Fri 24 Oct 2008
Airs and graces
In our bid to insulate homes is the need for ventilation being overlooked? Roger Hunt examines the standard of indoor air quality.The relationship between insulation and ventilation is a tricky one. As homes become increasingly well insulated and airtight 'fresh' air is inevitably in short supply and this means a whole new set of problems for the building and its occupants unless the issue is addressed.
Lack of ventilation can result in much more that simply a stuffy smell and condensation; it may mean that toxins build up within the home. Modern building products and materials, furnishings and cleaning materials, candles, tobacco, electronic products and toiletries all have the potential to pollute the indoor environment while moulds and other allergens, such as pet hairs and pollens, may also have an adverse effect on heath. Indeed, poor air quality has been linked to problems of 'sick building syndrome' and has been shown to reduce productivity in offices and learning in schools.
For these reasons specifiers of 'eco' buildings need to look not only at ensuring there are adequate air changes within the indoor environment but also consider the products and materials used within the building's fabric and furnishings. This is especially true in Europe where people spend about 90 per cent of their time in indoor environments and their exposure to many air pollutants is largely dependent upon the exposure that occurs indoors.
According to Derrick Crump, technical director at BRE, good indoor air quality depends upon a wide range of parameters including the presence in air of volatile organic chemicals (VOCs), gases such as nitrogen dioxide, ozone and carbon monoxide, particulate matter such as 'PM10' and 'PM2.5' and fibres, and biological particles including bacteria, fungi and pollen.
"The perception of these components can be influenced by the temperature and humidity. Effects on health depend upon the toxicity of the pollutant, its concentration and the exposure period. The effects may therefore range from odour to irritation as well as carcinogenic and non-carcinogenic toxic effects," says Crump.
Neil May, chairman of the Good Homes Alliance, a group of developers committed to building and promoting sustainable homes and communities in the UK, is among those who believe that the materials selected for building are often not considered carefully enough given the range of different chemicals they contain.
"Of the 55,000 different materials used by housebuilders, only three per cent are tested to identify the impact on human health. This figure is astonishing. We really do need to take these issues into consideration when designing our new homes and look for allergy-free solutions."
The Government of Canada's Super E programme was one of the first to address such matters and now draws from more than 20 years in the industry with over 10,000 Super E homes having been built in Canada. The first Super E energy efficient timber frame home arrived in the UK in 2000 and recently the 500th home was constructed by Berkeley Homes. It is the largest builder of Super E homes in the world and is currently constructing them at its Holborough development near Maidstone in Kent.
Super E homes have high standards for indoor air quality and builders must choose strategies to reduce or eliminate toxic emissions from the materials used in their construction, thereby limiting the dangers of off-gassing - the release of volatile chemicals into the air through evaporation. This is a process that can continue for years after products such as paints, varnishes, stains, carpet, insulation, flooring, plywood and particleboard are initially employed.
"If a home has low air permeability, then controlled ventilation with heat recovery and the careful choice of building materials become crucial to control pollutants as well as moisture levels to ensure occupant health," explains Jeff Culp from the Super E house programme.Within the highly insulated 'leak proof' Super E homes, a heat recovery ventilation system acts as the building's 'lungs', ensuring a continuous supply of healthy, fresh air. Filtration removes dust and pollen particles and the system also helps maintain optimum humidity levels. High humidity levels can lead to the growth of moulds and encourage the proliferation of mites. Low humidity levels can lead to respiratory irritation, and promote respiratory infections.
Super E construction emphasizes moisture control, mainly to prevent damage to the building's fabric. The spin-off effect is that the insides of the walls in Super E houses are very dry, preventing the growth of moulds within the walls. Mould spores are one of the most common causes of sick building syndrome.
A number of studies have been done in Canada comparing the health of occupants of houses using Super E construction techniques to those living in conventional housing. In one study, 13 different symptoms were compared. In every category of physical symptom, occupants of Super E houses reported more healthy results than conventional new housing. Overall, 94 per cent of occupants in Super E homes reported a significant improvement in indoor air quality over their previous residence.
"It is vital that we do not overlook the unintended consequences in our drive to improve the air tightness and insulation of new homes," emphasises Neil May. "If precautions are not taken, high humidity levels are a considerable risk, allowing dust mites, moulds and bacteria to thrive, causing and exacerbating cases of asthma and other respiratory diseases. Asthma alone costs the UK £2 billion each year in fatalities, medication and lost working days.
"To prevent trapped moisture and keep relative humidity levels below 70 per cent, installing an efficient assisted or mechanical ventilation system is essential. It is also important that the interior wall materials are hygroscopic or breathable so that they are able to help absorb moisture present in the air."
Chris Marriott, managing director of ADM Systems, an independent specialist in air management, explains that there are various methods of providing ventilation in dwellings and, when these are combined with the latest heat recovery systems, they provides for a very thermally efficient and healthy property.
"The latest building regulations for improved thermal efficiency and the mandatory requirement for building energy labelling mean that specifiers are increasingly looking at integrated systems that provide clean fresh air as well as contributing to more sustainable and efficient dwellings."
Marriott says that the relatively simple means of ventilation such as central (mechanical) extract ventilation (MEV), passive stack ventilation (PSV) and positive input ventilation (PIV) are very good at reducing condensation and, in particular, preventing the build up of smells and harmful gases.
"However, many clients now prefer mechanical ventilation with heat recovery (MVHR) because it helps achieve a better energy rating on a dwelling in addition to providing more comfortable living spaces," explains Marriott.
Ian Mitchell, residential product manager for Vent-Axia, the low energy air movement solutions provider, explains that, in order to meet the Code for Sustainable Homes, the ventilation system for new homes must be installed to maintain indoor air quality while limiting energy and heat loss. "Controlled mechanical ventilation using heat recovery is essential if the Code requirements are to be achieved."
MVHR combines supply and extract ventilation in one system and incorporates a heat exchanger. Warm stale air is typically extracted from wet areas in the house, such as the kitchen, laundry and bathrooms, via a series of ducts and passed through the heat exchanger before being exhausted to outside. Fresh incoming air drawn from outside the building is preheated via the exchanger and supplied into the living areas of the house in a process which saves and transfers between 70 and 95 per cent of the heat energy that would otherwise be wasted.
"Changes to Part L and Part F of the Building Regulations conspire to make central extract or whole house heat recovery the preferred ventilation solution in the new build market place where adequate levels of air tightness are achieved," says Mitchell. "MVHR is the only system where no background ventilators are required, therefore there are no heat losses and the opportunity for noise ingress will be reduced."
Whole house systems can prove very effective at reducing the heat load in energy efficient homes while single room systems are proving an increasingly popular choice for small scale renovation projects. Both types of system offer dual speed options, either continuous trickle ventilation or high speed boost extract flow, and the more airtight the property, the more energy efficient the heat recovery system will prove.
The downside with these systems is that, although they recover heat that would otherwise be lost, they use electrical energy to drive the fans that make them work. One way of solving this problem has been pioneered by ZEDfactory. It has created rooftop wind cowls which instead of employing electric fans to move the air rely on the wind to create both positive pressure at the inlet and negative pressure at the output to ensure a throughput of air. Ducting that forms part of the system passes through a heat exchanger which is claimed to be 70 per cent efficient.
Posted by Roger Hunt
in ADM Systems, BRE Trust, Chris Marriott, Derrick Crump, Good Homes Alliance (GHA), Ian Mitchell, Neil May, Sustainability, Vent-Axia on Fri 24 Oct 2008
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