Condensation on the outdoor surface of window glazing — Calculation methods, key parameters and prevention with low-emissivity coatings

The market share of thermally insulating, triple glazing installed as window glazing increased to nearly 50% in Germany in 2011 and is continuing to rise in 2012. Dew and frost on the outdoor surface of insulating glazing units with thermal transmittance values U_g below 1.0 W/(m²K) present a severe problem, because unimpeded visual contact, one of the essential features of glazing, is disturbed. The report describes calculation methods to determine the occurrence and prevention of outdoor condensation and the key parameters influencing it. From this parameter study, it follows that condensation on the outdoor surface of window glazing in the form of frost and dew is prevented for all the glazing by low-emissivity (low-e) coatings with a certain thermal emissivity value ε_o between 0 and 1 on the outdoor surface if the outdoor air temperature t_o is lower than the room temperature t_i, normally given for buildings in the Central European climate when outdoor condensation occurs. However, today weather resistant low-e coatings with sufficient transmittance applied on outdoor surfaces of window glazing can only be manufactured on the basis of indium tin oxide with ε_o ≈ 0.2. The study shows that with such low-e coatings frost can be prevented as well as the frequency of dew occurrence can be reduced on all vertically installed glazing with thermal transmittance values U_g ≥ 0.47 W/(m²K) and on the outside of skylight glazing with U_g ≥ 0.70 W/(m²K). This is sufficient for all window glazing marketed nowadays. Weather-resistant, cost-effective coatings on glass with ε_o < 0.2, which are required for energy-neutral glazing with U_g values < 0.5 W/(m²K), are the subject of ongoing development. - Highlights: ► Condensation on the outside of highly insulating window glazing ► Calculation methods for the occurrence of this condensation ► Prevention with low-e coating on the outdoor of window glazing ► Limits for these coatings ► State of the art of the development of such coatings