About Moisture and Humidity
Relative Humidity is usually measured as a percentage of water vapour in the air. The amount of vapour contained in the air can vary with air temperature and pressure.
‘Comfort Level’ – The Effect of Humidity with Temperature Variations.
The effective ‘comfort level’ in a home or office can change with temperature and humidity swings.
When a temperature is below 14° C, humidity has little effect on temperature feel but in a normally heated living area, temperatures of about 22°C, there can be a significant difference in ‘comfort level’ when humidity is high. When humidity is low, a normally warm temperature can feel cool due to faster evaporation of body perspiration. In higher humidity situations, perspiration is slowed providing a warmer feel at the same temperature. High humidity acts like insulation for the human body.
Heating, Ventilation and Air-Conditioning (HVAC) systems
HVAC systems should provide air at a comfortable temperature and humidity levels and filter indoor air contaminants. HVAC systems that are poorly commissioned or maintained inadequately can contribute to poor indoor air quality. In commercial buildings depressurisation in parts of the building can suck in air from undesirable locations like underground car parks.
Effect on Buildings and Their Contents
Many materials (wood, plaster, decorative/paintings etc) absorb water vapor from the air. Materials can absorb more moisture in high humidity situations and release moisture as the relative humidity lowers inside the building envelope.
Moisture soaked plasterboard can, in some cases, damage paintings, clocks, musical instruments, electrical equipment and even building components.
The other consequence of high humidity is condensation occurring behind plasterboard walls that can result in structural damage and can provide the perfect conditions for the growth of mould and mildew. This is even more of a problem in Europe and in some parts of the USA, where it gets a lot colder during winter than in Australia.
Aquatic centres or homes with indoor swimming pools are a perfect example of where humidity can cause issues. A large body of water has consistent evaporation, and the more air that is moving over that water can exacerbate the condensation effect.
Ideally, during the winter months, the temperature of an office should be between 20°C and 24°C with a relative humidity between 30 to 60 percent. During the summer months, an indoor temperature between 23°C and 25°C with a relative humidity between 40 to 60 percent is considered normal.
Computers, office equipment, and paper products can be adversely affected by high humidity and low humidity and shorten their life.
A HVAC system must maintain desirable humidity levels inside the building envelope to prevent mould and mildew growth. In commercial buildings maintaining positive internal building pressure and humidity levels, below 60%, is essential to ensure proper indoor air quality in buildings.
Exhausting air from rooms like toilets, cafeterias and other areas without replacing it, creates a negative pressure that draws unconditioned air through exterior walls, windows, and doors. As the outside air is drawn into a building by the negative pressure that is created by air movement or exhaust systems, the moisture in the air may condense when it comes in contact with cold surfaces. Condensation provides the moisture mould & mildew requires for growth.
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