In accelerated testing, material specimens are exposed to completely artificial environments, where exposure to individual degradation agents is enhanced. In contrast, field-testing exposes specimens to multiple degradation agents. When performing a semi-natural test, the influence of a selected degradation agent may be increased while the specimen is still situated in its natural environment. Moisture is such an agent. In an experimental set-up for evaluating long-term optical performance of an building integrated Unglazed Solar Collector, a sub-task was to increase Time of Wetness (TOW) for several collector specimens for two reasons: to “simulate” an active cooling of the component as is the case for solar collectors; and to assess the role of TOW on optical degradation of the solar collector. Specimen cooling and increase in TOW was achieved by utilisation of a Direct-Air Peltier Element. TOW was estimated through measurements with WETCORR sensors (monitoring surface temperatures and moisture) and climate parameters (temperature and relative humidity) at site. At this particular test site in Gävle, Sweden, TOW was more than doubled in comparison to non-cooled surfaces. The Peltier Element proved to be inexpensive and flexible for this purpose.