This document describes investigations of ventilation and condensation of solar collectors in anticipation of being able to improve the long-term durability of solar collectors. The intent of the present is improvement of the existing computer model is a part of the work within the working group Materials in Solar Thermal Collectors of the IEA Solar Heating. It is important to avoid condensation in solar collectors, most of all because wetness of the absorber can damage the selective surface and cause corrosion on the absorber plate. During night time the cover of collectors will cool ambient temperature due to thermal radiation to the cold sky. In climates where the air during night time becomes saturated with humidity (the relative humidity is 100%), condensation will form on the outside and inside of the collector glazing. If too much condensation takes place on the inside of the glazing, it will start to fall off on to the absorber surface.