Condensation on windows is a fairly common phenomenon. From time to time you might notice your windows get foggy, or you may see it as droplets of water forming on the surface. This can happen on the exterior or interior of your windows. Why does this happen? What can you do about it? Will triple pane windows help?

Let us first understand why condensation happens. It must be understood that water vapour is a natural part of the atmosphere. Humidity is actually a measure of how much water vapour is contained in the air. At any given temperature, air can contain a certain maximum amount of water vapor. In other words, for a specific temperature, there is an upper limit to the possible humidity. That is when saturation is reached. If more water vapor tries to enter the system, it will condense to its liquid form. Air at higher temperatures allow for higher humidity levels. This is because air expands as temperature rises, which means there is more space to hold water molecules. Conversely, lower temperatures mean less humidity potential.

Based on this understanding, one would expect that as the temperature of the air goes down (ie. it becomes cooler), a point will be reached where there will be too much water molecules to hold in the air. This is known as the dew point. In other words, lowering the temperature will force saturation. It is at this point that condensation will begin. Essentially the water molecules become too compact to remain in the gaseous state, and must coalesce into its liquid state.

Condensation takes place on a surface that is cooler than the air which comes in its contact. This is expected since the cooler surface causes the interfacing air to become cooler, and if the dew point is reached, then water droplets will begin to form on the surface.

Condensation on the exterior of windows can therefore be anticipated when a cool night follows a hot day. During the day, the water vapour will build up in the air, and as the temperature falls in the cool of the night, the air will contract, causing the dew point to be reached, which results in condensation. Since windows tend to be cooler than the air, they provide a point for condensation. For energy efficient ones it becomes even more likely that condensation will take place on the exterior since there is less conduction of heat from inside to outside making the exterior temperature lower than that of windows that are not particularly energy efficient. So condensation on the exterior is actually a good indicator that your windows are performing as they should be.

Sometimes condensation will form on the interior of your windows. This will depend on the humidity inside, how energy efficient your windows are, and also the extent to which you have internal air circulation. Interior window condensation can be a nuisance especially when it gets to the point where the water begins to run down the wall and gather on the floor or elsewhere. This can erode the paint on the walls, or if you have wood in the path of the water, it might eventually do damage. It is best that you take measures to prevent interior window condensation.

The reason that the energy efficiency of your windows will impact whether condensation is formed on the interior, is that the less energy efficient windows tend to be significantly cooler than the air inside. This is so since the heat from inside is lost to the outside through the windows due to poor insulation, which make the temperature of the interior surface of the windows closer to that of the exterior. The more insulation a window provides, is the closer its interior surface temperature will be to the room temperature, making the conditions for condensation on the window surface less favourable.

Triple pane windows are noted for reducing condensation on the inside. Even double panes suffer from condensation issues. But triple panes provide the level of energy efficiency to ward off condensation. Bear in mind that not all triple panes are equal. Some might be no better than dual panes when it comes to the r-value. (see also energy efficiency of triple pane windows r value. But generally speaking, this means that one way to address any condensation concerns you may have, would be to install triple glazed windows. If that is not a viable option for you, you can try taking measures to reduce the internal humidity, or to improve the air circulation inside.