Condensation: A real & present danger, not a damp squib

Window Mould

Condensation forms when the humidity level in the indoor air reaches saturation point and that air comes into contact with a cold surface, turning into liquid water.  This can lead to damage, mould and respiratory problems. So what be done? Paul Williams of Domus explains. 

Whilst condensation can form throughout the year, it’s the winter months that present the greatest challenge.  As temperatures drop, residents turn their heating systems on and naturally close windows to prevent that heat from escaping.  This also prevents the moisture from escaping, increasing humidity levels and the risk of condensation. The colder the surface, the more likely condensation is to form, especially during the night when the heating goes off and the temperature drops and the air can no longer contain the moisture and finds all the cold spots to condense.

Surface condensation is caused by the high humidity linked to everyday home activities such as cooking, bathing, washing and drying clothes, and even just breathing.  Kitchens and bathrooms are usually the most affected areas as moisture levels are typically highest, but condensation can occur in other areas as well, especially in colder rooms. 

The Institute of Specialist Surveyors and Engineers – a not-for-profit, charitable scientific research and educational organisation – has calculated that if a person stayed indoors, slept, sat, and did housework for seven hours each, the average household of three could produce 4.8 litres / 8.4 pints every day!

Risks associated with condensation

Condensation isn’t just a nuisance.  Excess condensation left within a property can lead to the formation of damp patches and then mould growth.  The potential health impacts include asthma, eye irritation, respiratory problems like wheezing and difficulty breathing, chest tightness, cough, throat irritation, skin reactions and rashes, headaches and persistent sneezing.  Even more serious problems can result from exposure to mycotoxins – chemicals released by toxic mould.

The Building Research Establishment, in its briefing paper ‘The cost of poor housing to the NHS’, estimates that savings to the NHS per annum of £15,585,129 could be made if dampness was fixed in our homes.

Condensation also has consequences for the property.  Mould and dampness can lead to musty smells, staining, peeling wallpaper, cracked wall surfaces, plaster degradation, warped window frames and even structural damage over time, all of which come at a cost to repair.

How widespread is the issue?

The English Housing Survey Headline Report 2020-21 stated that in 2020* 2% of homes had problems with condensation and mould (namely where there are extensive patches of mould growth on walls and ceilings and/or mildew on soft furnishings).  When you consider there are an estimated 24 million households in England, that 2% equates to 480,000 homes suffering from serious condensation and mould.

The problem is most prevalent in private rented dwellings where 6% had some type of damp problem, followed by 4% of social rented dwellings and 2% of owner-occupied dwellings.

The good news is that all damp-related problems in housing have been on a downward trajectory since the 1960s.  The bad news is that condensation and mould reduction have stalled, with only a slight reduction in the figures over the past decade.

Condensation: who’s to blame?

So, what can we do to reduce condensation in our homes? Over the years, the onus has been very much placed on residents to make lifestyle changes.  These have included drying clothes outside when possible or using a tumble dryer, covering pans when cooking, shutting the bathroom door when showering or bathing, and ensuring extraction fans are switched on.

However, for many low-income households that are struggling to adequately heat their homes, and therefore have a higher level of condensation, these measures are not always economically viable.  Furthermore, recent research has verified that construction and external factors play more of a role in the development of these issues than residents’ behaviour: dampness and mould are directly linked to badly insulated and poorly ventilated houses. 

CMX Multi-H MEV in ceiling void

So, it’s time to stop laying the blame at the door of the resident and start making our homes more condensation proof.

Ventilation: the way forward

If we improve the ventilation in a home, we lower the humidity.  It really is as simple as that.  Circulating the air and extracting excess humid air outdoors can be accomplished through a number of different ventilation solutions, depending on the budget available, the nature of the property and the seriousness of the issue.

For existing properties, the most cost-effective options are bathroom and kitchen intermittent extraction fans or single flow, continuous running decentralised mechanical ventilation fans, such as the new Domus Ventilation D-dMEV.  Positive Input Ventilation (PIV) units can also be used which gently pressurise a dwelling to expel stale and humid air through natural ventilation points. 

For new builds, the options are more widespread and ultimately more effective.  In addition to intermittently operating extract fans, Mechanical Extract Ventilation (MEV) units and Mechanical Ventilation with Heat Recovery (MVHR) can be deployed.

MEV units, provide constant low-level ventilation by extracting waste, and damp air from a dwelling through a system of ducting.  These are best suited to apartments and small to medium-sized houses. 

MVHR systems go one step further by combining extraction with supply.  The extracted air is passed across a high-efficiency heat exchanger located inside the MVHR unit, to remove and transfer the otherwise wasted heat.  The exhaust air is then expelled into the atmosphere.  At the same time, fresh filtered supply air is drawn across the opposite side of the heat exchanger and pre-warmed before being distributed through ceiling-mounted grilles to habitable rooms.

The new breed of MVHR units on the market is highly energy-efficient, with models available with integral humidity sensors.  By accurately measuring air humidity, the extract speed automatically changes from background to boost as the level of humidity increases, thereby providing optimal ventilation performance. 

These mechanical systems provide a long-term and cost-effective solution rather than short-term problem ‘hiders’ such as dehumidifiers. 

Time to take action

Our climate has changed over the last few years, with milder winters and wetter summers, but the issue of condensation in our homes remains and is not improving fast enough.  Expecting people to change their lifestyle to prevent excess condensation is not realistic (it certainly hasn’t worked up until now, so it’s unlikely to work in the future), nor will it reduce condensation to comfortable levels.  Building adequate ventilation into our homes is by far the more realistic and long-term solution.

Paul Williams is Product manager at Domus Ventilation

Related links:
Related articles:



modbs tv logo

‘Bittersweet’ honour for air quality champion

Air quality and child health campaigner Rosamund Adoo-Kissi-Debrah has been made a CBE for services to public health in the King’s New Year Honours. 

CIBSE recognises rising stars and exceptional employers at the Young Engineers Awards 2022

CIBSE Young Engineers Awards (YEAs) 2022 recognised future industry change makers, as they address the big questions around the role of engineering in delivering a safe, healthy, and sustainable built environment for all.