WERS 101: what and why of testing windows’ thermal performance in Australia

Trial and test window system performance

We test the EDGE Architectural window and door systems in accordance with AS2047, the standard for windows and doors. It entails testing a physical product at full proportions (within 10 per cent, generally, of maximum size). The test entails deflection, to the ‘serviceability’ – expected wind load for the product in service, air infiltration (air leakage) and water testing at different pressures usually till it fails (giving you a product’s limits). Then we do an ultimate test which has to be 50 per cent higher than serviceability which ensures the product doesn’t fail (collapse or permanently distort) to simulate a 100 year storm.

WERS is a theoretical software calculation. Developed from the US NFRC program, WERS uses its modelling and thermal calculation software. WERS is based on a fixed sample size, which is usually calculated by energy raters using a BCA spreadsheet or energy rating software like First Rate or Accurate.

To understand why WERS was developed, first we must dive deeper into the background of windows in Australia.


Windows are possibly the most complex and interesting elements in the fabric of our homes and buildings. They provide light and fresh air, and offer views that connect our interior living spaces with the great outdoors. However, windows can be a major source of unwanted heat gain in summer and significant heat loss in winter.

Energy efficient windows

Remarkable new framing and glazing materials changed the energy performance of Australian windows in a radical way. Energy efficient windows make a home or building more comfortable, increase usable space within the structure, dramatically reduce energy costs and help to create a brighter, cleaner, healthier environment.

Window standards: building classification

Building classifications are defined in the Building Code of Australia. To understand the rating on any window, understanding the intended application is crucial before considering whether the product is suitable.

Specific building types are relevant to the three different applications for windows:

  • Housing (residential homes)
  • Residential (buildings outside the definition of Housing as described in AS4055)
  • Commercial (commercial buildings)

Leading up to WERS

In 2007, the Department of the Environment, Water, Heritage and the Arts (DEWHA) commissioned the Australian Bureau of Statistics (ABS) to produce a report modelling the relationship between the energy efficiency rating (EER) of houses and house prices in the Australian Capital Territory (ACT). This report and the Department’s overview of it represents the first study of its kind in Australia. It illustrates that the ACT housing market, which in 1999 became the first jurisdiction in Australia to introduce mandatory energy disclosure for all houses on the market, places a higher value on energy efficiency and suggests ‘it replaced the traditional real estate mantra of ‘location, location, location’ in the ACT.

Almost overnight, this market converted to double glazing, seeking to achieve higher energy ratings for homes. This was the beginning of the advent of WERS as a national code across Australia for housing, which commenced around 10 years ago. WERS for commercial use was discussed and included in the BCA (Building Code of Australia) in 2006, however not implemented until May 2011.

Once enacted, a phasing-in period of 12 months usually takes place (as buildings approved under the previous rules could still be constructed). Only now, the impact of energy ratings on commercial buildings is starting to bite and whist it has been required in homes, it is now a very real need in commercial buildings. As energy ratings become more stringent, building materials suppliers need to respond with more energy efficient products. In terms of windows, glass makes the largest difference, but more energy efficient window systems, including thermal break, become more prevalent to achieve the requirements.

Window Energy Rating Scheme (WERS)

WERS rates the energy impact of residential and commercial windows in homes and buildings in Australia. With up to 40 per cent* of a home’s energy for heating lost through windows and up to 87 per cent heat gain through windows, non-thermal performance windows significantly increase heating and cooling loads. Improving window systems’ thermal performance reduces energy costs and Australia’s greenhouse gas emissions. The aim of the WERS scheme is to provide consumers with a tool to evaluate the relative energy performance of different types of windows, allowing them make an informed decision suited to their needs. WERS is an independent scheme owned and managed by the Australian Window Association with the full support of the window industry. Independent of any one manufacturer, WERS is a fair, rigorous and credible system for rating the energy performance of windows.

WERS follows all AFRC protocols and processes.

How Does the Rating System Work?

WERS rated windows carry a label which certifies the window has been simulated by an independent accredited simulator and approved by a WERS auditor.
The window’s cooling and heating performance are rated separately on a scale of 0 – 10 stars; the more stars, the better the rating. These ratings indicate the impact the window will have on the energy performance of a whole house. In support of the rating labels, the window manufacturer can issue a certificate for that window type, spelling out in clear detail its energy performance.

“The correct selection of window frames and glazing, is one of the major items in achieving thermal comfort in a building.”

Commercial buildings account for at least 7 per cent of national energy consumption; 10 per cent by some estimates. They require operational energy for many functions such as gas heating, electric heat pumps, electric resistance heating, evaporative cooling, refrigerative air-conditioning, lighting, motors, equipment and plug loads.

  • Almost all these end-uses require electricity.
  • In Victoria, each kilowatt-hour of brown coal-fired electrical energy consumed releases about 1.3kg CO2-equivalent into the atmosphere.
  • While commercial buildings are responsible for 7–10 per cent of national energy use, they account for at 13 per cent of national greenhouse emissions (Maria Atkinson, Lend Lease, 2009).
  • All commercial buildings and most residential buildings use energy for heating, cooling and lighting.
  • Purchased energy is used to compensate for unwanted heat gain, unwanted heat loss and as a substitute for natural light.
  • Supplying these needs as efficiently as possible, with the lowest overall energy consumption, is the essence of a high-performance building.
  • The best high-performance buildings are a new generation of Zero-Energy Buildings which produce no greenhouse gas (GHG) emissions in their operation.
  • ClimateWorks (www.climateworksaustralia.com, 2010) estimates that Australia’s commercial buildings can contribute three-quarters of the total, potential building-related GHG reductions between now and 2020. This translates into 16 million tonnes (16 Mt) of emissions saved – and at a net savings to society.

Energy performance characteristics

  • Regardless of outside temperature, heat can be gained through windows by direct or indirect solar radiation.1
  • The ability to control this heat gain is measured in terms of the Solar Heat Gain Co-efficient (SHGC).
  • Solar heat gain can be beneficial in winter but undesirable in summer: SHGC range is 0.1 to 1.0, lower the rating the better and range is +/- 10%
  • Heat loss and gain also occur by air leakage through cracks in the window assembly and is measured in terms of the amount of air that passes through a unit area of window.
  • Air leakage can contribute to summer cooling loads in some climates by raising interior humidity levels.

Definition of WERS terms

What is a U-value or U-factor?

U-value measures how well a product prevents heat from escaping. It is a measure of the rate of non solar heat loss or gain through a material or assembly. U-value ratings generally fall between 2.0-10.0 W/m2.K for Australian products. The rate of heat is indicated in the terms of the U-value of a window assembly which includes the effect of the frame, glass, seals and any spacers. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value. The U-value for a window takes account of the various U-values for the components making up the window, so you may see these in technical literature; UW is the value for whole window and because of its importance is usually abbreviated to U, UC is the value at the centre of glass, UF is the value for the frame.

What is the difference between R-value and U-value?

R-value and U-value are essentially two sides of the same coin. R-value is usually cited when discussing things such as wall and ceiling insulation values. The term does not translate well to windows and other fenestration products. That industry prefers U-values. The two are actually inversely related. The higher the R-value, the better insulated are the walls and ceilings. The lower the U-value, the better job a window does in keeping out the heat and cold.

What does Solar Heat Gain Co-efficient (SHGC) mean?

SHGC measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.

What is Visible Transmittance (VT)?

Visible transmittance measures how much light comes in through a product. It is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the number, the more light is transmitted.

Why do I need the air infiltration figure from my AS2047 test report for an energy rating?

Air infiltration (or leakage) is indicated on a test report and expressed as L/s m2. Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the air infiltration figure, the less air will pass through the cracks in the window assembly, improving the energy rating.

What is Condensation Resistance (CR)?

CR measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the rating, the better the window is at resisting condensation formation. The CR is expressed as a number between 0 and 100. Whilst this rating cannot predict condensation, it provides a method of comparing the potential of various window types.

What is considered a standard aluminium window frame?

Assume that aluminium frames are standard unless the manufacturer provides a Window Energy Rating (WERS) which demonstrates that better energy performance will be achieved by using the particular frame.

What is considered a thermally improved aluminium window frame?

An aluminium improved frame is defined as one which is broken but has a gap of less than 5.30mm between the pieces of aluminium. This may be achieved by a variety of design improvements, some (or all) of which may be present. To be classified as broken, a window must conform to the following definition by the U.S. National Fenestration Rating Council (ref: NFRC 100-2001).

What is considered a thermally broken aluminium window frame?

To be classified as thermally broken, a window must conform to the following definition by the U.S. National Fenestration Rating Council (ref: NFRC 100-2001).

“Thermal break: a material of low thermal conductivity that is inserted between members of high conductivity in order to reduce heat transfer. Thermal barrier material conductivity shall be no more than 0.5 W/m.K.

Thermally broken (TB) members: system members with a minimum of 5.30mm separation provided by a low-conductance material (where thermal conductivity is less than or equal to 0.5 W/m.K) or open air space between the interior and exterior surfaces. Such systems include members with exposed interior or exterior trim attached with clips and all skip/debridged systems.”

What is spectrally selective glazing?

A coated or tinted glazing with optical properties that is transparent to some wavelengths of energy and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared radiation.

What effect does the window type have on energy performance?

For operating windows, the operating type has little direct effect on the U-value or SHGC of the unit, but it can have a significant effect on the air leakage and ventilation characteristics. Hinged windows, such as awnings and casements, generally have lower air leakage rates than sliding windows, either horizontal or double hung. A compressive seal and latch increases the effectiveness of the weather stripping in preventing air leakage through hinged windows.

Are there any recommendations I should consider to deliver comfort to my customer?

It is better to install efficient windows than to rely on heating, ventilation and air conditioning (HVAC) systems to solve thermal comfort problems. Not only do HVAC systems create non uniform interior conditions, only partly relieving thermal discomfort, but power outages on peak days can lead to extremely uncomfortable conditions.

In very cold climates, look for windows with very low U-values in order to minimise discomfort. If summer discomfort is expected look for very low SHGC values, but bear in mind that lower SHGCs lead to increased comfort in the summer at the expense of less winter solar warmth.