Triple glazing: what you need to know about heat and moisture

Insulated glazing has great capabilities that improve the heat retention of your home. But how does it affect the surface temperatures and relative humidity levels that cause condensation? Let’s look at a few of the heat, moisture and condensation facts of triple glazing.

How does heat retention work in triple glazing?

An insulated, triple glazed window unit is made up of 3 layers of glass with 2 gas-filled spaces between the glass.

The outer panel is made of low-iron glass which is an extra clear glass that allows the maximum amount of short wave solar gain into the home. This not only maximizes sunlight, but also the amount of heat that can flood through the windows.

The central and inner panes both have a low-E or low-emissivity coating on the glass. This microscopic film reflects long-wave infrared energy, or heat. That means that the warmth that has built up inside the home, gets reflected back into the room instead of escaping to the outside through the glass.

How does condensation form?

The heat that comes from outside into the home through the glass as well as the heat that is being reflected back into the home, have an effect on the forming of condensation against the glass.
Condensation happens when a surface cools down to dew point and then comes into contact with humid air. This causes a thin film of water or droplets to form against windows.

DEW POINT: the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

Why does condensation form on the outside of my windows?

The dew point varies according to relative humidity and atmospheric pressure, so it greatly depends on the specific location of the window. Click here for a handy dew point calculator.

Dew point in the greater London area typically varies between -1 and 5 degrees from October to May. Considering that triple glazing greatly reduces heat loss through windows, it then follows that the outer glass panels will likely be at or below dew point during winter, causing condensation on the outside of the home.

What can be done about interior condensation?

Because the inner panels are preventing heat loss, triple glazing is not prone to interior condensation as long as the interior temperature is being managed optimally. Some studies show that surface temperatures on the interior panel remain at around 18 degrees with a room temperature of 21 degrees, which is very high compared with a standard single glazed window which has a surface temperature of 1 degree, thereby causing condensation to form on the inside during winter.

Water vapour in warm, heated air will condense against a window with a temperature below dew point. Unless the window surface is warm, condensation will take place.

When there is a problem with water forming on single- or double glazing, heating and ventilation are the only ways to prevent it. Even heating throughout the home will prevent the glass surface temperature from dropping below dew point, and sufficient ventilation will expel moisture-laden air to the outside.

To open a window to improve ventilation is not the most efficient way since this also allows heat to escape. Installing windows with trickle vents in the frames will quickly and effectively take care of the ventilation.

Clouding up

If you have a clouding up on the inside of your windows, it means that the seal of the insulated window unit was most likely damaged and the glazing has to be replaced.

Can I avoid condensation on the outside of my windows?

Unfortunately, there is not much that can be done to prevent condensation on the outside of you windows. The difference in surface temperature – which is an indication that your insulated glazing is working correctly – is largely unavoidable. But rest assured that this is perfectly normal and an indication that your windows are working correctly and not letting the warm interior air escape to the outside of your home.

Published on : 2nd September 2016

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