How can you prevent Condensation in your home this winter.
Condensation causes problems for many people in NZ homes whether this is frustration at dealing with water over winter, damage to furnishings, mould and mildew, or actual health impacts. Accordingly, this is an issue which many people want to understand and manage to enhance their comfort and enjoyment at home.
What is condensation?
Condensation is a natural phenomenon whereby liquid water is shed from air when it is cooled. We see this outdoors as dew, and indoors as condensation forming on cold surfaces.
What causes condensation to happen?
There are 3 main factors which influence condensation …
1. The temperature of the air
Warmer air can carry more water vapour.
If air is laden with the maximum water vapour possible at its current temperature, then it is said to be ‘saturated’.
2. The amount of moisture in the air
The level of moisture present is identified as relative humidity.
Relative humidity expresses the existing moisture level in the air as a percentage of the moisture level at saturation. So, the relative humidity at saturation is 100%.
3. The temperature that the air is cooled to
As air cools, its saturation capacity reduces so the relative humidity increases.
If air is cooled sufficiently to reach 100% relative humidity, any further cooling will cause water to condense. This is the ‘dew point’.
The following table helps to show the relationship between these factors. For each combination of air temperature and humidity, the dew point is shown. If surfaces in the room are at the dew point temperature or less, then water will condense.
Dew Point Relationship to Air Temperature and Humidity
The blue band identifies a ‘comfortable’ temperature range
The light blue band identifies a humidity range generally considered suitable for comfort and health.
The orange area outlined shows expected dew points for indoor conditions involving desirable temperature and humidity levels.
The cells with red borders illustrate increasing relative humidity as temperatures fall. In the examples shown, relative humidity increases by around 10% for each 2°C reduction in air temperature. These examples all contain the same amount of water vapour (which will result in saturation if the air temperature drops to 10°C).
The table indicates that, even with desirable air temperature and humidity (the orange zone), water will condense on any surfaces in the room that are colder than 8 to 12°C.
Factors affecting condensation
A number of factors will influence how much condensation occurs:
Higher humidity will cause more water to condense
Colder surfaces will cause more water to condense.
Larger cold surface areas will allow more water to condense.
Passing greater air volumes over cold surfaces can cause more water to condense.
Conversely, the following factors will influence towards less water condensing
Warmer room temperatures which can be achieved by
Elimination of draughts
Minimising heat loss with effective insulation and double glazing
Appropriate heating
Reducing excess water vapour by
Extracting major water vapour generators at source – bathrooms, cooking, clothes drying etc
Appropriate ventilation
Preventing dampness underfloor or around the structure of the house.
Warmer internal surface temperatures by
Separating internal surfaces from outdoor temperatures
Using thermally efficient window and door frames
Double glazing
Why does so much condensation happen on window glass?
Glass is an efficient thermal conductor.
When outside conditions are cold, heat is drawn rapidly through the glass away from the indoor surface. The indoor glass surface temperature gets cold; often much closer to the outside temperature than the room temperature.
Air close to the cold glass is cooled and if the dew point is triggered, condensation results.
Windows comprise a significant portion of the external wall area. So they are large cooling elements.
Also, a convection cycle is set up which carries a stream of air over this cold surface. The cooling air becomes denser and falls as it cools. This causes the coldest air (and most condensation) to be at the bottom of the window. More warm air is drawn in to replace the cooled air so that a cascade is established allowing water to condense from a much larger volume of air than that initially in contact with the glass.
How does double glazing reduce condensation so effectively?
Double glazing directly influences 2 of the 3 main condensation factors (the room temperature and amount of cooling).
By thermally separating the internal and external glass surfaces, the internal glass surface is kept warmer and warm air temperatures are more easily maintained.
High performance double glazing does this significantly better again.
It can be demonstrated that internal glass temperatures (measured in the centre) with cold outside conditions may be improved from under 5°C with single glazing to over 16°C with high performance double glazing. Widespread condensation on the glass at this surface temperature is much less likely and would only occur if very high water vapour levels were present.
Can condensation occur on double glazed windows?
Double glazing allows influence over the factors which cause condensation.
High performance double glazing does this even better.
This does not mean that condensation is not possible. It means that condensation can be avoided or minimised over a much a wider range of conditions.
So, occurrence of condensation on inside glass surfaces with double glazing in place is less likely, will be less frequent, and will be less severe unless other factors are at play. Examples of such factors could include
Ambient humidity levels being very high. Inadequate extraction for cooking, showering or clothes drying and lack of ventilation can give rise to this.
Extremely cold weather conditions.
Heat loss through a less efficient spacer causing the perimeter of the glass to be cooler and less resistant to condensation.
Heat loss through joinery cooling the edges of the glass.
Condensation on joinery. Conventional aluminium joinery is also thermally conductive and will condense water irrespective of the performance of the glass.
Blinds or window coverings which effectively seal the window off from the room can lead to condensation. If the window space is sealed off, residual heat loss, even though at a much lower rate, will result in the window space getting progressively colder. In this situation the dew point threshold can still be triggered. The solution to this can be as simple as leaving a small gap for air from the room to circulate through this space
Will double glazing stop condensation on my window frames?
If double glazing has been fitted to existing frames, the glass will not change the thermal performance of the frames. So, the condensation performance of the frames will also be unchanged.
However, the surface area of the frames is usually much less than the glass and frames condensation is therefore somewhat easier to manage when it is not widespread across the glass.
To change the condensation performance of frames, replacement with a more thermally efficient frame would be required.
What happens with convection currents?
Although cold convection currents are not as easily seen or understood, the secondary win from eliminating these problems can be at least as beneficial for homeowners as reduction in condensation.
When people feel cold floor temperatures and draughts around their ankles, they don’t often consider the windows as the problem.
The reality is these things are symptoms of the thermal processes that are happening around the window in the same way that condensation is.
In many instances, solving the condensation problem also results in much warmer feet or an end to cold air dropping onto beds. A very welcome bonus!
Why does condensation happen on the outside of my double glazing?
It is important to remember that dew happens outside too.
When heat loss is dramatically reduced by high performance double glazing, the indoor glass surfaces are kept much warmer and condensation (dew) is less likely to happen inside.
Conversely, when heat is no longer being transferred as quickly to the outside, the outside surfaces will be colder – much closer to the temperature of other surfaces outside.
Accordingly, when dew is forming on other surfaces outside, it will be more likely to form on the outside of the windows too.
This is a visual confirmation that the double glazing is working really well. Much more heat is now being kept inside the home. The more effective the double glazing is, the more likely it will be to see dew form on the outside surface of the glass.
Summary
If condensation is a problem for you then we are here to help you understand the causes and options to help manage the issue.
You can book a free consultation to discuss your particular situation and the control options that are possible for you.
Further reading
You can learn more about condensation from the following sources: