Flat collectors are usually not air or water tight. Therefore, they can exchange air and moisture with the environment. Moisture can accumulate in the collector, especially when the thermal insulation material acts as a storage for moisture (for example mineral wool). The moisture can increase the corrosivity of the micro-climate in the collector. An optimised ventilation rate, which is a measure of the air exchange between the collector and the environment, with respect to the moisture absorption properties of the thermal insulation materials helps to keep the collector dry. Optimum might be a relatively high ventilation if the collector case is not designed to be completely tight. (c) 2007 Elsevier B.V. All rights reserved.