Humidity inside the collectors is one factor that can be minimised to keep the most favourable microclimatic condition for the internal materials of the collector. This microclimate inside the collector is an important factor in determining the service lifetime of an absorber coating. During the design of the collector, the location and size of ventilation holes, properties of the insulation materials and dimension of the solar collector box are parameters that have to be taken into account for the optimisation in order to achieve the most favourable microclimate to prevent corrosion. Simulation of the microclimate in solar thermal collectors can be a valuable tool for optimisation of the collector with respect to ventilation. A computer model has been established for fulfilling this. By using this tool the producers can be advised whether their solar collectors ought to be additionally tightened, or whether more ventilation openings should be made and what influence the insulation material has. Guidelines for collector designers are proposed. The design guidelines provide some suggestions to be considered during the design of solar collectors. The work was carried out within the framework of the working group Materials in Solar Thermal Collectors of the International Energy Agency-Solar Heating and Cooling Programme. (C) 2003 Elsevier Ltd. All rights reserved.