This mis-information often leads home owners to spend a lot of money trying to treat the symptoms, rather than actually tackling the cause. In fact, addressing the causes of condensation is often a lot cheaper than installing a system to constantly manage the symptoms.
So, where to start:
Moisture and Relative Humidity
First we need to understand the basics about water vapour in the air.
At any specific temperature the air can hold a certain level of water as vapour - the warmer the air is, the greater the potential amount of water vapour it can hold. For example:
Air at 10ºC is saturated (or at 100% RH) when it contains 7.6g water per kg dry air.
Air at 20ºC is saturated (100% RH) when it contains 15.3g water per kg dry air.
Relative Humidity (RH) is a measure of how 'saturated' the air actually is, expressed as a percentage, i.e., what is the proportion of actual water vapour compared to the maximum amount that can be held at a given temperature.
So, using the examples above, air at 10ºC, containing 7.6g water per kg dry air has a relative humidity of 100%
But, if this air is warmed to 20ºC, and it still only contains 7.6g water per kg dry air, then it's relative humidity is only 7.6 / 15.3 (water holding capacity) = 49.7%
So, when the total moisture levels don't change, as you warm air, it's RH drops. Conversely, as air cools, it's RH increases. This is an important thing to note, and goes a long way to explaining condensation.
The Dew Point
So we now know that cold air cannot hold as much water vapour as warm air, and that as the temperature drops the relative humidity increases. An often used illustration is that of a bucket of water.
Think of the air as a 'bucket' holding an amount of water. The size of the bucket denotes the temperature, higher temperature = bigger bucket. Lower temperature = smaller bucket.
As the air is cooled the 'bucket' get smaller and therefore the proportion of water increases. If air continues to be cooled, the 'bucket' shrinks until it's completely full of water (this is the DEW POINT).
Now, if the bucket gets any smaller (i.e. if the temperature goes any lower) the water will overflow and spill out of the bucket. This is what condensation is. It's the excess water that the cooled air can no longer hold.
So the DEW POINT = The temperature at which condensation begins = the point where the Relative humidity is 100%
Causes of Moisture
In a typical home, the main creator of moisture is us, humans. We breathe, we cook, we shower, we water pot plants, we dry clothes inside. Some of these things we can minimise or eliminate, others we can't. A watertight, unoccupied home will generally not suffer from condensation.
Windows and double glazing
Condensation usually forms on the windows simply because the air around those surfaces is usually the coldest. It's usually coldest because the cold air against the outside glass surface is very easily transferred through the glass to the inside surface. Glass is a great conductor of cold and heat, unfortunately. This is why double glazing helps to reduce condensation, it simply makes the inside glass surface less cold. It doesn't actually do anything about reducing the cause of condensation, which is the level of moisture in the air.
The importance of ventilation
Now that we've ascertained that condensation is caused by the temperature dropping to below the dew point for the level of moisture in the air, it should be clear that the two ways to prevent condensation are to either keep the temperature above the dew point, or to reduce the amount of moisture in the air.
If we only focus on keeping the home warm, we'll soon come unstuck. The average family is producing around 12 litres of water EVERY DAY (much more if you believe some advertisers). So as the moisture levels increase, we'd need to keep increasing the temperature to stay above the dew point until we ended up in a very uncomfortable environment indeed.
By far the best and most sustainable way to control condensation is to control the moisture levels in the air. This is done by minimising the amount of moisture is introduced into the home in the first place (i.e. not drying clothes inside, extracting steam from the bathroom before it gets into the rest of the house), and using ventilation to manage the remaining moisture, replacing it with drier air from the outside.
As we've learnt above, while the air from the outside is no doubt colder than inside, it's also a lot drier because:
a) it's cooler so can't hold as much water
b) it's not trapped in an enclosed space with moisture generating humans increasing it's humidity.
If you draw in this outside air to replace the damp inside air on a continuous basis, and at a level that doesn't mean losing more heat than is necessary, and if you maintain the indoor air temperature at a level higher enough to keep indoor surfaces above the dew point, then you will not have condensation.
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