Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings 3.6 Problems with Relative Humidity Relative humidity can be too low as well as too high. Low relative humidity can cause drying of the eyes, skin and mucosal membranes and contribute to dehydration and fatigue. These are symptoms which travellers might associate with passenger aircraft cabins, where relative humidity is usually not much more than 10%. Simple measures of human comfort often suggest 30% relative humidity as a lower limit and 60% as an upper limit (without mentioning a matching temperature range). Higher relative humidity can interfere with natural skin cooling by evaporation of perspiration and lead to a familiar “sticky” feeling. Inside the 30-60% band suggested for human comfort, problems can arise because other creatures also feel at home. House Dust Mites House dust mites are habitual companions in many human environments. They dine mainly on human skin scale and, in return, release allergens in their faeces which can affect susceptible individuals. Dust mites absorb the water they need directly from the air and are viable when relative humidity reaches 45-50%. They need exposure to this level for only an hour each day to become fully hydrated. At higher levels, they feed and multiply more rapidly. Controlling house dust mite population is not simply a matter of keeping relative humidity below 50% because they favour life in carpets, curtains and bedding which are difficult to keep dry enough. Carpets on concrete slabs are particularly prone to dust mite infestation because the surface temperature is likely to be lower than other room surfaces unless the slab is insulated. Figure 3.8 – The house dust mite (which is viable when it finds local relative humidity above 50%) (Source: BRANZ 2012) DRAFT HANDBOOK Mould, Mildew and Fungus Above 60% relative humidity, problems emerge which can more easily be seen. Moulds and fungus, for example, can develop on surfaces in a building or its contents when spores are present with a sufficient nutrient supply, temperatures stay between 4°C and 38°C and relative humidity rises above 70% at the surface. Left to develop, these organisms can produce toxins and irritants with suspected effects on respiratory health. The first two conditions for growth are Australian Building Codes Board Page 21
Information Handbook: Condensation in Buildings easily met because spores are always present in the air and most building materials or furnishings and contents can supply the nutrients. Keeping building occupants comfortable will also ensure that temperatures are amenable for mould, mildew and fungus. These realities require that mould growth be kept in check by controlling relative humidity adjacent to building surfaces. This means minimising water vapour levels and keeping temperatures up at the surfaces since relative humidity depends on both temperature and water vapour concentration. Although air temperature in a room might be at the preferred level, some parts of the walls, floor and ceiling or furnishings can be significantly cooler. This can apply over the whole area or be localised, depending on construction, local shielding from air movement by furniture and fixtures and on heating and cooling arrangements. In colder climates, mould can grow on room surfaces where they are cooled below the room temperature by heat loss to the outside. This can occur where insulation has too little thermal resistance for the severity of the climate or is locally interrupted by framing. Corners of walls and window and door openings are particularly susceptible because they are often constructed with multiple framing members, have limited space for insulation and air leaks through the external cladding is more likely. Surfaces inside cupboards and behind or under furniture that limit the circulation of warmed air or intercept radiant heat are also at risk. Windows, however, may show problems first because they often have the coldest surfaces in a room and respond quickly to falling temperature (Figure 3.9). Figure 3.9 – Surface temperatures of glazing in interior heated to 20°C (outdoors 5°C) Outdoors 0°C Indoors 20°C / 50% RH Wall surface 19°C / 53% RH Single glazing surface 12°C / 83% RH Double glazing surface 15°C / 69% RH DRAFT HANDBOOK Australian Building Codes Board Page 22
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Condensation in Buildings - Australian Building Codes Board

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