Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings 3.5 Relative Humidity Relative humidity measures how much water vapour is present compared to the most that the prevailing temperature could potentially support. Figure 3.4 compares limits as temperatures rise with a small fixed amount of water vapour. The fixed amount represents 100% relative humidity in an alpine temperature but smaller and smaller percentages at warmer temperatures. Figure 3.4 – Water vapour limits rising with temperature and relative humidity falling 100% RH in alpine winter 1x 1.5x 2x 3x Approximate increase in limit at each temperature 6x 4x -5°C 0°C 5°C 10°C 15°C 20°C 25°C 30°C 35°C 40°C Dry Bulb Temperature Changes in relative humidity as temperatures rise or fall are indicated on a standard psychrometric chart by curved lines of the sort shown in Figure 3.5. This version of the chart uses the same temperature range (0°C to 50°C) along the horizontal axis as the standard chart and the right hand vertical axis measures the water vapour content of the atmosphere, or its absolute humidity (AH), in grams of water vapour per kilogram of dry air. 8x 11x 15x 20x 7% RH in tropical wet DRAFT HANDBOOK Australian Building Codes Board Page 17
Information Handbook: Condensation in Buildings Figure 3.5 – Absolute and relative humidity on the psychrometric chart Absolute Humidity (from right hand scale) water vapour content 5°C 10°C Dew Point 12°C (100% RH) 15°C 20°C 60% RH at 20°C 25°C 44% RH at 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 To illustrate the difference between absolute and relative humidity, a dotted line runs horizontally across the chart from the 12°C point on the dew point curve. Where the line meets the right hand axis marks the absolute humidity for water vapour with that dew point. Points marked along the same dotted line highlight temperatures from 20°C to 35°C, in five degree steps. By the time the temperature has risen from 12°C to 20°C, the relative humidity has fallen from 100% (its dew point) to 60%. At 35°C, the relative humidity has declined to 25% although the amount of water vapour in the atmosphere (its absolute humidity) has not changed at all. Unlike a dew point temperature, a relative humidity percentage does not identify how much water vapour is present unless a temperature is also specified. 100% relative humidity, which triggers condensation, can occur at any combination of temperature and water vapour level along the curved dew point line of the psychrometric chart. This means that environments can have high relative humidity at low temperatures even if absolute humidity is low. Similarly, high relative humidity can fall away as temperature rises. Moving from the psychrometric chart to a building-related example, Figure 3.6 shows relative humidity in outdoor air decreasing in a heated interior. Water vapour introduced with fresh air from outdoors is heated from 7°C to 20°C, lowering its relative humidity from 70% outdoors to 30% indoors. The absolute humidity of the water vapour, indicated by its dew point of 1.9°C, remains the same indoors and out. 30°C 33% RH at 30°C Relative Humidity (from curved lines) varies with temperature 25% RH at 35°C DRAFT HANDBOOK 30 25 20 15 10 5 Absolute Humidity (g water vapour / kg dry air) Australian Building Codes Board Page 18
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