When carrying out an energy rating, assessors work with their clients to find a cost effective way of complying with the energy efficiency requirements of the Building Code of Australia. One of the first strategies employed is to add insulation to the roof/ceiling and to the external walls. Adding insulation to the roof/ceiling and the external walls often provides a noticeable improvement in the star rating.

However the thermal performance achieved by good levels of insulation is degraded by the provision of windows and doors. It is like punching a series of holes in the wall that allows unwanted heat flow to occur, in either direction, that has a significant impact on the star rating that can be achieved.

Today the trend is to design larger areas of glass into our homes that makes the task of achieving the star rating specified by the client more difficult.

The glazing in our homes is the weak link in the building envelope to achieving a good star rating.

This month’s article is a brief review of glazing and its main properties as they impact on the thermal performance of a building.

The thermal performance of glazing is determined by two main properties - the Solar Heat Gain Co-Efficient (SHGC) and the U-Value. Some manufacturer’s data refers to the Shading Co-Efficient (SC) of glazing that compares the glass to 3mm clear glass.

This property is not often referred to and has been superseded by glazing’s SHGC.

Glazing allows sunlight into the building, which is valuable for daylight and winter heating, but can be a problem if there is too much in summer. In winter, passive heating  from the sun that is gained  through the glazing must be greater than the heat lost from the house to the cold outside air to be worthwhile.

Glazing in a building can be a major source of heat loss in winter and a source of heat gain in summer when the temperature difference between inside and outside the building is high.

Other properties quoted for glazing that do not impact directly on thermal performance are visible light transmittance and air infiltration.

Solar Heat Gain Co-Efficient (SHGC)

Sunlight and heat (solar radiation) from the Sun passes through a standard clear glass window very easily. The amount of heat that is transmitted through the window depends on the angle of  the sun to the surface of the window. As the angle increases the amount of reflected heat increases and the amount transmitted decreases.

The amount of heat transmitted through a window is also affected by shading from the eaves and other shading devices.

The transmission of solar radiation from the sun through standard window glass is illustrated below.

The amount of heat absorbed by the glass is dependent on the thickness of the glass and its absorption coefficient.   The absorbed radiation is converted to heat, which increases the temperature of the glass.  Often the glass temperature will be greater than either the indoor or outdoor temperature.  Some of the heat absorbed by the glass will therefore be re radiated and convected into the building.

The ratio of this transmitted energy to the total incident energy is known as the solar heat gain coefficient, (SHGC).

Although the example above shows the incident radiation coming from the sky and making a glancing angle with the glass, the SHGC is actually measured with the incident sunlight perpendicular or at right angles to the glass.

The value quoted for the SHGC is often reduced by 10% to take into account transmission loss due to dirt build up.

U-Value

Heat energy will be conducted through the glass and frame due to the temperature difference from one side to the other. The greater the difference in temperatures – the greater the heat flow. Different frame and glass materials have different abilities to conduct heat, specified by the U-Value.

The lower the U-value – the less heat is transmitted.

U-Values are described individually for the frame or  the glass but industry uses the U-Value by combining the  glass and frame which is referred to as the system U-Value. The system U-Value depends on the U-Values of the frame and glass and the proportions of the area of the glazing unit occupied by each, which are referred to as the frame fraction and vision fraction respectively.

The heat transfer through a double glazed window is illustrated below.

Windows in Australia are certified for their energy performance by rating organisations who conform to Australian Fenestration Rating Council (AFRC) standards. In the AFRC system, performance is always certified for the whole system – glazing and frame combined – never the glass or the frame alone.

The Australian Windows Association maintains a website (www.wers.net) that lists the properties of glazing units of its members.

Design Considerations

In regions around Australia (generally the warmer and hotter climates), where no winter heating is required, the heat gained through glazing in all orientations should be minimised by eaves and shading devices.

In all other regions, windows facing north should have glass with a high SHGC to allow for heat gain in winter, whereas the east and west facing windows should have glass with lower SHGC values to minimise overheating in summer.  In sub tropical regions, south facing windows only receive diffuse and reflected radiation (except for short periods at the beginning or the end of the day in summer), so they don’t contribute to overheating to the same extent as east and west facing glazing.

The easiest way of reducing solar heat gain is to reduce the area of glazing in the wall.  If this is not possible or desirable then windows can be shaded.  Alternatively, tinted or reflective glass can be used to cut down on the solar heat gain.

In colder climates, windows with better U-Values and a higher SHGC should be considered to improve the thermal performance.

Next month’s article will examine the wide range of glass and frame options that are available in the market today and what works best in different climate zones.

Michael Plunkett is Principal of SmartRate and a Corporate Member of BDAV.