Why Do My Windows Condensate?

External Condensation {On The Outside Pane}

From time to time, we receive enquiries about the appearance of external condensation on glass.  This phenomenon is a natural occurrence and not a fault in the glass or window.

Moisture condenses out of the air onto a cold surface that is said to be below the dew point.  The dew point varies with the air temperature and the amount of moisture it contains.  Particularly in spring and autumn, the glass temperature can fall to a low level during the night and the dew point can be comparatively high in these seasons.  The glass temperature can be below the dew point under these conditions and moisture can condense onto the surface.

In order to save energy, maintain a comfortable internal environment and satisfy building regulations requirements, the windows we fit in our homes are much more thermally efficient than in the past.  With single glazing and older style double glazing, a large proportion of heat was lost to the outside through the glass.  With modern low emissivity glass, more of the heat is kept inside and the outer pane is not heated as much.

The more thermally insulating the glass is, the lower the outer pane temperature is likely to be and the greater the risk of condensation on the external surface.  In northern European countries, where levels of thermal insulation are higher than in the UK, the phenomenon is understood and accepted.

There is not much that can be done to avoid the risk of condensation to the outside.  In many cases the condensation does not last long.  A little heat from the sun warms the outer glass enough to evaporate the moisture and a gentle breeze or wind will help.

Surface condensation on the outside of glazing is a phenomenon that is occasionally seen at night and in the early hours of the morning on well-insulated glass in clear (cloudless) weather and when there is no wind. This is mainly caused by heat losses towards the clear sky.

Inside Condensation {On The Inside Window Pane}

What is this water on the inside of my windows?

Water or frost on windows is condensation. Condensation is formed when warm moist air comes in contact with cooler dry air. An example of this is when a bathroom mirror “steams up” after a hot shower. Just like that mirror, the inside or outside of your window can sweat or fog because of temperature differentials.

Are My Windows To Blame?

Faulty windows do not cause condensation. Glass is usually the first place you notice condensation because glass surfaces have the lowest temperature of any surface in a house.

It is important that this phenomenon is not considered to represent poor quality double glazed units, but rather proof of good thermal insulation.

Then What’s the Cause?

The moisture in the air causes condensation. The reason you may observe more condensation in your home is because of modern energy-efficient home building techniques and products. The insulation and construction materials used today are designed to keep cold air outside. This is especially true of new windows. While energy-efficient designs and weather-stripping keep cold air outside, they also keep warm moist air inside. Older window designs were less efficient and consequently allowed moisture to escape.

If you didn’t have as much condensation before replacing your old windows, it’s probably because they were draughty. Good windows and insulation all create barriers to the air exchange of a home. When combined with the additional water vapor (moisture) from showers, cooking, or from clothes dryers not vented to the outside, the result is excess moisture and a high relative indoor humidity level.

How Can Condensation Be Reduced?

The key lies in controlling the humidity inside your home. Install a hygrometer to monitor this. First, let’s understand where the moisture comes from. During the hot humid summer, your house absorbs moisture. The same principle applies to a newly constructed or remodelled home, due to the abundance of moisture from the building materials used in construction.

During the beginning of the winter when you start to heat your home, condensation occurs. After a few weeks, your home will begin to dry out and you’ll see less condensation. Opening a window briefly is a quick temporary solution. The drier cold air will enter the room while the moist air is allowed to escape.

Other solutions that may reduce condensation include:

  • Cracking open a window or door daily to air out your house.
  • Opening a window or running exhaust fans longer in the kitchen, bathroom and laundry room.
  • Opening drapes and blinds, allowing air to circulate against windows.
  • Turning off any humidifying devices in your home.
  • Installing and using a dehumidifier.