Published on January 14, 2021
Condensation arises because warm air can hold more moisture than cold air. For example, air at 25°C can hold about 20 grams of water per kilogram of air. This is its maximum water content so it corresponds to 100% humidity. If air in this state is cooled to 15°C, then its maximum water content falls to about half this value and about 10 grams of water must condense out of each kilogram of air. This will occur as a fog of liquid droplets if the air is cooled as a mass, or as condensation on a surface if that surface provides local cooling of the air around it.
Common examples of surface condensation include; condensation upon a window glass and/or aluminium frame as a result of the reduced temperature of the glass and/or aluminium frame in a warm environment or as a result of increasing moisture levels within the room.
Creating a healthy and comfortable living environment, by use of effective building design to reduce condensation and the likelihood of mould growth.
If air is cooled below its dew point (i.e. cooled to a temperature where its relative humidity would exceed 100%, so that it cannot contain all of the water originally present), then a fog or condensation will occur. If the air is relatively moist then the dew point temperature will not be far below the actual air temperature, so condensation occurs readily. Drier air will have a dew point which is proportionately low, so that condensation will only occur if it comes into contact with surfaces which are much colder.
Condensation in buildings
The ingredients for condensation are essentially one or more of the following:
- The presence of moisture levels which are too high.
- The low temperatures of building materials.
- Uncontrolled flow of water vapour from a source to a region of cold temperature.
Moisture levels within buildings are often higher than outdoors, and there are numerous reasons for this. One source of moisture is the ground. Concrete slabs generally provide a waterproof barrier to ground moisture, but buildings with suspended timber floors (typical of many houses) are quite susceptible to high indoor humidity arising from moist sub-floor spaces. Domestic activities produced by; cooking, bathing, showering, drying, high occupancy, high indoor plant concentrations, uncontrolled moisture ingress and domestic appliances such as gas or oil fired stoves, burners and heaters.
Typical quantities of water vapour produced in the home are (in litres per hour):
- Adult (breathing) 0.1
- Hot bath 1.5
- Clothes drier 5.0
- Hot shower 10.0
Condensation on inside surfaces is generally a winter (or heating season) problem. Insufficiently insulated surfaces become cold enough to fall below the dew point of indoor air and condensation necessarily occurs. Aluminium window frames are particularly susceptible because they ‘face out’ on a cold day/night.
As shown in our diagram (based on 21ºC internal temperature), the dew point (start of condensation), is when the outside temperature is at 8ºC for a standard aluminium frame and at -4ºC for a Dowell ThermaLine™ thermally broken aluminium awning window, when humidity inside a home increases due to activity such as people breathing, cooking, showering and drying – with typical internal humidity being around 50-60%. Even in extreme internal humidity level of 60%, Dowell ThermaLine™ dew point is not achieved until the outside temperature falls below 2.5ºC.
Dowell ThermaLine™ thermal barrier reduces moisture build-up and the unsightly mould growth that cause respiratory illness or asthma, chronic cough, rashes (dermatitis) and sinus problems, caused by internal surface condensation.
Reference source: Dowell management using, Dowell ThermaLine™ data and Bureau of Meteorology, extracts from CSRO ‘Building Technology File 2001’ and Dept. of Health, WA ‘Mould and condensation in your home’.