Condensation in Buildings - Australian Building Codes Board

Information Handbook: Condensation in Buildings
Examining which of the curving relative humidity lines pass through the two comfort zones
provides some useful observations. The Birdsville case suggests that building occupants
acclimatised to summer in a hot-dry climate may tolerate relative humidity lower than 20% and
up to about 60% (from bottom-right corner of the comfort zone up to top-left). People here would
notice discomfort before relative humidity reached levels likely to promote mould growth on
building surfaces. By contrast, adaptation to the cold Thredbo winter climate allows relative
humidity approaching 100% to feel acceptable. In colder climates, problematic relative humidity
levels may develop without providing any sensory alert to occupants.
Outdoor climate data is added to the psychrometric chart, using twelve straight lines to
represent average monthly conditions. In Figure 4.11, which has Richmond (NSW) as an
example, the lines are red for summer months, orange for autumn, blue for winter and green for
spring. Each monthly line connects two points on the chart. The points are defined by:
1. the average (or mean) minimum temperature for the month and the mean dew point
temperature at 9am (which, it is recognised, might not occur at the same time);
2. the mean maximum temperature and the 3pm dew point temperature.
A midpoint on each line, with a white centre, shows the average dew point temperature for the
month and the average temperature.
Figure 4.11 – Climate Analysis on the Psychrometric Chart: Richmond (NSW) – Mild Temperate
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Australian Building Codes Board Page 41