Why thermally insulated glass units (IGU) condense on outside surface of the window?


In cold winter mornings, when the sky is clear and there is no wind, the outside surface of the window can start to condense. Occurrence of condensation is normal, it depends on outside climate conditions and is directly correlated to good thermal insulation and the quality of insulated glass unit. It occurs on triple as well as on double insulated glass units, but most people are familiar with condensation on car glass in the morning. Condensation occurs when the outside glass surface temperature drops below the outside temperature of air and the ‘’dew point temperature‘’ (temperature at which air humidity converges from gas to liquid state) of outside air is higher than outside surface temperature of glass. In other words, the air that has high humidity hits against the cold surface of the outside glass. A good example of its occurrence is a cold, windless and clear morning after a clear night. Outside glass is cooled down, the temperature and air humidity of outside air begin to rise. In such case, the outside glass can’t heat up as fast as the outside air, because of the good thermal insulation of glazing unit (low Ug value) so the heat transfer from the warm room inside is to slow to heat up the outside surface glass. Outside glass stays cold, and at these conditions the condensation on glass occurs. It is present on most of the outside glass area except on the edges of glass, where the glass gets normally warmer due to lower thermal insulation on edges due to the effect of glazing bars or interpane spacers. At this part of the window the heat transfer from the warm side to the cold side is faster and because of that the condensation effect is not present. The condensation effect on the outer surface of glass therefore depends on the physical properties of glass and climate conditions outside. Condensation can not be fully avoided, as the outside glass surface temperature is always exposed to the ever-changing climate conditions.

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