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This indicator examines the provision of adequate ventilation for good indoor air quality in residential houses. The Building Code of Australia (1990) states that all buildings (including residences), must have adequate cross ventilation and air quality. This must be provided by natural ventilation from permanent, openable windows, doors or other devices with an aggregate openable size of not less than 5% of the floor area of the room to be ventilated. If windows are closed, air enters the buildings by infiltration (leakage of air through cracks, spaces and ventilators in the building envelope). The rate of infiltration is determined by pressure differences between the inside and outside of the building (Brown 1997). These pressure differences drive air through the building envelope either into or out of the building. Therefore infiltration rate is the minimum air exchange rate that can occur within a building. Buildings, such as residences, usually have low infiltration rates. This is mainly due to alternative construction techniques that, through time, have made houses less 'leaky'. An example would be the introduction of construction techniques that seal the building tightly so there is no air leakage, changing standards (i.e. removal of the requirement of wall vents), and newer more synthetic building materials that do not allow the building to 'breathe'. As a consequence, there is less infiltration within buildings, and odours, vapours and other gases accumulate into the building because there is no mechanism for their removal. This is especially the case when windows and doors are shut. There are no minimum air exchange rates required in Australian houses, therefore acceptable infiltration rates should be taken from proposed international standards. The proposed rates are 0.5 air exchanger per hour in Norway, Sweden, Finland, Denmark and Iceland, 0.8 in Germany, and 0.5 - 1.0 in the UK. A quantitative assessment of residential buildings in Tasmania is important to determine their ventilation rates. The assessment should include the use of tracer gas techniques or the pressurised infiltration rate technique (to measure the air flow rate). The latter technique is preferable because it is a more standardised technique. Studies on air infiltration rates in Australia have been few. One study measured 'natural' infiltration rates in unoccupied houses in Melbourne and found that background infiltration rates were 0.33 air exchanges per hour (ACH) (Biggs et. al. 1987). This study was then extended to other Australian states to estimate average ACH rates (undertaken on test houses using empirical data equations and wind data). Estimates showed that Canberra had an ACH of 0.44, Sydney and Hobart had an ACH of 0.55, and 0.57 ACH in Melbourne. Harrison 1985 also found similar findings in a study of nine houses in Perth (0.05-0.41 ACH). Ferrari 1991 on the other hand demonstrated that air exchange rates were more varied. Rates ranged from 0.2 to 2.3 ACH in 41 Sydney dwellings. There has been no study undertaken on air infiltration rates in Tasmanian residential buildings. When examining 'pressurised' infiltration rates (50 Pa), Biggs and Bennie 1988 found that Australian ACH rates were similar to those found in other countries. Infiltration rates (ACH) through houses for 50 Pa pressure difference Country Number of houses Mean air exchange/hour Australia
10 26.3 Australia
12 12.2 New Zealand 10 11.0 Netherlands 130 12.0 United Kingdom 19 13.9 Canada 60 4.4 Sweden 205 3.7 Source: Biggs et. al. 1987 It has been demonstrated that the use of certain building construction techniques and materials has an effect on infiltration rates. Biggs and Bennie (1988) state that infiltration rates were approximately halved as a result of the use of weather strips on doors, and the elimination of fixed wall vents. Because so many new residential buildings are being constructed in Australia following 'energy efficient' construction methods it is highly likely that infiltration rates in some new dwellings may well be below levels considered essential for reasonable air quality. Further information is available in the New Dwelling Completions Indicator. National Human Settlements Key Indicator 7.4 (Newton et. al. 1998) |
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