We installed 5 new windows (3 panes) in our house. 2 of them regularly get exterior (outside) condensation, the others are fine (or get considerably less). This happens when outside humidity is high (regardless of time of day, sometimes in the morning, sometimes in the evening).

Most sources I found googling mention that this is a "good thing" as it's a sign of good insulation. I suspect that this might not be fully accurate.

My hypothesis is that warm humid air from inside leaks outside from the edge of the window (due to poor installation), which causes the condensation. Currently the two windows are wet, inside air is around 20°C with 65% RH, while outside is 8°C and 90% RH.

Does my hypothesis make sense? or could there be an alternative explanation for this?

  • Why would you think that the less humid air escaping would cause the condensation on the OUTSIDE of the window?
    – Solar Mike
    Commented Oct 11, 2022 at 18:41
  • It's a new triple-pane window, so it's well insulated, so it's not as warm outside as a poorly insulated window. When below dew point, condensation will form. So long as condensation is on the outside surface, all is well. When it's between the panes you have a problem.
    – Ecnerwal
    Commented Oct 11, 2022 at 20:45
  • @SolarMike 20°C with 65% RH air contains more water than 8°C and 90% RH air. The dew point of the warm air is around 13°C.
    – eimrek
    Commented Oct 12, 2022 at 10:05
  • @Ecnerwal why would this happen only for 2 out of 5 windows?
    – eimrek
    Commented Oct 12, 2022 at 10:10
  • Different exposures to sunlight or clear skies.
    – Ecnerwal
    Commented Oct 12, 2022 at 12:37

1 Answer 1


I would suggest that the window is presenting a cold surface ie below the dew point to the outside due to the internal room temperature.

That causes the humidity of the outside air to condense on the glass surface.

  • Why would the outside of the window be colder than the outside temperature "due to the internal room temperature"? Also, why would this only happen in 2 of the 5 windows?
    – eimrek
    Commented Oct 12, 2022 at 10:03

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