Product Certification - Energy

Windows Energy Rating Scheme (WERS)

WERS is a 5-star rating system that ranks windows in terms of their annual energy performance and provides certified data.

The Window Energy Rating Scheme (WERS) rates the energy performance of residential windows.  The rating process produces star ratings for heating and cooling performance. The WERS star ratings are performance rankings based on the predicted annual energy demand of the “model house” when fitted with the chosen windows.

WERS ranks the window in terms of its annual energy impact on a house. The energy loads are the amount of annual (heating) energy that must be added to a house, and the amount of annual (cooling) energy that must be removed, to keep the house within a comfortable temperature range. This generates star ratings for cooling (summer and solar control performance) and heating (winter performance). The system uses separate scales of 0-5 stars for heating and cooling impact, in half-star increments. The star ratings are based on the relative, whole-house energy improvement caused by the use of a given window compared with using the base-case product (single glazed clear, standard aluminium frame).

The star rating process is understandable to the non-technical consumer and the window specifier who need to know if the window will enhance or degrade the house in energy and comfort terms.

WERS operates on three levels in conveying information about product.

• Star ratings for heating and cooling

• Indicative % reduction in heating and cooling needs and interior fading damage

• Thermal, solar and optical performance data

WERS is based on extensive international research and experience and has been developed specifically for Australian conditions.  Under the NatHERS, Australia has been classified into 3 distinct climate zones - "cooling", "heating" and "mixed". The climates vary from alpine through to humid tropical.

Star ratings are constant across the country to within the half-star resolution used by WERS.  The rankings are valid for all orientations, a wide range of window sizes and both raised timber and concrete slab-on-ground floors. If the homeowner does not heat or cool, the rankings still indicate which windows will yield the most comfortable house. Thus the choice of window is based on the relative importance of heating vs cooling in each location.

However, WERS rankings will be less accurate where there are:

• Very large glass areas
  (total glass area more than 35% of floor area)

• Large areas of overhead glazing
  (including sunspaces, attached conservatories and large skylights).

WERS aims to:

• Provide a fair, rigorous, scientifically based and easily used method for comparing the relative energy performance of different windows
• Reduce Australia’s energy consumption for residential heating and cooling
• Cut greenhouse-gas emissions by the use of energy-efficient windows.
• Provide links to other building energy rating schemes, codes and standards
• Educate the community about the benefits of energy-efficient windows.

The operational energy efficiency of Australia’s housing stock is improving steadily and it is envisaged it will increase dramatically as energy-efficient windows become more readily available.

Climates with a significant heating fraction (“heating” and “mixed” climates) account for 70% of Australia’s population.  Numerous studies show that in such climates more advanced windows return a net energy benefit over a whole year, regardless of which direction they face. 

• It is possible for a window’s energy gains to exceed its losses even if it faces south. 
• A house with large windows can outperform one with smaller windows and the milder the climate the easier it is for the windows to reach or exceed the break even point.
• A well-designed home incorporating energy-efficient windows, needs significantly reduced energy for space heating or cooling.

The energy performance rating of a window establishes the basic solar, thermal and optical properties of the glazing unit and window frame, based on either:

•  Measurements obtained in a laboratory alone, or

•  A combination of laboratory measurements and computer simulation

Air infiltration must be measured according to procedures meeting AS2047 for residential windows.  If air leakage data is not available, conservative default values are used as input to the rating process.

The solar/thermal properties of the window are the first outcome of the procedure and comprise:

• U-value

• Solar Heat Gain Coefficient (SHGC)

• Shading coefficient (obsolete but included for the sake of familiarity)

• Visible Transmittance  (Tvis)

• Fading Transmittance  (Tdw)

The energy properties of a window are an area weighted average of the corresponding properties of its component parts or regions.  These parts are the:

• Centre-of-glass

• Edge-of-glass

• Frame

Energy efficient windows offer significant benefits to house owners and occupants.  In addition to reducing energy costs and green house gas emissions, a properly selected window plays an important role in improving a homeowner’s comfort and well being.

For a typically insulated house, windows are the biggest heat loss (or gain) building element.  

WERS in a nutshell . . .

• WERS acts as a fair, rigorous and credible system for testing performance claims.
• WERS ranks residential windows for their energy performance in typical housing anywhere in Australia. It will tell you whether a given window is suitable for the climate or not.
• Rated windows get from 0 to 5 stars for each of cooling (summer) and heating (winter), depending on how they rank against the alternatives.
• WERS will complement manufacturer's existing standards for wind, water penetration and safety (AS 2047).

Windows for Good Energy and Acoustic Efficiency

A Primer
Peter Lyons, Project Manager, Window Energy Rating Scheme, AWA Inc.
© Australian Window Association, April 2002

Introduction

Technological advances in the last 25 years have made it possible to design windows which insulate against heat and cold up to four times better than conventional windows. Dramatic improvements in sound insulation are possible. To a point, similar technologies can serve both needs. This Technical Note summarises the objectives, the technical solutions and the trade-offs that building designers and industry need to understand.

Energy Efficiency

An energy-efficient window is one that helps to minimise the use of artificial heating and cooling in a building. In many parts of Australia, the priority is to keep solar heat out of the home, except during colder months, when ‘free’ solar heat gain and retention of warmth in the house become important. Heat gain or heat loss take three forms: radiant heat transfer, conductive heat transfer and heat transfer via air infiltration. Ideally, this calls for strong solar protection on east and west windows, but deliberate use of free sunlight (via clear glass) from northerly windows. This means different window solutions for different orientations. Alternatively, it is possible to use the same uniform high-performance window type on all sides of a home – provided it is correctly chosen. From an energy point of view, this is not quite optimal but it simplifies the specification process and still results in very substantial energy savings and improved comfort, compared to using clear single-glazed windows.

Low U-value

In most climates, windows with at least four heating or four cooling stars, on all sides of the house, will ensure that conductive heat losses and heat gains are minimised. This means the windows must have a low U-value. It is possible to reduce the glazing U-value by means of single glazing having a low-e coating. Low-e coatings are near-invisible. While a low-e coating on simple single glazing reduces that part of the heat transfer which is due to radiation, it does nothing to reduce conductive and convective heat flow.

A much better solution is to reduce all three forms of heat transfer. To do this an insulating glass unit (IGU) is required. The IGU is the vehicle for all high-performance windows, in all climates – hot, cold and mixed. The IGU should preferably have some sort of low-e coating on at least one pane and have argon gas fill between the panes, to give the lowest overall heat transfer. U-values as low as 1.8 are possible, compared with about 5 in the case of a single-glazed clear window. To complement the glazing system, a frame with a low U-value assists in reducing the whole-window U-value. Frames that use thermal breaks or composite metal / timber design, or timber or uPVC frames, outperform standard aluminium windows. This frame performance difference shows up in the WERS stars as an extra half star or so.

The most common filling gas between the panes of an IGU is dry air. The narrowest air gap used is 6 mm but this should be avoided unless there is no alternative. The use of wider gaps (10-20 mm) will improve the U-value and increase the WERS heating star rating by at least half a star. Contrary to common belief, a very wide air gap is not disadvantageous, apart from being impractical for an IGU. Although convection cells are bigger and more active, this is counteracted by the greater thickness of air which provides additional resistance to conduction. The use of (cheap) argon gas instead of air in the IGU space lifts the heating rating by about a third to half a star in most cases. Table 1, the WERS table of Generic Windows, gives indicative performance.

Wider gaps also give better acoustic insulation; see the section on acoustics later in this document.

High versus Low Solar Heat Gain Coefficient: Low-e Coatings

As mentioned, all low-e coatings assist energy efficiency by suppressing radiation heat transfer. This means they act like ‘heat mirrors’ and reflect heat back into a room in winter, which reduces the amount lost to the outside. Some low-e coatings are essentially transparent to solar radiation; these mostly take the form of so-called ‘hard’ or ‘pyrolytic’ coatings. They are best suited to cooler climates or on the north side of a building where ‘free’ winter heating can be exploited. Other low-e coatings (frequently referred to as ‘soft’, ‘multilayer’ or ‘spectrally selective’ coatings) are available, which block up to half the invisible, radiant solar heat while still preserving daylight. Windows with spectrally selective low-e coatings reduce the solar heat gain coefficient (SHGC) by up to 60% compared with clear 3 mm glass. This reduces or in some cases eliminates the need for wide eaves or shading systems. When the Generic Table was developed, spectrally selective low-e coatings were unusual on the Australian market. However their performance is well approximated by windows 22 – 27 in the table, which achieve similar results by a different construction.

Good Weather Seals

Heat leaves or enters a home through gaps and cracks around sashes and frames. When a window is shut it should be shut. WERS-rated windows must satisfy Australian Standard AS 2047 for air infiltration performance. Most easily exceed it and achieve air leakage figures below 1 litre per second per square metre of window area. Traditionally, windows with compression seals, as fitted to awning and casement windows, tended to have superior long-term infitration performance. However recent advances in some sliding window seals have reduced the gap. The difference between a window that just meets AS 2047 (5 L/s.m2) and a very tight window is about half a heating star. The effect on cooling performance is much less.

Good/Better/Best Windows…and the BCA

The Australasian Window Council (AWC) has submitted a proposal on energy-efficient windows to the Australian Building Codes Board (ABCB) for adoption in the Building Code of Australia (BCA). The submission has been prepared by the AWC-ABCB Working Group. The idea is to present the designer with a simple, effective selection guide to energy-efficient windows that meets the Deemed-to-Satisfy requirements of the BCA. To achieve this it is necessary to identify window options which offer increasingly better insulation and/or sun protection compared with the starting point of clear single glazing and fixed eaves. In many cases, eaves can be cut back or even eliminated, with no loss of summer comfort. At the same time, insulating glass reduces conductive heat losses and gains. The overriding philosophy is to offer the builder value-added window solutions which are cost-competitive with the conventional approach of single-glazed windows and fixed shading.

At the time of writing, house energy ratings (HERS) attainable using the AWC method are being evaluated. The general target is NatHERS 4 stars or better, since this is the standard applying or pending in Victoria, the ACT, parts of NSW and most likely in SA and WA before year’s end. An ideal result for the window and glass industries would be to demonstrate that the majority of project homes, with windows up to about 25% of their gross floor area, could meet or exceed NatHERS 4-star level using high-performance windows and reduced eaves and at a total cost no greater than the wide-eaves, single-glazed house.

To assist this submission, the AWC-ABCB Working Group is seeking accurate prices on complete window schedules for six house designs. The combined window costs and energy savings will be used to improve the Cost-Benefit Tool currently available from the Australian Greenhouse Office.

Copyright© 2002-06 Australian Window Association  Phone: intl+ 61 2 9498 2768   Email: Info@AWA.org.au

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