Tackling condensation in social housing

Condensation, damp and mould in social housing are the bane of the surveyor’s life. Tenants often claim their health suffers as a result of damp living conditions and yet some abuse the corrective measures installed to address the problem. Meanwhile the Government continues to press local authorities to implement energy efficient systems. John Davison of Airtech Environmental Systems looks at the root cause of the problem and suggests ways to combat condensation that will both suppress claims for compensation as well as cut energy costs.

Most surveyors know that Relative Humidity (RH) defines how much moisture there is in the atmosphere. To be exact it is the ratio between the measured water vapour content of a volume of air to the maximum water content that could be held as vapour at a certain temperature. The higher the temperature, the more vapour can be held. When the air temperature cools, for example on meeting a cold surface, it gives up the vapour as condensation. This is known as the dew point and it varies with the RH and the prevailing temperature. As condensation occurs RH falls, raising the dew point until equilibrium is achieved. If the ambient temperature is consistently below the dew point there will always be condensation.

Steam, or vapour pressure, is a key factor. Normally in a dwelling this pressure is around 11.6 millibars but if a higher pressure exists elsewhere in a property then steam will to seek it out and give its vapour up as condensation; for example in a cold bedroom. This is why it is important to capture and extract the moist air as near to its source of origin as possible, e.g. a cooker, shower or tumble dryer. If RH increases along with a rise in temperature then vapour pressure rises still higher. Vapour pressure is a good indicator of how much steam is being generated by a tenant’s lifestyle. Both RH and vapour pressure can be measured using a hygrometer.

Effecting a cure
Air bricks, or passive vents as they are called, are a vital aid to air circulation especially in properties where tenants never open their windows. Modern passive vents have one way filters, and shutters to prevent draughts and although they help to equalise pressure they do not tackle the root cause of condensation.

One of the most effective ways of controlling condensation is to install an extractor fan that incorporates a microprocessor controlled sensor that continuously monitors air quality and operates to adjust RH only when necessary. Couple this with a data processor that logs real time RH, temperature, RH set point, vapour pressure, dew point, power interrupts and running time every two hours on a rolling 420 days basis, as featured on the Energysaver Datamatic fan from Airtech Environmental, and you have a complete monitoring and control system.

Although this type of fan automatically detects rising RH and switches on the extractor, if it is fitted in a window the cold glass may not give a true reading of the atmosphere in the room. In this case a remote sensor may need to be fitted elsewhere – ideally between the main source of vapour production, for example a cooker, and the door.

A significant benefit of this type of product is that surveyors can download, view and print out data such as hours run, starts/stops to check if the fan has been tampered with – i.e. switched off – as this would seriously affect the RH and subsequently the generation of condensation in the property. Equally, it reveals the temperature in the dwelling and amount of steam being generated by the occupant’s lifestyle.

As electricity bills rise there is a real danger that tenants will switch off extractor fans with disastrous consequences. A data log could provide crucial evidence in the event of a claim for compensation due to damp housing conditions.

Meeting HECA requirements
The Home Energy Conservation Act (HECA) 1995 requires all UK local authorities with housing responsibilities to identify practicable and cost effective measures likely to result in significant improvement in the energy efficiency of all residential accommodation.

Fans that operate only when necessary to deal with condensation can make a considerable contribution to meeting the HECA requirements because they offer energy savings of up to 80%. Although actual fan running costs are around one tenth of the cost of a 60W light bulb the real savings are in lower fuel bills as heating systems do not have to work so hard to maintain temperatures.

Normal conditions
Generally fans should be set to activate at around 67%RH which allows 17g/m3 of water vapour at 20°C: the accepted comfortable living temperature for most people. Airtech surveyors have, however, recorded indoor temperatures of 35°C in some flats which amounts to 47g/m3 of water vapour and this can cause serious condensation problems.

As night time temperatures fall RH and dew points rise the fan would normally kick in. However, a fan with a setback device controlled via a sensor allows RH to rise to 77% at 10°C before the fan is activated.

During the day a fan might run continuously and this can generate phone calls to the housing manager or surveyor’s office. The problem is that a fan will run until it has brought down the internal RH sufficiently to help dry out the walls. In severe cases this may last a few weeks, but the fan is only doing its job. Once equilibrium has been achieved it will settle down and only operate when steam producing activities occur.