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- Hans Eek - The Vision & the Visionary of the Passive House Concept
Causes of Condensation
Condensation happens, when the warm air inside our homes touches a cold surface, such as windows, tiles and even walls. The reason is that warm air holds more moisture (or vapour) than cold air. As soon as this warm air (which contains vapour) hits a cooler surface, condensation will form unless the warm air is removed with ventilation.
When condensation is trapped and cannot evaporate, the prefect conditions exist for mould to grow.
If we insulate and warm our homes thoroughly, you might think that condensation can be avoided, however, the moisture content of the air still needs to be controlled or released. To control the moisture content, we need to balance between heating with ventilation.
Installing trickle vents to windows and extractor fans to kitchens and bathroom are common methods, but also cause temperature changes in the home which can feel draughty.
Another method that stabilizes the temperature throughout the home is to install mechanical ventilation or air handling units. This will remove the damp air while also bringing in fresh air. The fresh air is heated and distributed throughout and maintains an even temperature.
Condensation on Glazing
There are various causes of condensation on glazing, depending on whether it occurs on the external face, internal face or between the panes of glass.
Condensation to Internal Face
This is link to external climate, internal temperature, humidity inside the home, ventilation flow rate and the surface temperature of the glass.
The best way to reduce surface condensation is to extract the air (which holds vapour) and take it out of the building. It is also advisable to heat and ventilate the premises adequately.
Condensation to External Face
This occurs when the surface temperature of the glass is lower than the air temperature outside and if the dew point (temperature at which vapour becomes liquid) of the outside air is higher than the glass temperature.
The surface temperature of the glass will depend on:
- The heat passing through the glass from inside to outside. This depends on the temperature variation within the glass itself and the U-value of the glass
- climate outside
- Heat loss
Various studies show that heat exchange by radiation is relatively limited when the sky is overcast. However, when the sky is clear at night, there are significant heat losses.
The effect of radiation from a glazed surface to the sky, can be compared with a car parked outside on a clear night – in the morning, some areas of glass will be wet or frosted over, even if it has not rained. If the car has been parked by a building, the windows closest to the building are never wet because the building reduces the heat exchange by radiation, between the car windows and the sky.
By choosing a window with lower U-values (better insulated) this reduces the transfer of heat to the external surface of the glass (heat loss from the building). The external glass is now colder, which actually increases the risk of condensation on the outside of the window. However, this means the condensation is kept outside, rather than inside the building. And as the sun rises or the wind picks-up, the condensation will evaporate.
*It is important that condensation to the external face is not considered to represent poor quality double-glazing, but rather proof of good thermal insulation.
Condensation between both panes of glass
This is an indication the cavity of the glazed unit of the window, is no longer sealed. The desiccant will rapidly become saturated and any damp air penetrating via the seal will reduce visibility when condensation forms.
The double-glazed unit must be replaced, as this cannot be reversed.
This occurs in;
- Periods of high humidity
- Areas such as bathrooms or kitchens
- Confined spaces (furniture or curtains are too close to windows)
- Or during exceptionally cold weather
This is normal, as long as the condensation is not permanent.
Timber is naturally a very good insulating material and minimises thermal bridging, when used for window frames. Engineered timber frames use the latest lamination and finger-jointed technology that ensures the frame is durable and stable, by eliminating natural defects like knots.
A window is made up of many components, but namely the frame and the glazed unit. If these are poorly insulated, heat from inside the home escapes and where the glass meets the frame, is called a thermal bridge. This is a path where heat loss occurs.
Warm edge spacers reduce thermal bridging and condensation that can occur inside, at the corners of a window.
The cavity is filled with gas, usually argon gas and should be approximately 16mm with a low-emissivity coating. This gas is sealed between the panes of glass and the low-e coating ensures the reduction of heat loss that occurs through the glass itself. It also maintains an even temperature across the entire surface of the glass and helps to create a thermal performance that is evenly distributed, throughout the whole glazed unit to reduce any condensation forming at the centre of the glass.
Another result of this is reduced sound transmission.
Triple glazed windows are now becoming the norm and can be similar in price to double-glazed windows.
*Windows that do not state the U-value (W/m²K) are unlikely to have been assessed for their thermal performance and are even less likely to have the benefits noted above.