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Indoor air quality loses out in healthy trade off

The last couple of decades have seen unprecedented investment in the energy efficiency of buildings. However, David Fitzpatrick believes this has led to a lack of focus on indoor air quality
Environmental concerns and the rising cost of gas and electricity have created a tidal wave of measures to improve the energy performance of buildings - and that is as it should be. However, in the rush to plug fabric leaks and minimise heat losses indoor air quality (IAQ) has definitely lost out.

Governments worldwide regularly act decisively to tackle pollution of outside air, but the issue becomes invisible as soon as the air disappears inside a building. Indoor air quality is more usually judged on what is 'acceptable' and if people don't become ill then building managers have, more or less, met their obligations. Surely, we can aim higher than that.

Improved energy standards under Part L of the Building Regulations have made our buildings increasingly airtight, which means they need a consequently higher level of ventilation.

While this is clearly stated in Part F of the regulations, many buildings are falling short of the required standards.

BSRIA principal test engineer Mark Roper told the recent B&ES/ADCAS Ductwork Conference that IAQ was the 'elephant in the room'.

Tightness
Eighty nine per cent of the 10,000 air tightness tests carried out by BSRIA passed and met standards set by Part L, which shows that building quality is improving. 'So we have made our buildings air tight, but are we ventilating right?' asked Mr Roper.

BSRIA believes there is a serious problem with the quality of mechanical ventilation systems. 'Many are not meeting the standards - often because installers don't have the right experience,' said Mr Roper.

'Flexible ductwork can also be a problem. It is easy to install, but if you stuff the excess back into the wall it has a dramatic impact on flow rate. For example, a fan rated at 25 litres per second (l/s) could end up delivering only 5l/s,' he added.

Building ventilation systems should be able to meet the basic brief of reducing the chance of overheating, particularly in hot summer weather, while maintaining good IAQ all year round. Other factors like noise control and smoke and fire safety will also have a big influence on the choice of solution.

Sadly, financial pressures created by the long recession led to a culture of specifiers trading one value off against another in a bid to reduce capital and running costs. 'Out of sight out of mind' ventilation systems have always been an easy target for reduced investment in upgrades and maintenance when budgets are tight.

However, the UK ventilation industry is perfectly capable of delivering high air quality without increasing operating and capital costs - or driving up carbon emissions. Developments in the design of natural ventilation systems, in particular, mean it is very possible to significantly reduce lifecycle costs while actually improving the IAQ.

With natural ventilation there is less need for expensive hardware and the service and maintenance is also considerably reduced in comparison to conventional mechanical systems. In a new build situation, natural ventilation also allows the architect greater flexibility by reducing the space required for plant.

Natural ventilation will help secure planning permission and meet the energy efficiency reductions called for by the Building Regulations. The ability to use 'free' night cooling can dramatically cut energy costs because the building is allowed to cool down naturally overnight, which can minimise or even eliminate the use of air conditioning during the day.

It is this ability to harness, or work in tandem with, nature's elements rather than trying to engineer them out that holds the promise of really significant progress in low energy building designs. However, for natural ventilation to work effectively there needs to be a very high level of design accuracy.

Modelling
Engineers should make use of the detailed computer modelling techniques now available to test their theories thoroughly before putting a system together. Factors such as the orientation of the building; thermal mass; shading; and the
size of the openings in the building fabric all have an impact on how a natural ventilation system will perform. It is not suitable for all buildings.

Occupants also worry about being more vulnerable to outside air pollutants as well as the security implications of open windows or easily accessible louvre systems. Any responsible engineer and equipment supplier will keep these factors at the front of their minds when putting a system together, but these problems can be overcome through good design.

The extra effort needed to design a natural ventilation solution is worthwhile as there is strong evidence that naturally ventilated buildings are healthier because they have lower levels of airborne contaminants.

There are three main approaches: Wind; stack and mixed mode. The first works on the principle that wind blowing against the (Windward) side creates a positive pressure on one side of the building and a negative pressure on the opposite side (Leeward). Air then flows from the windward to the leeward sides via carefully positioned ventilators, usually used in conjunction with a roof terminal.

Stack (or chimney) effect relies more on the buoyancy created by the warmer air inside the building, which is less dense than the outside cooler air. Warmer stale air rises up and exhausts through high level ventilators and/or a roof terminal and this movement also draws in outside air at low level. This approach is generally thought to be more reliable than windbased systems.

A mixed mode approach is often a popular compromise, which involves some mechanical elements such as fans and fan coils. It is particularly useful for peak demand periods during winter heating and summer cooling when the natural system might struggle to provide the optimum internal comfort conditions. The
controls are the critical element in this 'hybrid' solution as they must ensure the motorised items only operate when needed to ensure low energy consumption.

Whichever approach we adopt, it is critical that humidity, temperature and CO2 levels are managed closely. These are the main factors that determine whether an internal environment is healthy and productive or not.

Ultimately, clients should be dissuaded from focusing solely on energy saving at the expense of providing air of suitable quality to maintain the health and productivity of their occupants. With good design there really is no need for such an unhealthy trade off.

// The author is the sales director of Ruskin Air Management //
11 March 2014

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