Window Energy Rating Scheme (WERS)
The Window Energy Rating Scheme (WERS) is a 10-star rating system that ranks windows in terms of their annual energy performance and provides certified data.
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 0-10 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 10 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).
To participate in WERS windows must meet all Australian standards. WERS forms part of the quality assurance that smart manufacturers offer their customers. It is all about certified performance. To realise their full potential, WERS generic and custom ratings are designed to 'plug in' to NatHERS, Australia's Nationwide House Energy Rating Software.