Window energy ratings explained: U-value, SHGC and WERS

Why window ratings confuse people

Window brochures and quotes quote performance figures in different ways. One gives a U-value for the glass. Another gives a star rating. A third says the window is energy efficient and leaves it there. Two numbers that look directly comparable can describe different things, and a product that looks better on paper may simply be quoting a more flattering measurement.

The Australian Government's Your Home glazing guide warns that the various ratings and descriptions can be confusing or misleading, and advises making sure declared U-value and SHGC values follow the requirements of the Australian Fenestration Rating Council. It points homeowners to the Window Energy Rating Scheme, known as WERS, which rates windows, skylights and glazed doors using AFRC procedures.

This page explains the main figures in plain terms, then shows how to check that the rating you are given belongs to the window you are being sold. We avoid quoting target numbers, because the right figure depends on the house, the room and the orientation, and on whether your project has to meet a formal energy rating.

Whole window versus centre of glass

A window is not just glass. It has a frame, sashes, seals and a spacer around the edge of each sealed unit, and heat moves through all of them. The WERS frequently asked questions explain that the U-value of a window assembly includes the effect of the frame, glass, seals and spacers, and they distinguish the whole-window value (Uw), the centre-of-glass value (Uc) and the frame value (Uf).

A centre-of-glass figure describes only the middle of the pane, away from the edges. It is useful to glass makers, but it leaves out the parts of the window that usually conduct heat most readily, such as the frame and the glass edge. That is why a centre-of-glass U-value often looks better than the whole-window figure for the same product.

When you compare windows, compare whole-window figures. If a quote or brochure gives a single U-value without saying which kind it is, ask. A supplier who knows their product will be able to tell you straight away, and should be able to show you where the whole-window rating is published.

U-value: how readily heat passes through

The U-value measures how readily heat is conducted through the window. Your Home describes the whole-window value, Uw, as describing the conduction of the glass and frame together, and explains that the lower the U-value, the greater the window's resistance to heat flow. In practice that is the number that matters most on cold winter nights, when heat inside the house is trying to escape.

U-values are the inverse idea of the R-values used for wall and ceiling insulation. The WERS FAQ notes that the window industry prefers U-values, and that the two are inversely related. A higher R-value indicates greater thermal resistance; a lower U-value indicates less heat transfer under the same rating conditions. Neither alone settles daylight, solar gain or suitability for the room.

Keep the frame description beside the rating, rather than treating it as a separate marketing claim. Ask whether a quoted upgrade changes the full frame system or only the glass. If two options use different frames, give each its own row in your comparison. You can then discuss the difference in cost without losing track of which product each figure belongs to.

SHGC: how much solar heat comes in

The solar heat gain coefficient, or SHGC, measures how much of the sun's heat striking the window ends up inside, both passing straight through and absorbed then released inward. WERS expresses it as a number between 0 and 1. The lower the number, the less solar heat the window lets in.

Unlike U-value, a lower SHGC is not automatically better. On a west facing window that overheats a room on summer afternoons, a lower SHGC helps. On a north facing window that catches welcome winter sun, a very low SHGC can take away free warmth. Your Home also points out that the actual solar gain varies with the angle at which the sun hits the glass, which changes with orientation, season and time of day. A rating is measured under standard conditions, so it is a comparison tool rather than a prediction for your particular room.

Record what causes discomfort in the room before choosing a solar-control option. Note the season, approximate time and whether existing shading is open or closed. A west-facing study used during the afternoon may have a different brief from a rarely occupied spare bedroom. Give those observations to the supplier or assessor instead of asking every room to meet one preferred number.

Visible light, air leakage and condensation resistance

Visible transmittance, written as VT or Tvis, measures how much visible light comes through the window, from 0 to 1, with higher numbers meaning more light. In the WERS tables the whole-window figure is the glazing value multiplied by the share of the window that is glass, so a window with a wide frame lets in less light than its glass figure alone suggests. Toned glass and some coatings reduce VT, which matters in rooms that are already dim. The privacy glass and daylight guide covers the trade-off with obscure glass.

Air infiltration describes leakage through the closed product under stated test conditions. Compare like test conditions and the same opening type. Ask whether the report covers the slider or awning actually quoted, rather than accepting a result for a fixed pane with no opening sash.

Condensation resistance, when supplied, is a comparison rather than a promise that a room will remain dry. Record where moisture appears and when, and discuss ventilation and other moisture sources separately. A window quote should not turn an unresolved leak or ventilation problem into an assumed glazing cure.

What WERS star ratings do and do not tell you

WERS gives windows separate star ratings for heating and for cooling. According to the WERS custom ratings page, the stars compare a product against a base case of plain clear glass in a standard aluminium frame, and relate to the annual energy impact of the window on a model house. The scheme also publishes the thermal, solar and optical data described above.

Use the stars as an introduction to the choices, then request the matching product data for assessment. Save the complete report rather than cropping a star badge from a brochure. Your file should let another person identify the window without guessing what the missing page said. If a quote presents a modelled percentage saving, ask what baseline and assumptions it uses before treating that percentage as an estimate for your household.

Keep the rating reference, issue date and supplier confirmation with the quote. When an older brochure and a current directory entry disagree, send both to the supplier and ask which applies to the supplied configuration. Do not silently substitute the more attractive number. If the supplier cannot identify the matching entry, mark that evidence as outstanding in your comparison.

Matching a rating to the window on your quote

A rating belongs to a specific configuration: a particular frame system, a particular opening type and a particular glass make-up. Change any of those and the rating changes. The WERS tables are also calculated for standard sizes, so the figures describe the product at those sizes rather than every window in your house.

To check that a rating applies, you need three pieces of information on the quote for each opening or group of openings: the frame range, the opening type and the glass make-up written pane by pane. With those, the supplier should be able to point you to the matching rating in the WERS directory or on their own published rating sheets. If a quote gives one rating for every window in the house, including sliders, awnings, fixed panes and doors, ask which configuration it was taken from.

If your project needs a building permit or an energy rating, the energy assessor or building surveyor will tell you what glazing information they need. Give them the same written specification, not the brochure. For projects that do not need a formal rating, the figures are still the most reliable way to compare quotes, as described in the guide to comparing double glazing quotes line by line.

Finally, remember what ratings cannot do. They compare products under standard conditions. How warm a room feels after new windows depends on the rest of the house as well: insulation, draughts, shading, heating and how the windows are installed and sealed. A good rating is a sound starting point, not a promise about comfort or bills.

What a rating does not cover: installation and sealing

A window rating describes the product as manufactured and tested. It does not describe the gap between the frame and the wall, the flashings, or the sealant around the opening. Those are part of the installation, and a well rated window installed with gaps around it can still let draughts and moisture in.

When you compare quotes, ask how each installer seals the frame into the opening, inside and out, and what they do if the opening is out of square or the existing packing is poor. This matters most for full frame replacement, where the whole perimeter is new. For a retrofit that keeps the old frame, ask how the existing frame's seals and weather strips will be checked or replaced, because the rating of a new sealed unit says nothing about the condition of an old frame around it.

It is also worth asking the supplier whether they supply the product as rated, or whether any part of it is substituted locally, such as the glass or the seals. A rating should describe the window you receive, not a similar one.

Hypothetical example: two quotes, two kinds of figure

A homeowner has two quotes for the same openings. Quote A lists the frame range, gives each opening type with its glass make-up, and attaches whole-window U-value, SHGC and VT figures for the sliding windows and the awning windows separately. Quote B describes the glass as double glazed Low-E argon and quotes a single U-value taken from the glass manufacturer's brochure.

Quote B's figure is lower, so it looks better. On asking, the homeowner learns it is a centre-of-glass value and that no whole-window rating has been given for the frame. The homeowner asks Quote B for whole-window figures for each opening type. Only then are the two quotes compared on the same terms.