Water droplets on the outside of a pane can impair visibility, even if a glass surface has previously been cleaned. For property managers, the question naturally arises as to whether cleaning agents have been left behind. Another hasty explanation is that external condensation is proof of particularly good thermal insulation. Both judgments go further than the visible observation. Jonas Gebäudereinigung explains the physical principles based on public sources; the article describes no specific customer case.
The glass surface matters, not just the air temperature
The British weather service, the Met Office, defines dew as water droplets that form through condensation. The decisive factor here is the temperature of the surface in relation to the dew point of the immediately adjacent air. If the surface reaches this range, water vapour from the air can turn into liquid water there. The moisture does not, therefore, have to have been applied to the pane beforehand as water or a cleaning product.
Air humidity and air pressure influence the dew point. Therefore, there is no fixed temperature value here that applies to every pane and every weather condition. Nor does the temperature of the outside air alone answer the question: the actual glass surface can have a different temperature. This article does not calculate a dew point or a measured value for any specific building.
Two different thermal conditions
Pilkington describes a nightly process in technical bulletin ATS-161 of 14 January 2013: glass can cool down due to thermal radiation towards a clear sky. Heat from the ambient air counteracts this. In calm air, the surface can nevertheless fall below its dew point. If little heat from the warm building interior reaches the outside, the outer pane is heated less.
The same bulletin mentions another case: with very hot, humid outside air, poorly insulating single glazing on air-conditioned office lobbies can be cooled so much from the inside that condensate forms on the outside. External condensation alone therefore provides no clear proof of good insulation performance. The cases mentioned are manufacturer explanations, not observations at Jonas properties. They also do not constitute a suggestion to adjust an air conditioning system or replace glass.
Why adjacent panes can look different
On its current explanatory page on external condensate, Pilkington points out that orientation and objects in front of a window can influence the surface temperature. For this reason, not all panes of the same window have to be affected at the same time. Subsequent warming can cause the moisture to evaporate again. Different condensation on adjacent glass surfaces is therefore, in itself, not proof of inconsistently careful cleaning. However, no definitive explanation for every actually visible spot follows from this.
What this means for assessment after cleaning
A correct description first distinguishes between visible moisture and its suspected cause. Newly formed water droplets alone do not prove the presence of cleaning agent residues. Conversely, possible external condensation does not rule out existing residues or damage. No concrete cleaning result can be exonerated or faulted based on the general mechanism.
If moisture disappears and later reappears, the expectation must also be clarified: dirt removal and permanently condensate-free vision are different results. Removing existing moisture does not prove that the conditions for its re-emergence have been altered. On the existing Jonas page for glass cleaning, the agreed glass surfaces and supplementary components are described separately. This scope of service does not replace a building physics diagnosis.
Jonas Gebäudereinigung is active in the commercial building cleaning sector in Oldenburg. This specialist article was created by an AI agent and checked against the stated sources. No human expert verification is claimed.
Sources
Met Office, Dew and dew point: https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/temperature/dew
Pilkington, ATS-161, as of 14.01.2013: https://www.pilkington.com/-/media/pilkington/site-content/usa/window-manufacturers/technical-bulletins/ats161firstsurfacecondensation20130114.pdf
Pilkington, External condensate: https://www.pilkington.com/en/gbl/architectural-and-technical-glass/product-categories/enhanced-visibility/pilkington-anti-condensation-glass/external-condensation
Jonas, existing glass cleaning: https://xn--jonas-gebudereinigung-e2b.de/glasreinigung/
Contact
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Alexanderstraße 101 B, 26121 Oldenburg, Germany
Phone: +49 441 24923021
Email: info@jonas-gebäudereinigung.de
Website: https://xn--jonas-gebudereinigung-e2b.de/