Preventive conservation is described by Thomson (1986) as both the control of the environment to minimize degradation by ageing, the destruction of art exhibits and material, and the treatment of exhibits and materials so that they are stabilised and degradation slowed. This paper concerns the control of the environment, which in conservation means mainly the control of humidity. The aim must be to keep relative humidity (Rh) within the acceptable limit. What comprises a suitable limit is still the subject of debate, as the literature quotes quite a wide range of figures from around 40-70 percent Rh, depending on the geographic location, the building, and the objects being preserved. In the projects described here, an upper limit of 65 percent Rh has been used. The large technical survey on museum storage rooms in Sweden (Holmberg and Johansson 1996) showed that unheated but dehumidified buildings could be an acceptable alternative to traditional buildings with mechanical and electrical systems for indoor climate control. This paper describes measurements of air infiltration, relative humidity and temperature at three unheated historic palaces in Sweden: Skokloster, Läckö and Salsta. The descriptions are aimed at conservators, curators and other museum staff, rather than engineers, although some technical explanations have been included. As a result of the measurements, three solutions for improvements were tried: at Skokloster, almost nothing was done; at Läckö, the building envelope was improved by installing inner windows, and dehumidification was installed; and at Salsta, the building envelope was improved by a "Tempereirung" system, in which copper pipes carrying warm water were installed within the walls.