thereby decrease the risk of damage by mildew and rot is to decrease the outdoor air ventilation of the attic. This is possible only in cases where the roof has no air leakage from the inside. This means that the ceiling must be airtight and that there always is an underpreassure compared to the attic. Measurements have been made in a test roof space employing six different forms of structures. Two structures, one with mineral wool insulation, the other with cellulose fibre insulation, are ventilated with fans to a degree of 2 ach, two structures are naturally ventilated and two are not ventilated at all. The study shows that low ventilation gives a drier climate in the attic and that lower ventilation rate gives smaller differences in RH during day and night. There is only a slight difference in performance of the attics with cellulose fiber insulations compared to those with mineral wool insulation. Four researchers have made calculations of temperature and RH outgoing from outdoor climate and data of materials and constructions of the six structures. In some cases the agreement with measured results was very good, in other cases not so good.