Condensation on the outer surface of window glazings - Causes, effect on heat loss and method for prevention

Abstract

Fogging on the outside surfaces of window glazings increases if the thermal insulation of the glazing is improved, i.e., the k value is decreased as realized in the heat-insulating glasses currently on the market. The main reason for this is the radiation exchange with the open sky. The most endangered Windows for fogging are the sky lights and glazings in the skin of buildings, e. g. structural glazing facades, because they are exposed to the largest portion of the open sky. The conditions of fogging are investigated and discussed, as well as its influence on the heat loss of glazings is demonstrated. By radiation exchange with the open sky, the heat loss of window glazings can be increased by more than a factor 2 in comparison to the standardized k value. However, fogging reduces heat losses in dependence on the outside air humidity. Fogging and heat loss can be considerably reduced by a low-emissive coating on the outer surface of glazings. The demands on such a coating are discussed. Also a method of double coating consisting of a low-emissive subcoating and a hydrophobic and anti-sticking top-coating is presented, which has a self-cleaning effect. Lowemissive and, at the same time, self-cleaning coatings, deposited on the outer surface of glazings are a new function of flat glass and may be a challenge to the glass industry in the next years.