Glazing

Key points

  • Glazing – the glass and frames in windows, external doors and skylights – has a significant effect on thermal performance. Up to 40% of a home’s heating energy can be lost and up to 87% of its heat gained through windows.
  • Improving the thermal performance of your glazing will increase your home’s comfort and reduce energy consumption, therefore lowering costs and greenhouse gas emissions.
  • The thermal performance of a window, door, or skylight depends on how well the glass and frame conduct heat (conduction or U value) and how well the glass and frame transmit heat from direct sunlight (the solar heat gain coefficient or SHGC).
  • There are many different types of glass and frame, and different combinations, to choose from.
  • Both single and double or triple glazed units will allow good solar heat gain, but double or triple glazed units are better at preventing heat loss.
  • Use the Window Energy Rating Scheme ratings to compare the performance of windows, doors and skylights. You can also use the Australian Glass and Window Association tool to calculate the potential savings that may be achieved by energy-efficient glazing.
  • Glazing needs to work together with other passive design features of your home to ensure good thermal performance. You will need to pay attention to orientation, thermal mass, insulation and weather sealing to get the best performance.
  • Glazing decisions are usually made when you are buying or building a new home. But improving your glazing through renovation is one of the easiest ways to significantly improve the thermal performance of your home without rebuilding.
  • Window furnishings (for example, louvres, blinds, curtains) can be used to improve the thermal performance of your windows, and are especially useful to add to existing windows.

Understanding glazing

What is glazing?

Glazing simply means the windows in your home, including both openable and fixed windows, as well as doors with glass and skylights.

Glazing actually just means the glass part, but it is typically used to refer to all aspects of an assembly including glass, films, frames and furnishings. Paying attention to all of these aspects will help you to achieve effective passive design.

Why is glazing important?

Glazing lets in light and fresh air and offer views that connect interior living spaces with the outdoors. Energy-efficient glazing makes your home more comfortable and dramatically reduces your energy costs.

However, inappropriate or poorly designed glazing can be a major source of unwanted heat gain in summer and significant heat loss and condensation in winter. Up to 87% of a home’s heating energy can be gained and up to 40% lost through windows. Improving your glazing’s thermal performance will reduce energy consumption, therefore lowering costs and greenhouse gas emissions.

Glazing is a significant investment in the quality of your home. The cost of glazing and the cost of heating and cooling your home are closely related. An initial investment in energy-efficient windows, skylights and doors can greatly reduce your annual heating and cooling bill. Energy-efficient glazing also reduces the peak heating and cooling load, which can reduce the required size of an air-conditioning system by 30%, leading to further cost savings.

Tip

The Australian Glass and Window Association has developed a tool to calculate the savings that may be achieved through energy-efficient glazing. This tool compares window selections to a base level aluminium window with 3mm clear glass.

Glass properties

Understanding some of the key properties of glass will help you to choose the best glazing for your home.

Solar heat gain is transmitted through a window from direct sun light and through conductive heat.

Key properties of glass

Source: Adapted from the Australian Window Association

Visible light transmittance

The amount of light that passes through the glazing is known as visible light transmittance (VLT) or visible transmittance (VT). A low VLT can reduce heat gain from the sun but if the VLT of your glass is too low, it will be too dark inside the room. This may lead you to switch on lights, which will result in higher energy costs.

Conduction

Conduction is how readily a material conducts heat. This is known as the U value. The U value for windows (expressed as Uw), describes the conduction of the whole window (glass and frame together). The lower the U value, the greater a window’s resistance to heat flow and the better its insulating value.

The amount of heat conducted through a glazed unit (in watts; W) is calculated as Uw × T × A:

  • the U value (Uw)
  • multiplied by the number of degrees difference in air temperature on each side (T)
  • multiplied by the area of the glazing unit (A).

For example, if your home has 70m2 of glazing with aluminium frames and clear glass with a U value of 6.2W/m2 °C, on a winter’s night when it is 15°C colder outside compared with indoors, the heat loss through the windows would be:

6.2 × 15 × 70 = 6510W

That is equivalent to the total heat output of a large room gas heater or a 6.5kW room air-conditioner running at full capacity.

If you choose a window with half the U value (3.1W/m2 °C) (for example, double glazing with an argon-filled gap and less-conductive frames), you can halve the heat loss:

3.1 × 15 × 70 = 3255W

Solar heat gain

The solar heat gain coefficient (SHGC) for windows (expressed as SHGCw) measures how readily heat from direct sunlight flows through a whole window (glass and frame together). SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits to the house interior.

Glazing manufacturers declare an SHGC for each window type and design. However, the actual SHGC for windows is affected by the angle that solar radiation strikes the glass. This is known as the angle of incidence. The angle of incidence is influenced by the orientation of the glazing and the position of the sun according to location, season and time of day.

When the sun is perpendicular (at 90°) to the glass, it has an angle of incidence of 0° and the window will experience the maximum possible solar heat gain. The SHGC declared by glazing manufacturers is always calculated as having a 0° angle of incidence.

As the angle increases, more solar radiation is reflected, and less is transmitted. It falls sharply once the angle exceeds 55°. Also, as the angle increases, the effective area of exposure to solar radiation decreases.

A diagram shows the sun’s radiation hitting a window at midday in summer where the angle of incidence is high, and the solar gain is low. It shows the sun hitting a window in the late afternoon where the angle of incidence is low, and the solar gain is high.

Solar gain varies according to the sun’s angle of incidence

Source: Adapted from Association of Building Sustainability Assessors

The same window can have different solar gain depending on its position and the season. A north-facing window in summer, when the sun is high in the sky, may have an angle of incidence of 8° (depending on location). In winter, the angle of incidence at midday is about 35° and the glass is exposed to a greater effective area of solar radiation. That window can transmit more solar heat in winter than in summer.

A west-facing window on a summer’s afternoon has an angle of incidence from near 0° up to 30° with a large effective area of solar radiation. A north-facing window, in summer, has a high angle of incidence and a low effective area of solar radiation, so can transmit less heat than a west-facing one.

Achieving good glazing

Glazing decisions are usually made when you are buying or building a home. But you can quickly and easily improve the thermal performance of your home by replacing your windows. This is one of the most effective methods of renovation to achieve improved thermal comfort.

Glazing choices

There are thousands of types of glass and frames to choose from. Choosing the right ones is important to improving the energy efficiency of your home. Specific products have been designed to keep heat in or out and have varying impacts on daytime lighting, noise control, maintenance and security.

Glass

There are many different types of glass products to choose from.

Single glazing

Single glazing uses a single pane of glass. Single glazing with clear glass is not very efficient when it comes to heat loss or gain. To improve performance, you can use single glazing with a more energy-efficient type of glass such as low emissivity (low-e) glass.

Double or triple glazing

Double or triple glazing (also known as insulated glass units or IGUs), is the combination of 2 or more layers of glass sealed into a frame with a gap between the layers. Multiple layers can be assembled with sealed cavities between each sheet of glass.

IGUs generally offer better energy performance than single glazing, because they transmit less energy. However, the energy performance of IGUs also depends on:

  • the properties of each layer of glass. Different glass types (for example, clear and low-e glass) can be put together in an IGU. Different combinations of clear, toned and low-e glass can deliver a wide range of SHGC and VLT values to suit your performance needs
  • the contents of the cavity. IGU cavities can be filled with air or a more inert, low-conductivity gas such as argon
  • the width of the cavity. Cavity thickness is usually 6 to 18mm. Wider cavities provide lower (better) U values, with 12mm normally accepted as the preferred gap
  • how well the cavity is sealed. Cavities must be dry and well sealed to prevent moisture getting in. Manufacturers have special dry rooms for IGU assembly. If argon is installed to the cavity in place of air, moisture is reliably excluded
  • the level of desiccant (drying agent). The spacer (metal or polymer strip) that separates the glass layers contains a desiccant to absorb any moisture. Inadequate desiccant may cause moisture to condense on the glass surface in cold conditions, reducing thermal performance.

It is sometimes wrongly assumed that insulated glazing is only for cold climates. In fact, IGUs can deliver better energy performance for all climates, especially in heated and air-conditioned homes.

An illustration demonstrating the properties of single, double and triple-glazing units.

Cross-section detail of single, double and triple-glazing units

 

Low emissivity glass

Low emissivity glass (commonly known as low-e glass) reduces heat transfer. Low-e glass may be either high or low transmission:

  • High transmission low-e glass has a coating that allows daylight from the sun to pass into the house to achieve good solar heat gain, but reduces the amount of the long wavelength infrared heat that can escape back through the window.
  • Low transmission low-e glass has a coating that reduces the amount of solar heat gain while still maintaining good levels of visible light transmission.

Low-e glass has either a pyrolytic coating or a vacuum-deposited thin film metal coating. Pyrolytic coatings are durable and can be used for any glazing; vacuum-deposited coatings are soft and are only used within IGUs.

Low-e coatings can significantly improve both U value and SHGC; however, they must be used correctly or they will either deteriorate or fail to perform as required. They are often more susceptible to surface damage than standard glass. Low-e coatings can be used in combination with clear, toned or reflective glass.

Low-e coatings

Low-e coatings on glazing can reduce heat transfer where needed

Photo: Department of Industry, Science, Energy and Resources

Toned glass

Toned glass has colouring additives included during manufacture. It is available in various colours, usually bronze, grey, blue and green. Different colours will change the amount of visible light transmitted (VLT) and the SHGC; however, the colours do not change the conduction (U value) of the glass.

Toned glass options include ‘supertoned’ glass, which has heavier colouration that transmits visible wavelengths while filtering out solar near-infrared wavelengths. This provides improved energy performance by lowering solar heat gain but does not affect light levels.

Laminated glass

Standard glass will readily break into long shards and small sharp slivers. Laminated glass has a plastic glazing layer, called an interlayer, which is adhered permanently between 2 sheets of standard glass. This reduces the danger of the glass breaking, and if it does break, keeps all shards in place so they do not form loose dangerous shards.

Laminated glass is often used in areas in the home most prone to injury from human impact such as bathrooms, doors, around staircases and in areas close to the floor (it meets the requirements of ‘safety glass’ that is mandated for use in these areas by Australian Standard AS 1288 Glass in buildings).

Careful selection of different interlayer types can also address noise concerns and energy efficiency requirements to some extent, but it is not a substitute for double glazing.

Toughened glass

Toughened glass has been ‘tempered’ by being reheated and quickly cooled again. This process makes it much stronger than standard glass – it can resist higher impact loads before breaking. It also makes it safer because, when it does shatter, it breaks into many small cubic pieces rather than dangerous shards. It can be used as ‘safety glass’ as mandated in Australian Standard AS1288, and may be mandated in bushfire-prone regions. However, toughened glass has no thermal or acoustic benefits over other glass of the same toning or thickness.

Secondary glazing

Secondary glazing is where single-glazed windows are retrofitted with a transparent acrylic or glass sheet attached to the inside of the frame or openable sash with a secondary frame or with magnetic strips. This creates an air space between the 2 layers, which reduces the U value and air infiltration. Secondary glazing will not perform as well thermally as a manufactured IGU, since it is impossible to totally seal the perimeter, but it can provide good noise control.

Films

Window films are a thin polymer film containing an absorbing dye or reflective metal layer, with an adhesive backing. They stick to your glazing to change its colour or make it reflective. They can be a cost-effective way to improve the thermal performance of existing windows or doors.

Applied to existing glass, some window films can halve the overall SHGC of the window by absorbing and/or reflecting solar radiation. This can be particularly beneficial in hotter climates where cooling is the main concern, or on east and west elevations directly exposed to long periods of sunshine.

However, window films may also reduce visible light transmittance. In addition, glass panes with applied films exposed to direct sun become hotter than untreated glass and industry guidelines must be followed to avoid thermally induced cracking. For this reason, it is generally best to use an accredited installer of window film.

Frames

Frames have a significant impact on the thermal performance of windows and doors, because energy can be gained and lost through the frame, as well as through the glass. Different types of frame will allow different levels of heat gain and loss, so careful choice of frame is important for effective passive design.

Energy performance of common window types

Window type

VLTw

Uw

SHGCw

Aluminium frame, single glazed with 3mm clear glass

0.80

6.9

0.77

Timber or uPVC frame, single glazed with 3mm clear glass

0.72

5.5

0.69

Aluminium frame, double glazed with 3mm clear glass/6mm air gap/3mm clear glass

0.72

4.2

0.69

Timber or uPVC frame, double glazed with 3mm clear glass/6mm air gap/3mm clear glass

0.65

3.0

0.61

Timber aluminium composite window, triple glazed with 4mm glass/16mm argon gap/4mm low-e glass/16mm argon gap/4mm low-e glass

0.36

1.00

0.25

SHGCw = solar heat gain coefficient for windows (lower values mean less heat gain); Uw = conduction for windows (lower values mean better insulation - more resistance to heat flow); VLTw = visible light transmittance for windows (lower values mean less available light)

Source: Window Energy Rating Scheme

Aluminium

Aluminium frames are light, strong and durable, and come in a variety of powder-coated and anodised finishes. However, aluminium is also a very good conductor of heat and will decrease the insulating value of a glazing unit, unless specifically engineered to reduce this.

Thermally broken aluminium

A ‘thermally broken’ frame is made up of 2 aluminium sections connected by a structural insulator (typically a low-conductivity structural polymer). This ‘breaks’ the thermal connection through the aluminium and reduces the heat flowing through the frame. Thermally broken frames are among the highest performing frames available, and are appropriate for almost any climate. They can be expensive, but prices are decreasing as they become more common.

Timber frames

Timber frames are a good natural insulator that can suit some home designs. Timber frames should be made from species that have naturally high durability or be treated to prevent decay and deformation. Check that the timber is sourced from a sustainably managed forest.

Because timber is an organic material that swells and shrinks with changes in atmospheric humidity, window and door frames require larger tolerances in openings. However, this can result in gaps that allow air infiltration unless good draught sealing (weather stripping) is installed.

uPVC

uPVC is a form of plastic (unplasticised polyvinyl chloride, also known as rigid PVC). uPVC frames provide excellent thermal performance, often better than timber or thermally broken aluminium. uPVC is long lasting and requires very little maintenance, and can be moulded into complex profiles that provide excellent air seals. uPVC frames are increasingly common in Australia, and are already very common in Europe and North America.

uPVC doors

uPVC doors and windows have excellent thermal performance

Photo: Ben Wrigley (© Light House Architecture and Science)

Composite frames

Composite frames use aluminium profiles on the outer sections with either a timber or uPVC inner section. These combine the low maintenance and durability of aluminium with much improved thermal performance.

Fibreglass and other materials

Materials such as fibreglass are currently of limited availability in Australia. Steel has had a small resurgence as a window and door framing material, especially in buildings needing high fire ratings. Some steel frames are set up specifically for double glazing with thermal breaks, and thus may be good insulators.

Frame styles and sash design

Windows and doors come in a wide range of styles or configurations that can affect energy performance in several ways. For example, different styles provide different opening areas, which determine how much cross-ventilation can be gained. For a 2-pane window, maximum opening for different styles are:

  • casement 95%
  • louvre 90%
  • tilt and turn:
    –    turn mode 95%
    –    tilt mode 20%
  • sliding 45%
  • 2-panel sliding door 45%
  • double hung 40%
  • awning 20–30% (depending on height and operator mechanism).

Awning windows have chain winders to control operation with a maximum throw of less than 300mm – so they do not open very wide at all. These will not be very effective at encouraging incoming breezes, unless the breeze is quite strong. However, they are very effective at keeping rain out. They can also be useful at allowing air to exit a building, especially if not too tall, so that each sash can open to at least 45°.

Casement windows oriented so they open towards the welcome breezes ‘reach out’ and steer the passing breeze inside, and so are very effective in forcing cross-ventilation. If useful cooling breezes always come from known directions, such as a coastal sea breeze, then casements on the windward side and awnings on the leeward side can be a good combination.

Sliding and double-hung windows limit the area of a window that can be opened (unless they slide beyond the window across the wall, but these are highly bespoke windows and are often very draughty). They also cannot ‘reach out’ to passing breezes and are poor at preventing rain entry.

There are an increasing number of European-style ‘tilt and turn’ windows available, generally with good Uw values and superior air tightness. These work by a double-action handle mechanism that alternatively allows the top of the sash to tilt inwards while hinged at the sill, or turn inwards like a casement, while hinged at the side. The flyscreen is generally fixed on the outside. Their ability to ‘reach out’ to passing breezes is limited, but can be balanced by their good performance in other ways.

" "Diagram illustrates 6 different window types; Awning, Casement or hinged, Tilt'n'Turn, Double Hung, Louvre and Sliding.

Window frame and sash styles can provide a variety of openings

 

Window furnishings

Window furnishings, blinds and curtains can improve the overall thermal performance of your window systems, and can be an effective way to overcome problems with existing windows.

Reduce convective heat transfer through windows with snugly fitted blinds and curtains with pelmets that trap a layer of still air next to the window. Eliminate air gaps around all perimeters of the curtain and pelmet to improve performance.

Curtains with a built-in pelmet trap

Curtains with a built-in pelmet trap a layer of still air next to the window

Photo: Getty Images

Heavy fabrics and multiple layers of fabric help to increase the insulation provided by curtains by reducing the amount of heat flow between the air in the room and the air adjacent to the window. This benefit is reduced if there is air movement around the curtain.

In double and triple glazing, internal blinds between the glass layers can reduce some solar heat gain by blocking direct solar radiation and reflecting incoming heat back out through the window. To help reflect solar heat, the external surface of blinds should be white or near-white, and some manufacturers offer a metallic, reflective film on the external surface with a decorative fabric facing in. However, internal blinds are much less effective than preventing the solar heat from entering the window in the first place.

Blinds hang within a window space, with two layers of fabric joined into cells to create an insulative barrier.

Tightly fitted honeycomb blinds are a popular window furnishing

Photo: Felicity Woodhams

Glazing and thermal performance principles

The choice and design of glazing is of critical importance to the thermal performance, comfort, and economy of your home. The impact of glazing on the thermal performance of a building is complex. It involves the interaction of the following:

  • climatic conditions in your location — temperature, humidity, sunshine, and wind
  • building design — the orientation, form, and layout of the building
  • building materials — the amount of thermal mass and insulation
  • the size and location of glazing assemblies and shading
  • thermal properties of the glazing system.

All climates

A lowest possible U value for glazing is better in all climates and orientations, because it is the U value that measures the ability to retain heat in winter and cold in summer.

Some simple principles can be followed when choosing glazing to optimise the thermal performance of your home:

  • Locate and size glazing and shading to let sunshine in when the temperature is cold and exclude it when it is hot.
  • Use thermal mass to store the sun’s heat and provide night-time warmth in cold conditions.
  • Locate window and door openings to allow natural cooling by cross-ventilation.
  • Provide effective seals to openings to minimise unwanted draughts.

The implementation of passive solar design principles can be more challenging on some sites. For example, winter sun might be blocked by neighbouring buildings, or views may be to the south or west, often leading to the inclusion of windows with poor orientation. In these instances, choosing glazed elements with improved thermal performance can compensate for aspects of the building design that are detrimental to its thermal performance.

Hot climates

In hot climates such as Darwin and from Brisbane north on the east coast, SHGC is more influential than the glazing U value. There is little room for compromise: a low SHGC is best. A combination of a small U value and proper shading can be a good option to decrease the cooling demand.

In those climates, if passive solar gains are desired in the coolest months, they should be obtained from north-facing glazing only. These can have a slightly higher SHGC if they are protected by proper shading. East- south- and west-facing glazing should be totally shaded or, if that is unachievable, very low SHGC values.

Temperate climates

In temperate and cool temperate climates such as Sydney, Perth, Melbourne, Adelaide and Hobart, north-facing glazing should have a high SHGC. However, these windows and doors need to be correctly shaded – this means fixed or moveable (active) shading designed to shade as little of the glass as possible in winter and as much as possible in summer.

East- and west-facing glazing should have deep overhangs or active shading devices that effectively protect all of the glass from summer sun early and late in the day. The SHGC of south-facing glazing is less critical because there are lower solar heat gains from that direction in winter or summer in these latitudes (but not in the tropics).

It is a common misconception that north-facing glazing should be single glazed to obtain the best house energy rating, while other aspects should be double glazed. In reality, both single and double glazing will admit a lot of direct solar radiation. This means they can be used for heat gain in winter, and should be well shaded in summer. The difference is that double glazing will prevent heat loss. This is particularly useful in temperate and cool climates.

Cool climates

In cool climates such as Melbourne, Hobart and Canberra, the key driver of annual energy consumption due to glazing is the U value, because it governs the level of heat loss. However, passive solar gains from glazing with high SHGC can significantly reduce the heating demand during the day and into the evening. The need for heating may not be totally eliminated, especially if the potential solar gain is partially blocked by trees or neighbouring buildings.

Design and product considerations

Orientation

It is common practice to use the same windows on all elevations, but optimising windows by orientation is likely to yield a better performing home (by more than half a star).

Different types of glass should be considered for different orientations in your home. Care should be taken if windows within the same room have different glass types, to ensure any differences (for example, in tone) are not too visible. Using thermal modelling like the Nationwide House Energy Rating Scheme (NatHERS) software tools can add valuable design information, by running a series of scenarios to determine the best performance value of different glazing options.

Thermal mass

For thermal mass to work effectively (for example, provide beneficial evening heat in cool climates) glazing needs to let in solar radiation during the day to warm the mass. You should ensure that you have the right ratio of glass to mass to make sure enough solar radiation reaches the mass during the day.).

If thermal mass is used in warm and hot climates to absorb heat during hot days, natural ventilation should be used on cooler evenings to let out the stored heat (night purging). In these climates, the thermal mass should not be directly exposed to any solar gain or it will act as a continual heat bank.

Seals

Glazing that is poorly sealed, allowing cold draughts in winter or hot draughts in summer, will compromise the thermal performance of the window. Ensure your windows and doors are installed with good seals between the frame and the surrounding wall.

Also ensure your window design has good seals between sashes and their surrounding frames. In general, awning windows and casement windows, which seal by compression, control air leakage much better than do sliding windows and doors, whose seals tend to lose their shape and wear out gradually from constant friction.

Noise control

Increased thickness and/or multiple layers of glass (laminated or double glazing) is the most effective way to control noise. Sealing cracks and gaps around windows and doors and elsewhere will also help.

Sealed double glazing reduces transmission of medium to high-frequency noises such as the human voice. To reduce low-frequency noise such as traffic and aircraft, choose thicker or preferably laminated glass, preferably double glazed with the largest air gap possible; but note that such large gaps may allow convection to occur between the panes and thus reduce insulating properties.

Condensation

Water will condense onto cool surfaces. The inside of efficient glazing assemblies will stay at the same temperature as the room, preventing condensation and the risk of mould. Less-efficient glazing will transmit exterior temperatures more or less directly to the interior surface, thus creating greater differences between room temperature and glass surface temperature and causing condensation to form. The easiest way to prevent condensation is to choose energy-efficient windows and doors.

Fading

Exposure to sunlight causes many modern interior furnishings to fade. The wavelengths most responsible for fading are the ultraviolet (UV), violet and blue wavelengths. Appropriate glazing can block some of these wavelengths and reduce fading, but does not prevent it completely. Glass that is transparent to visible light absorbs nearly all UVB, the wavelength that causes most fading, but does not absorb as much UVA, so some fading will always occur.

Ratings and regulations

Ratings

All glazed window and door systems in Australia are rated according to guidelines recognised by the Australian Fenestration Rating Council (AFRC). The testing conditions and documentation procedures recognised by the AFRC are based on the procedures of the US National Fenestration Rating Council (NFRC).

When you look at products, you might find ratings from AFRC, NFRC or the Australian National Average Conditions (ANAC). You might also encounter marketing terms such as ‘energy-efficient’. The various ratings and descriptions can be confusing or misleading. Be absolutely sure, when selecting and specifying products, that the declared U value and SHGC values are according to the AFRC requirements or you could end up with products that do not meet performance expectations and may not comply with regulatory requirements. Look for evidence that the ratings are AFRC approved. If you are not sure, question the supplier, or refer to the Australian Glass and Window Association.

Window Energy Rating Scheme

The easiest way to find out details on products you are interested in is to check the Window Energy Rating Scheme (WERS). WERS rates the energy and energy-related performance of windows, skylights and glazed doors in accordance with AFRC procedures.

WERS provides the U value and SHGC values of the window system, as well as air infiltration and visible light transmittance details. It also provides a simple star rating of window systems according to their heating and cooling performance. It includes thousands of specific products from most manufacturers, listed according to the types of frame and glazing.

Window Energy Rating Scheme logo

Regulations

Most states and territories have regulations on glazing performance, and you are advised to check how these affect your project at an early stage because they can affect glazing area and budget, which are fundamental design considerations.

All states except New South Wales and the Northern Territory use the glazing provisions contained in the National Construction Code (Part 3.6 Glazing and Part 3.12 Energy efficiency).

New South Wales uses BASIX as the regulatory instrument, which in turn relies on either the ‘DIY tool’ which emulates full computer simulation, or computer simulation using one of the Nationwide House Energy Rating Scheme (NatHERS) accredited tools (AccuRate, BERS Pro, HERO or FirstRate 5).

The Northern Territory uses a 2009 version of the Building Code of Australia, because of the traditional prevalence of naturally ventilated buildings in the territory.

National Construction Code

Under the National Construction Code (NCC), window manufacturers are required to supply windows and glazed doors that meet mandatory minimum specifications for structural sufficiency and water penetration resistance under Australian Standard AS2047 Windows in buildings, selection and installation. Energy-efficiency provisions state that windows performance data must be determined in accordance with the guidelines of the Australian Fenestration Rating Council (AFRC).

The NCC includes a requirement that all openable windows with a sill height below 1.7m above the floor and with internal floor levels more than 2m above adjacent external ground level must be fitted with window locks that stop the window being opened more than 125mm or have reinforced screens to prevent children from falling from a height. This requirement limits the use of traditional windows that open wide for cross-ventilation, and can compete with the NCC provisions for natural ventilation. The NCC also mandates minimum daylight in buildings. It is wise to obtain expert professional advice in relation to these matters.

Some window types are better able to comply with this regulation, such as louvres, but in single glazing these have very high Uw values. Double glazed louvres are now available, and more products are likely to follow soon.

Other window types, such as casement (Queenslander style), will require either strong security screens fitted, or have limiters installed. Both of these can only be removed with a tool rather than by hand, and must be able to withstand horizontal impact force.

Passive House certification

Glazing requirements for the Passive House design standard are far more stringent than the NCC, in recognition of glazing’s potential as a source of significant unwanted heat transfer. Standards for air tightness are also high. There are a small number of Australian and imported engineered glazing products that meet ‘Certified Passive House’ standards.

Specifying and documentation

Because windows have a major impact on the thermal performance of buildings, and because there are thousands of different types, they should be clearly specified and documented in your building plans. Inadequate specification and documentation can lead to products being used that do not meet the intended performance standard and may fail to satisfy regulatory requirements — leading to potentially expensive errors.

Glazing specification should include:

  • size – height × width
  • head heights and sill heights above floor
  • whole system U value and SHGC
  • Window Energy Rating Scheme (WERS) rating
  • glass thickness (this is often included in thermal specifications; the requirements of Australian Standard AS 1288 Glass in buildings must take precedence for safety and wind loading)
  • opening types
  • details, and aesthetic or maintenance considerations.

Ensure that any sales advice you receive is consistent with the information contained here. Refer any queries to a qualified professional, or the technical advisors at the Australian Glass and Window Association.

References and additional reading

Learn more

Authors

Original authors: Dr Peter Lyons, Bernard Hockings 

Contributing authors: Chris Reardon, Chris Reidy

Updated: Tracey Gramlick, Richard Hamber, 2013; Dick Clarke, 2020