Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings In warmer climates or seasons, when air conditioning is used, cooling warm outdoor air will increase the relative humidity. Where water vapour levels in the atmosphere are high, relative humidity indoors is likely to exceed the 70% needed for mould growth. Excessive cooling, without effective dehumidification, can create 100% relative humidity (and dew point) at the cooler surfaces. Example: Increasing relative humidity by cooling In Weipa, during February, the outdoor air has a mean maximum temperature of 31.4°C and a 3pm dew point of 24.3°C, indicating a relative humidity of 66%. If this outside air is cooled to 25°C (with the temperature moving leftwards across the psychrometric chart below), relative humidity rises to 96%. Cooled slightly further, it will reach its dew point and water vapour will begin to condense. 5°C 100% RH at 14°C 10°C 15°C Dew Point 100% RH at 24.3°C 20°C 25°C 0 0°C 5°C 10°C 15°C 20°C 25°C 30°C 35°C 40°C 45°C 50°C Dry Bulb Temperature Cooling to any lower temperature will unavoidably remove water vapour by condensation and lower the absolute humidity. Energy will be used to reduce the temperature of the air and to remove the heat (latent heat of condensation) released by the condensing water vapour. The relative humidity at the lower temperature will always be 100% but the absolute humidity (on the right hand scale) will have fallen significantly. 30°C COOLING WEIPA, February 3pm 66% RH at 31.4°C Water vapour removed DRAFT HANDBOOK 30 25 20 15 10 5 Absolute Humidity (g water vapour / kg dry air) Air will often be chilled below the target indoor temperature, to compensate for mixing with warmer air in the room. If poor diffuser design or placement allows the chilled air to blow against a wall or floor surface (Figure 3.10), it can create cold patches on the surface where the relative Australian Building Codes Board Page 23
Information Handbook: Condensation in Buildings humidity will be higher than the general level in the room. The cooling effect can extend through the lining into surfaces concealed inside the building fabric. Water vapour from outdoors leaking through the envelope construction can then trigger mould growth where it cannot easily be detected, but can still harm indoor air quality. Figure 3.10 – Localised variation in surface temperatures with artificial cooling 17°C 100(+)% RH 19°C 93% RH 21°C 82% RH 23°C 73% RH 25°C 65% RH Conditions at the thermostat: 25°C / 65% RH DRAFT HANDBOOK Australian Building Codes Board Page 24
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Condensation in Buildings - Australian Building Codes Board

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