In last month’s article we reviewed the role of glazing in achieving the desired star rating and its two main properties - Solar Heat gain Co-Efficient and U-Value.

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.

Assessors can choose from a wide range of glass types and frames when determining what is the best solution for a particular design.

Let examine what glass types and frames are available today.

Glass types

Toned or tinted glass has colouring additives included in the glass during manufacture. A range of colours is available with tinted grey being a common choice of builders seeking a cost effective solution to reduce solar heat gain into the house. The different colours and tints provide better SHGC values than clear glass and some variation in visible light transmittance. Glass tinting does not change the U-Value of the glass because glass conductivity and emissivity are unaffected by the presence of a pigment in the glass.

Supertoned or heavily tinted glass has higher levels of additives in the glass to filter out the solar near-infrared heat energy whilst maintaining a reasonable level of visible light transmittance.

Reflective glass has either a vacuum-deposited thin-film metal coating or a pyrolytic reflective coating added during manufacture. They must be kept clean to work effectively.

High transmission low emissivity (low-e) glass has a thin metallic coating added during manufacture that allows daylight from the sun to pass into the house but reduces the amount of the longer wavelength infrared heat (radiated by heated objects within the house) that can escape through the window.

Low transmission low-e glass has a coating added during manufacture which reduces the amount of solar heat gain while still maintaining good levels of visible light transmittance and may provide an improvement in both U-Value and SHGC of glazing units.

The thickness of glass has virtually no impact on its U-Value and SHGC. It does though, have a significant impact on noise transmission, the strength and safety of the glazing.

Glass is available in a single sheet, or as two sheets with a laminate between the glass. The performance of laminated glass is determined by the type of glass in each layer.

It is often wrongly assumed that double glazing is only for cold climates. In fact, the best performance levels in both U-Value and SHGC can only be achieved by double-glazing, particularly in air conditioned homes. Multiple layers of glass can be assembled with sealed cavities between each sheet.

This is commonly called double or triple glazing but is now increasingly referred to as an Insulating Glazing Unit (IGU).

Insulating Glazing Unit:  The insulating value of the air gap in an IGU increases the resistance to heat flow.  Hermetically sealed units usually have a desiccant (a material that absorbs moisture) in between the glass panels to stop internal condensation. The gap between the two panes can be filled with a gas that is less conducting than air such as argon.

Glass has an emittance of about 0.9 that leads to a significant amount of heat being radiated across the gap in double glazed systems. Thin low emittance films can be added to the internal surfaces of double glazing to reduce the  heat radiated from the glass across the air gap.

Windows incorporating these films are referred to as being "low E".

Window films can be an cost effective option for significantly reducing solar heat gain through existing windows. Applied to existing glass, some window films can halve the overall SHGC of the window by means of absorption and/or reflection of solar radiation. They may also cause an equal reduction in visible light transmittance which must be considered when choosing a film.

Window films do not generally have significant impact on the glazing U-value because they do not add thermal resistance nor reduce the emissivity of the glass.

Frames: Aluminium frames are common in many windows used in Australia but conduct the largest amount of heat per unit area.  To improve the performance of the window, use frames such as timber, PVC, or aluminium incorporating a thermal break to reduce heat loss through the windows in winter and heat gains in summer.

Even though the frame is a small fraction of the total window area, around 10 - 15%, the impact on the conducted heat flow through window as a whole may be large and varies with type, size and material of the frame.

However, it is the high transmittance of heat through glass that is of more concern.  Six times more heat is conducted through glass than an equivalent area of wall.

Commonly available frames in Australia include :

Aluminium window frames are light, strong, durable and easily extruded into complex shapes, but aluminium is a good conductor of heat and can decrease the insulating value of a glazing unit by 20 to 30 percent. Aluminium frames, especially dark coloured ones in full sun, absorb a lot of solar heat and conduct it inside.

A thermal break is often used to reduce the heat conducted through aluminium frames. It separates the exterior and interior pieces of the frame using a low-conductivity component.

Timber frames are a good insulator but requires more maintenance than aluminium. Timber species must have naturally high durability or be treated to prevent decay and deformation.

Composite frames use thin aluminium profiles on the outer sections with either a timber or uPVC (unplasticised polyvinyl chloride) inner section. These provide the low maintenance and durability of aluminium plus improved thermal performance.

uPVC frames are petroleum derived products which are relatively new in Australia but common in Europe and North America. Their insulating properties are similar to timber and they can be moulded into complex profiles that provide excellent air seals.

The colour range is more limited than powder coated aluminium.

(Sourced from Your Home Technical Manual - Fourth Edition as amended - 2010)

What should assessor look for when choosing glazing to model in the software tool?

Broadly, assessors should follow the following principles.

In cool to cold climates - select a glazing unit with a low U-Value and high SHGC.

In warmer climates units with a high U-Value and low SHGC may be better.

In mixed climates - find the best balance between U-Value and SHGC, use eave width and external shading devices (vertical to the east and west elevations and horizontal to the north) to achieve the optimum result.

There is no single solution or guide available to assessor when specifying glazing. Historically glazing to the whole house has been the same specification. In the future, glazing will be specified differently to each part of the house according to its orientation and the role it plays in achieving good thermal performance.

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