Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings Figure 3.6 – Warmer indoor temperature lowering relative humidity of fresh air from outdoors Outdoors 7°C / 70% RH (dew point 1.9°C) Indoors 20°C / 30% RH (dew point 1.9°C) In reality, outdoor air is not the only source of water vapour that will influence the relative humidity indoors. All occupied buildings have their own internal sources of water vapour due, in part, to the breathing of the occupants. An adult breathes out about 200 grams of water vapour each hour while awake (and engaged in sedentary activities) and about 20 when sleeping. Evaporation from the skin will provide some more. In a residence, daily hygiene and housekeeping involve many activities that release water vapour. The amount of water vapour that might be added to the interior of a dwelling depends on the number of people (and pets) living there, how active they are, how much cooking, dishwashing, laundry, showering and bathing they do, what they choose to store or keep around them and what heating or cooling arrangements they use. How much of the added water vapour accumulates inside the building will depend also on the rate of ventilation and the use of any extraction systems to capture water vapour near its source and discharge it directly outside. DRAFT HANDBOOK A family of four could easily generate between 7 and 22 kilograms of water vapour per day, even allowing for absences at work and school. Which part of the range applies would depend on how much of the water vapour is diverted outdoors from sources such as bathrooms, laundry, cooking and gas heating. Figure 3.7 shows how extra water vapour, near the higher end of the range, could affect the 30% indoor relative humidity indicated in Figure 3.6. Even for a well-ventilated house, the relative humidity at 20°C would rise to 50%, high enough to support rapid expansion of dust mite populations and their attendant health risks. At lower temperatures, which might occur when heating is turned off overnight, relative humidity rises further and mould germination becomes possible at 14°C. Australian Building Codes Board Page 19
Information Handbook: Condensation in Buildings Figure 3.7 – Warmer indoor temperature lowering relative humidity of fresh air from outdoors Indoor Relative Humidity 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 12°C condensate forms Mould likely on surfaces at 14°C or cooler mould germinates dust mites proliferate 13°C 14°C 15°C 16°C 17°C 18°C 19°C The psychrometric chart in Figure 3.5 illustrates relative humidity falling as temperatures increase but the risks for buildings stem from the rising relative humidity which develops as temperatures fall. The temperature fall can be due to a cold external environment or cooling in an air conditioned interior. As suggested by Figure 3.7, unwelcome effects can emerge before relative humidity reaches 100% and the dew point when condensation begins. 20°C Water vapour added indoors increases RH to 50% which supports dust mite proliferation 21°C 22°C 23°C Indoor Surface Temperatures 24°C 25°C 26°C 27°C 100% DRAFT HANDBOOK 70% 50% 30% 28°C Australian Building Codes Board Page 20
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