Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings 2 Overview Even a dry building contains water. Some of it is confined in pipes, drains and tanks installed to make the building workable. Some is bound up in the microscopic pores of absorbent building materials and contents. Those forms of water are simply out of sight. A crucial and often overlooked part is truly invisible. It is the colourless gas, water vapour, which is always present in the atmosphere, mixing freely with air and going wherever air goes. As it happens, air travels virtually everywhere in and around the rooms of our buildings (Figure 2.1). Since fresh air is vital for health and amenity, outdoor air is encouraged to flow through open windows and doors (or drawn in through mechanical ventilating and air conditioning systems). Driven by fluctuating pressures around buildings, air also leaks inwards and outwards through interior linings by way of holes made for light fittings, light switches, power outlets, pipes and the like. It can also find its way through small gaps under architraves around windows and doors and behind ceiling cornices and skirting boards. In addition, outdoor air is deliberately circulated through roof spaces, wall cavities and sub-floor spaces to help them dry. All of that mobile air carries water vapour with it. Figure 2.1 – Air and water vapour movement through the building interior and the building fabric wall/ceiling leaks wall/floor leaks roof space ventilation outdoor air ventilation sub-floor ventilation downlight leaks leaks around openings leaks at lights switch plate leaks DRAFT HANDBOOK Indoors, people release more water vapour into the air by breathing and transpiring and by daily activities such as cooking, washing, bathing and laundry. In a room where both temperature and humidity are closely controlled, the amount of circulating water vapour might vary from under 0.5% of the moist air mass in a cold climate up to about 1% in a warm humid climate. That is not a lot of water vapour but not much more is needed to cause problems. Chapter 3 discusses the behaviour and effects of accumulating water vapour and points out that even a low cavity ventilation Australian Building Codes Board Page 5
Information Handbook: Condensation in Buildings concentration, at a low enough temperature, can represent a high relative humidity (RH). Relative humidity, which is considered in detail in Chapter 3, is the amount of water vapour present compared to the maximum that could be present at the same temperature. It is always expressed as a percentage. When relative humidity rises towards 50% and beyond, undesirable effects begin, including: • dust mite proliferation, particularly in bedding, carpets and soft furnishings, producing inhalation allergens (when relative humidity reaches 45-50%); • mould and fungi growth, including wood rotting fungi (when relative humidity exceeds 70%). The presence of microbial infestations compromises indoor air quality and can seriously affect asthmatics and others susceptible to allergic reactions. As part of reducing these risks, international guidelines on indoor air quality recommend complete replacement of the air in a dwelling every two to three hours. This means that outside air will flush out the indoor volume between eight and twelve times in every 24 hours. Figure 2.2 represents how much air is involved to achieve the higher ventilation rate. The diagram also highlights that, even in a contemporary “airtight” dwelling, about half of the air (and water vapour with it) will travel through accidental pathways formed by gaps, cracks and holes in the building fabric. Figure 2.2 – Fresh air ventilation from outdoors for acceptable indoor air quality Indoor air volume replaced up to 12 times in 24 hours Air change by accidental leakage Air change by intentional ventilation DRAFT HANDBOOK Since fresh air supply cannot be reduced below an essential minimum, water vapour brought in with outdoor air can limit attempts to lower water vapour levels indoors (unless an air conditioning system with substantial dehumidifying capacity is involved). Once that limit is reached, relative humidity must be managed by raising the temperature, using active or passive heating. The benefits of controllable heating are illustrated in Figure 2.3. At each one degree temperature step across the chart, the darker blue lower column shows how much outdoor air for ventilation contributes to the relative humidity indoors. The lighter blue Australian Building Codes Board Page 6
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