Over the last few months, I’ve been talking with a homeowner who’s been involved in a lawsuit with a window installer who had installed retrofit high-performance double glazing in his home. His complaint was that the windows condense on the outside.
First thing in the morning, when he wakes up, he looks out the window to take in his beautiful morning view. But with the high-performance glazing installed, this view is sometimes blocked by condensation (fog) on the outside of the windows.
The science behind external window condensation
This external condensation on his windows is dew—like dew on the grass. And it’s caused by the same thing as dew on the grass. It’s what we call night sky cooling. When you have a clear night sky, the grass can see outer space, and gets colder than the surrounding air. Water in the surrounding air then condenses on the grass as dew.
The same thing is happening to the homeowner’s windows. The outside pane of glass is getting down to below the dew point of the surrounding air. So, he has dew fogging his windows.
The primary factor is a clear night sky. Nights with an overcast sky will very almost never have condensation on the outside of the windows in the mornings as they see the warmer clouds rather than the cold of outer space.
The next most important factor is high relative humidity. With a high relative humidity the glass only has to cool slightly to cause condensation on the surface. Areas near the sea and other bodies of water can have local pockets of very high relative humidity.
Another aggravating factor is the low wind speeds on calm nights. With the air transferring less heat to the glass, the surface of the glass can get colder due to the radiation cooling to the night sky.
Although the focus of this discussion is windows, roofing iron and other materials can also cool to the night sky and experience condensation. The photo below shows some roofing iron literally dripping and soaking the surrounding paving with water condensed from the air overnight – to be clear this is not from rain.
External Condensation and Performance of Different Windows
Different windows are affected differently by night sky cooling. Windows that have no overhangs such as that of buildings on a cliff top, which can see a large amount of sky, have a high view factor (i.e. they see more sky) and are more susceptible to greater amount of external condensation.
But the main determiner for window performance in terms of external condensation is the temperature of the glass itself. If you have a house that’s not heated very much or a portion of the house that’s not heated, then you are much more likely to get external condensation than if the house is well heated. As you heat the house and the heat transfers through the glass panes and heats the exterior glass surface up, condensation is reduced or eliminated.
As you install higher and higher performance windows, you increase the incidence of external condensation as you lower the external surface temperature. Although external condensation is more common on high performance windows, I’ve seen it on single glazing as well.
How Do We Deal with External Condensation on Windows? 
These are several ways you can reduce the likelihood of external condensation:
- Put the low-e coating on the inside of the exterior pane of glass rather than the inside of the interior pane of glass
If you are going to put a high-performance window in, you can check where the low-e coating goes. You want the low-e coating on the inside of the exterior pane of glass. This way, the exterior pane will be slightly warmer than if the low-e coating is on inner pane of glass.
- Add overhangs or exterior blinds
Another thing you can do is to reduce the night sky view. If you have a window with an overhang, typically the part that’s shaded from the night sky view will not have external condensation on it. Only the lower part of the window that can see the night sky will have external condensation. Exterior blinds that you close at night will keep the glass from getting cold and thus reduce/eliminate exterior condensation.
- Switch to lower performance window
If a view is absolutely critical, you could opt for a lower performance window that intentionally keeps the outer pane glass warmer. It depends on what is more important to you, the view or the thermal comfort and possible internal condensation with resultant mould inside your home.
- Heat the glass – not likely but can be done
If you had to guarantee no external condensation, you could heat the exterior pane of glass. Crazy idea in terms of energy usage but that is how those fancy no-fog bathroom mirrors work.
- Low-e coating on the outside surface of the external glass surface
In the 1970s it was common for office buildings to have a pyrolytic low-e coating on the exterior of single pane glass windows. This reduces night sky cooling and thus reduces exterior condensation, and is also very pretty with an iridescent look to the glass. Someday it may be possible to buy glass that has a pyrolytic coating on the outside surface of the exterior pane of glass and a high performance low-e coating on the inside of the exterior pane of glass. This would reduce exterior condensation and provide high performance.
- Set client’s expectations
The most important point is to understand that exterior condensation on surfaces is just a part of physics. It’s part of the environment. If you don’t want to have heat loss out of your building through your windows, and especially if you’ve chosen to have high performance windows installed, some exterior condensation is to be expected. It’s just a function of the surface temperature of the glass and the environment it’s in.
When putting high performance windows in a building, we need to manage our clients expectations. The building will be much more comfortable, easy to heat and healthier but they need to know that there is the possibility that some of the windows in the building are going to have exterior condensation under certain conditions.
With all of the high performance building owners I’ve met, they don’t really give external condensation on windows much thought. To them, the windows clears up pretty early in the morning. It’s not an issue.
But for others who treasure their morning view, they may be unhappy with the trade-off. So it’s important that clients know what they are getting and that we give them appropriate advice.