Why does condensation form on the outside of some windows and not others in the morning?
Thursday, June 21, 2018
The dew point is the temperature at which moisture in the air will condense on a surface. It fluctuates with the outdoor temperature and relative humidity, which have an inverse relationship to each other. Because cold air can hold less moisture than warm air, as the temperature falls the relative humidity rises and, when the temperature of a surface falls enough, it reaches the dew point—which is 100% relative humidity—and condensation forms.
The window glass of an air conditioned home will be cooler than the outdoor air on a warm morning and reaches the dew point temperature before other outdoor surfaces. Occasionally, we get asked the question “Why do I have condensation when my windows are insulated?” An insulated window still has some heat/cold transmission, and the exterior glass surface will still be slightly cooler because of the chilled indoor air.
Trying to determine why one window has condensate on it and a nearby one does not can get complicated due the variables at the different locations. Here’s few things that can affect the formation of condensation:
- The direction the window is facing.
- The level of shade from an overhang or tree.
- Minor leakage of the gas between the panes of an insulated window will deteriorate its performance and allow the outside surface to be slight cooler than an adjacent window with no leakage.
- Moisture is constantly rising out of the ground and, if a window is over damp soil, the higher humidity above the soil may cause condensation sooner than a window on a screen porch on the same wall.
- The indoor temperature of one room of the house may be slightly cooler than another room and decrease the temperature of the glass.
- Any combination of these variables.
If you suspect that the condensation is due to the loss of the inert gas between the panes of an insulated window, eventually the problem will show itself as a cloudiness on the glass. It forms on the surfaces of the panes of glass that face the inert gas space, so the haze cannot not be cleaned away.
See our blog post What causes sweating (condensation) on the inside of windows in the winter? for condensation on the other side of the glass.
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