Key Takeaways
- U-value: it measures how easily heat passes through a whole window, frame and glass together, and a lower number means better insulation.
- R-value: it is the reciprocal of U-value (R = 1/U), and a higher number means better insulation.
- Whole window: energy reports use whole-window values, not the glass on its own.
- Proof: ask your supplier for a WERS certificate and keep a copy.
The U-value of windows is one of the two numbers on most energy reports, alongside SHGC. This guide explains what it means, how it differs from the R-value you know from insulation, and how to check that the windows you are quoted will meet it.
What Is U-Value in Windows?
U-value measures how readily heat passes through a window. It is expressed in watts per square metre per degree (W/m2.K), and a lower U-value means the window insulates better. It describes the whole window, with the frame and glass working together.
The Australian Glass and Window Association (AGWA) notes that ordinary windows can be a major source of unwanted heat gain in summer and significant heat loss in winter. U-value is the figure that shows how well a window resists that heat loss through conduction.
U-Value vs R-Value: What Is the Difference?
U-value and R-value describe the same thing from opposite ends. R-value is the reciprocal of U-value (R = 1/U), and it is the rating most people know from ceiling and wall insulation.
| Measure | What It Shows | Better Result |
|---|---|---|
| U-value | How readily heat passes through the window | Lower number |
| R-value | How strongly a material resists the flow of heat | Higher number |
| Converting | A U-value of 0.5 equals an R-value of 2 | Both mean better insulation |
Windows are usually rated by U-value, so you will rarely see an R-value on a window. Use the conversion when you want to compare a window with the rest of your home’s insulation on the same scale.
What Is a Good U-Value for Windows?
A good U-value is a low one that meets the figure set for your home. AGWA explains that an energy report lists performance values for windows, and that a window complies when its U-value is lower than or equal to the one specified. Because the target comes from your own report, the right figure depends on your design and climate.
- U-value: the window must be lower than or equal to the figure on the report.
- SHGC: the window must be within 10% of the figure on the report.
- Whole window: both values apply to the frame and glass together, not the glass alone.
- Climate: AGWA splits Australia into three climate areas because each needs different window properties. Most of Victoria and the ACT sit in the heating area, which covers BCA climate zones 6, 7 and 8.
What Changes a Window’s U-Value?
Frame, glass, spacers and sealing all change the result, which is why two double glazed windows can perform differently.
- Frame material: aluminium conducts heat readily, while uPVC is a low-conductivity material with a multi-chambered design.
- Glazing: double glazing traps air or an insulating gas between two panes. Argon is denser than air, so it insulates better.
- Low-E coating: this transparent coating reflects radiant heat back into the room in winter.
- Spacers: the strip between the panes can create cold edges if it is metal. Our pane spacers are not made of metal.
- Sealing and installation: even a high performing, thermally broken, double glazed window can underperform without correct installation and sealing.
Lower U-values also make for warmer glass inside your home. Cold glass draws heat from people and pulls cool air down to the floor, so a window with a lower U-value feels more comfortable in winter.
How U-Value Affects Comfort and Condensation
A lower U-value makes a difference you can feel, not just a number on a report.
- Winter comfort: windows with lower U-values have a warmer inside surface, so you lose less body heat to the glass and notice fewer cold draughts.
- Condensation: condensation forms when warm, moist air meets a cold surface. A warmer window surface greatly reduces the chance of it forming.
- Energy use: less heat escaping means your heating and cooling do not have to work as hard.
U-Value and SHGC: Why You Need Both
U-value covers heat conducted through the window, while SHGC covers heat that arrives as sunlight. A window with a low U-value but a high SHGC can hold in warmth while still letting in too much summer sun. To see how the second number works, read our guide on what SHGC means for Australian homes.
How to Check a Window’s Energy Rating
Follow this process before you accept a quote. It is based on AGWA’s guidance on the Window Energy Rating Scheme (WERS).
- Find your targets: read the U-value and SHGC figures on your energy report.
- Ask for the certificate: request the WERS certificate for the exact window you are buying.
- Match the descriptions: the glass and frame on your quote, including frame material and operator type, should match the certificate.
- Compare whole windows: never compare a glass-only figure with a whole-window figure.
- Keep the certificate: it records the make-up of your window, which helps if you need glass replaced or repairs later.
Frequently Asked Questions
What is a good U-value for a window?
A good U-value is a low one that meets or beats the figure on your energy report. Lower numbers mean better insulation. The target depends on your design and climate zone, so check your report and ask your supplier for a WERS certificate showing the whole-window value.
Is a higher or lower U-value better for windows?
Lower is better, because heat passes through the window more slowly. A lower U-value also keeps the inside glass warmer in winter, which improves comfort. R-value works the other way, where a higher number is better, so do not mix the two scales when comparing products.
How do I convert U-value to R-value?
Divide 1 by the U-value. A U-value of 0.5 gives an R-value of 2, and a U-value of 2 gives an R-value of 0.5. A lower U-value means a higher R-value, and both describe a window that resists heat flow better.
Is U-value the same as SHGC?
No. U-value measures heat conducted through the whole window, while SHGC measures how much of the sun’s heat passes through, on a scale from 0 to 1. A lower U-value means better insulation. The best SHGC depends on your climate and which way the window faces.
Where can I find the U-value of my windows?
Check your energy report for the target, and the WERS certificate for the window you are buying. WERS also keeps a Certified Product Directory. If you cannot find a rating for existing windows, ask the supplier or installer who provided them for the certificate they hold.
Get a Free Measure and Quote for Energy Efficient Windows
Every home is different, so the best way to meet your target is to talk it through with our team. We manufacture energy efficient windows in our Wodonga factory and install with our own teams, backed by a 12-year guarantee. Our quotes are itemised and fixed-price.
“They have reduced our power bills & north facing, we have significantly noticed the difference both in summer & winter, including noise reduction.” Nicole Bianca & Dennis, Ultimate Windows customers. Request a free measure and double glazed window quote online or call 1300 898 782.
