WERS window ratings help Australian project teams compare the energy performance of complete window systems—not just the glass. The most useful figures are whole-window U-value, which describes heat transfer, and whole-window Solar Heat Gain Coefficient (SHGC), which describes admitted solar heat. The right combination depends on the project’s climate, orientation and energy report.
Key takeaways
- A lower whole-window U-value means better resistance to heat flow.
- A lower SHGC means less solar heat enters through the complete window.
- Heating and cooling star ratings answer different questions; neither should be read alone.
- Compare the exact frame, glass and opening configuration—not a glass-only value or a family average.
- Use the project’s energy report and an accredited professional to confirm the required values.
What is a WERS window rating?
The Window Energy Rating Scheme (WERS) is an Australian system for rating the energy and energy-related performance of windows, glazed doors and skylights. It follows procedures recognised by the Australian Fenestration Rating Council. A WERS result describes a tested or simulated whole window: glass, spacer, frame and operating configuration work together to produce the result.
This distinction matters. Two windows using apparently similar double glazing can have different performance because their frame design, thermal break, seals, spacer and glass coatings differ. Likewise, a glass-centre value cannot be substituted for the whole-window value used in an energy assessment.
U-value: resistance to heat flow
Whole-window U-value (Uw) measures the rate of heat transfer through the complete window. It is usually expressed in watts per square metre per degree of temperature difference (W/m²·K). Lower is better for insulation because less heat moves through the system for the same temperature difference.
The Australian Government’s Your Home glazing guide illustrates the relationship with the equation heat flow = Uw × temperature difference × window area. Its worked example compares 70 m² of glazing at Uw 6.2 with Uw 3.1 across a 15°C indoor–outdoor difference: calculated heat flow falls from 6,510 W to 3,255 W. That example explains the physics; it is not a prediction for every house.
U-value matters in every Australian climate because windows can transfer heat in either direction. It becomes especially influential where winter heat loss is a major design load, including cool-temperate locations such as Canberra, Hobart and Melbourne.
SHGC: controlling solar heat gain
Whole-window SHGC (SHGCw) is a number between 0 and 1 that describes the proportion of incident solar heat admitted through the complete window. A lower number admits less solar heat; a higher number admits more.
“Lower is always better” is the wrong rule for SHGC. In cooling-dominated locations or exposed east- and west-facing openings, lower solar gain can help limit summer loads. In a cool climate, correctly shaded north-facing glazing may benefit from a higher SHGC so winter sun can contribute useful warmth. Orientation and shading therefore belong in the same decision as the glass specification.
The Australian Government windows guide recommends climate- and orientation-specific shading and notes that the best answer should be confirmed with an energy assessor or building designer. This is why copying a specification from another elevation—or another city—can give a poor result.
Heating stars and cooling stars
WERS also expresses comparative performance as separate heating and cooling star ratings. Heating stars indicate how the system performs when retaining heat is important. Cooling stars indicate how it performs when limiting unwanted heat gain is important. Treat these as quick comparison signals, then return to U-value and SHGC for project documentation.
| Metric | What it describes | Direction | Do not assume |
|---|---|---|---|
| Uw | Heat transfer through glass and frame | Lower means better insulation | It is not a glass-only value |
| SHGCw | Solar heat admitted through the whole window | Lower means less solar gain | The lowest value is not ideal for every orientation |
| Heating stars | Comparative heating-season performance | More stars generally indicates better performance | They do not replace the energy report |
| Cooling stars | Comparative cooling-season performance | More stars generally indicates better performance | They do not describe winter solar benefit alone |
| Air infiltration | Air leakage through a closed assembly | Lower leakage is generally preferable | Installation gaps are a separate site issue |
A five-step WERS specification check
- Start with the energy report. Record the required Uw and SHGCw for each opening or window group. The Australian Glass and Window Association advises that the selected Uw should be less than or equal to the specified value, while SHGC must remain within the allowed project tolerance.
- Match the exact system. Confirm frame series, opening type, dimensions, glass build-up, spacer and any Low-E coating. A fixed light and an opening sash in the same product family may not share one result.
- Check orientation. Separate north, east, south and west elevations where the design report does. Large west-facing openings often need a different solar-control strategy from shaded north-facing glass.
- Verify evidence. Ask for the relevant WERS or AFRC-recognised performance record rather than relying on labels such as “energy efficient”.
- Coordinate installation. Product ratings assume a complete system; poor perimeter sealing, incorrect flashing or installation outside the documented scope can undermine real-world comfort and water management.
For aluminium systems, frame design is particularly important because aluminium conducts heat. A properly engineered thermal break interrupts the conductive path between internal and external aluminium sections. See Forge’s guide to thermal-break aluminium windows, then compare the exact whole-window values for the proposed configuration.
Common mistakes to avoid
- Comparing centre-of-glass U-value with a whole-window Uw value.
- Choosing the lowest SHGC for every elevation without checking winter solar access.
- Assuming double glazing automatically produces the required WERS result.
- Using one product’s rating for a different size, frame series or opening type.
- Ordering before the energy assessor, builder and window supplier have aligned the schedule.
Frequently asked questions
Is a lower U-value always better?
For insulation, yes: a lower whole-window U-value indicates greater resistance to heat flow. However, the final window choice must also satisfy the project’s SHGC, structural, water, safety, ventilation, budget and design requirements. U-value is important, but it is not the only specification.
Should SHGC be high or low in Australia?
It depends on climate, orientation and shading. Cooling-dominated locations and exposed east or west glass often benefit from lower SHGC. Correctly shaded north-facing windows in cool climates may use a higher SHGC to admit useful winter sun. Confirm the target in the project energy model.
Does WERS rate the glass or the complete window?
WERS performance is for the complete window, glazed door or skylight system. The result includes the frame and glazing together. A glass-only figure cannot show the thermal effect of aluminium profiles, thermal breaks, sash configuration, seals and spacers.
Does a WERS rating prove AS 2047 compliance?
No. Energy performance and AS 2047 performance are related project inputs but they address different requirements. Ask for documentation covering the energy values as well as relevant structural, operating and water-penetration requirements for the proposed system and site conditions.
Specify the performance, not the label
WERS window ratings are most useful when they turn broad claims into checkable project data. Begin with the energy report, compare exact whole-window Uw and SHGCw values, account for orientation and shading, and keep the selected evidence with the window schedule. To discuss a project-specific aluminium window configuration, review the Forge Windows product range and contact the project team with your drawings and performance targets.