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Condensation on the outdoor surface of window glazing

Calculation methods, key parameters and prevention with low-emissivity coatings
: Gläser, H.J.; Ulrich, S.


Thin solid films 532 (2013), S.127-131
ISSN: 0040-6090
Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit BMU
Fraunhofer IST ()
coating; outdoor glazing surfaces; low-emissivity; outdoor condensation

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/(m(2)K) 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 epsilon(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 epsilon(o) approximate to 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/(m(2)K) and on the outside of skylight glazing with U-g >= 0.70 W/(m(2)K). This is sufficient for all window glazing marketed nowadays. Weather-resistant, cost-effective coatings on glass with epsilon(o) <0.2, which are required for energy-neutral glazing with U-g values <0.5 W/(m(2)K), are the subject of ongoing development.