During winter condensation is common in British homes. It can form anywhere, but is most commonly found as droplets of water on and around windows. So what’s the problem, why does it happen, and how can we stop it?
Condensation is bad for both human and building health
Condensation is water that forms on cold surfaces. It’s not particularly dangerous, and if it happened once or twice a year you could wipe it away and stop worrying. However, the reality is that if it happens once, condensation probably forms daily for a prolonged period, perhaps for 2 or 3 months during the winter. It’s this repeated moisture exposure that can cause problems.
- Aesthetic Damage – Damp paintwork tends to crack and fall away, a common sight on and around window frames.
- Structural Damage – At the more severe end of the spectrum, condensation can cause structural damage. Condensation can cause timber window frames to rot, and materials within the building fabric (e.g. timber joists) can suffer the same fate*.
- Mould and Health Damage – It almost goes without saying that living somewhere cold and damp isn’t good for us. More specifically, mould thrives on damp surfaces. Consequently, it’s common to see mould growth around windows in the same place as condensation. Mould can cause a variety of health problems, including respiratory illnesses such as asthma.
What causes condensation in homes?
There are two simple ingredients that combine to produce condensation in buildings.
- Moisture in the air – A humid environment will be more susceptible to condensation.
- Low surface temperatures – We all know that condensation forms on cold surfaces.
There are two ways of measuring humidity, absolute humidity (AH) and relative humidity (RH).
Absolute humidity is a measure of how much water there is in the air, with units of g/m3. If you had a cubic metre box of room temperature air with 1 litre of moisture in it, the AH would be 1000 g/m3. Warm the air up to 50⁰C, and the AH remains at 1000 g/m3 (provided nothing has gone in or out).
Relative humidity on the other hand is measured as a percentage. At 100% RH, air is at full water capacity and liquid moisture will appear (e.g. rain, or condensation). Because warm air can “hold” more moisture than cold air, RH does change depending on temperature, even when AH remains constant. A fixed amount of cool air at 100% RH could be heated up to produce warmer air at just 60% RH, even though both scenarios would have the same AH.
The relationship between air temperature, absolute humidity and relative humidity can be displayed on a psychrometric chart.
In most contexts, relative humidity is more useful than absolute humidity because it describes the situation regardless of temperature. For example, whether you heat your home to 18⁰C or 22⁰C, a relative humidity of 40-60% is a healthy range.
Condensation occurs on surfaces when the relative humidity at that surface is too high. Mould growth can start at 80% RH, with significant amounts of liquid forming as that number pushes up towards 100% RH.
A typical scenario would be a room at 20⁰C and 60% RH, but with the internal surface of a window at somewhere around 8⁰C. In the room there’s no problem, but when the air comes into contact with the cold windows it cools, the relative humidity shoots up, and condensation forms. This can happen on any surface, a wall, roof or floor, but by far the most common offenders are windows, particularly on and around frames.
Standard quality window frames (i.e. not high performance) will get cold
A typical insulated wall in a new UK home might have a U-Value of 0.25 W/m2K, or better if attention has been paid to energy efficiency. This means it’s relatively good at keeping heat in, and the internal surface will be warm. The same applies to most roofs and floors.
The limiting value required by Building Regulations Part L is that a window must have a whole-window U-value of 2.0 W/m2K or better. That’s eight times worse than the requirement for walls. To add to this, the glass itself almost always performs better than the frame. This is significant because a whole-window U-value is obtained by taking an area weighted average of different components, mainly the frame and the glazing.
Good double glazing might have a Ug-value of 1.5 W/m2K, so the frame can have a Uf-value of well over 2.0 W/m2K and the unit as a whole will still be comfortably compliant. In practice this means that you can have very good glazing, but the frame can perform worse as a result.
This is overwhelmingly the case with standard windows in the UK, which have uninsulated aluminium, timber or uPVC frames. With nothing other than a bit of plastic between the inside and outside, the internal surface of the frame won’t be far off the external temperature. When it’s freezing outside, that almost always means condensation inside – at least around the frames.
Even a good window can be installed badly
Installation of windows requires structure to fix to, often timber boxing or steel lintels. This structure can cause thermal bridging adjacent to cold frames, making the situation even worse. Clever architectural detailing and wrapping of insulation is all it takes to avoid this, but it’s rarely done well unless it has been specifically discussed with the architect, designer or window installer.
As well as the U-values of window frames and glazing, and the quality of the installation detailing, another source of heat loss is through window spacers. These are the things that sit between panes of glass in double or triple glazing, and are often made of highly conductive metal or plastic.
Any one of these issues is enough to produce a very cold surface on and surrounding window frames, causing the formation of condensation. Many windows suffer from all the issues combined.
Good ventilation will reduce condensation risk, but it can’t solve the problem alone
We’ve talked about low surface temperatures and what causes them, but the other key ingredient is the moisture itself.
The lower internal humidity levels are, the lower the risk of condensation. The risk won’t be zero unless there’s no moisture in the air, which isn’t an environment anyone wants, but the lower the RH the lower the dew point – the temperature at which condensation will form.
Just like windows, standard ventilation systems designed for compliance rarely do a good job. If your bathroom extractor fan doesn’t remove enough moisture after that morning shower, RH will rise and the risk of condensation will increase. The same goes for poor quality kitchen extractors.
Even if extraction systems work well in bathrooms and kitchens, other rooms or localised areas in a home can still be poorly ventilated. For example, trickle vents may be closed, blocked, or non-existent, and unfortunately placed furniture or blinds may restrict air flow to a particular place. Condensation often occurs in the morning after occupants have been sleeping and generating lots of moisture overnight, so ventilation to bedrooms is an important consideration.
The gold standard of domestic ventilation is mechanical ventilation with heat recovery (MVHR). These systems constantly supply fresh, filtered air to living spaces and bedrooms, and extract air from wet rooms. When used correctly, this whole-house solution has been shown to provide optimum indoor air quality, managing relative humidity more successfully than the alternatives and consequently minimising condensation risk. A heat exchanger also recovers heat from extracted air, making it the most energy efficient type of ventilation system.
How can I solve condensation issues?
There is rarely a single cause of condensation. It eventually boils down to a combination of low surface temperatures and high relative humidity, but the extent to which one dominates the other can be difficult to identify. However, there are a number of avenues you can explore to minimise the risk of condensation in your home, whether new or old.
Buy top quality windows
The U-value of your windows, particularly the frames, is really important. Consider installing triple glazing with insulated frames, ideally a Passivhaus certified product. This will increase the internal surface temperature and greatly reduce the risk of condensation. There is a cost uplift compared to a more standard product, but triple glazing comes with significant energy efficiency, comfort and acoustic benefits.
Consider installation detailing for new windows
Reducing thermal bridging around windows will also increase internal surface temperature, which is good. This detailing is the responsibility of the architect and is often straightforward, but all too often slips through the cracks.
Ventilate well, ideally with MVHR
Improving internal moisture management can often be a crucial step to eradicating condensation. Many extractor fans are old and under-perform, and background trickle ventilation can be inadequate in bedrooms. Consider replacing or upgrading existing systems to remove excess moisture more effectively.
Consider how occupant behaviour affects indoor humidity
Rooms in a home are designed to be used in a particular way, so using them differently can be problematic. Classic examples of behaviour that increases internal humidity and therefore causes condensation are drying laundry inside, lots of cooking or washing, or just over-occupancy. If there are 6 people in a home designed for 4, there will be more showering, more sleeping, and more laundry – all things that generate lots of moisture. Given the UK’s cramped housing stock these situations will often be unavoidable, but be as sensible as possible and don’t open a launderette in the living room.
* Interstitial condensation is when moisture forms invisibly within the building fabric, for example between a layer of insulation and masonry. It’s particularly dangerous because it’s hidden and is difficult to identify until it’s too late. However, the causes and solutions are usually different from those for more typical surface condensation discussed here.