We often hear people say, “These two windows have the same U-value, so they must perform the same.”
It’s a fair assumption, but it’s not always accurate.
Because window performance isn’t just about U-value.
Yes, U-value tells you how much heat flows through the window, lower is better. But SHGC (Solar Heat Gain Coefficient) tells you how much heat from the sun passes through the glass, and that’s just as important when you’re trying to control comfort and performance.
In Australia, most window ratings come through WERS (the Window Energy Rating Scheme). It’s a helpful system, but it uses assumptions that don’t always reflect real-world conditions, things like standard frames, install types, or generic shading.
If you’re building to a higher standard, or using PHPP for Passivhaus, both the U-value and SHGC are calculated differently. They’re assessed in context: the size of the window, the frame, the install method, the orientation, and how it all ties into the rest of the thermal envelope.
So when comparing windows, it’s worth asking:
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What testing system are the numbers from?
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Do those figures include the full frame and install method, or just the glass?
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Is this information being used in real design modelling, or just pulled from a catalogue?
At Enduro, we don’t look at window performance in isolation. We model it as part of the whole system. That means sometimes a window costs a little more, but saves a lot in comfort, thermal stability, and long-term efficiency.
It’s not about picking the most expensive product.
It’s about choosing the one that actually works for your home.
Frequently Asked Questions
Do two windows with the same U-value perform the same?
Not necessarily. U-value only tells you how much heat flows through the window. SHGC (Solar Heat Gain Coefficient) tells you how much of the sun’s heat passes through the glass, and it matters just as much for comfort and performance. Two windows can share a U-value yet behave very differently once SHGC and orientation are considered.
What is the difference between U-value and SHGC?
U-value measures how much heat flows through the whole window, and lower is better. SHGC measures how much solar heat passes through the glass. You need both to judge real performance: U-value for keeping heat in or out, SHGC for controlling how much sun the window lets in.
Are WERS window ratings reliable?
WERS (the Window Energy Rating Scheme)(opens in new tab) is Australia’s standard rating system and a helpful starting point, but it uses assumptions, such as standard frames, install types and generic shading, that don’t always reflect real-world conditions. For a high-performance build, the figures need to be assessed in the context of your actual home.
How are window ratings assessed for a Passivhaus home?
When building to a higher standard using PHPP for a Certified Passivhaus, both U-value and SHGC are calculated in context: the size of the window, the frame, the install method, the orientation, and how it all ties into the rest of the thermal envelope, rather than generic catalogue numbers.
What should you ask when comparing window performance figures?
Ask three questions: what testing system the numbers come from, whether the figures include the full frame and install method or just the glass, and whether they are used in real design modelling or simply pulled from a catalogue. The answers reveal whether you are truly comparing apples to apples.
Why does Enduro sometimes recommend a more expensive window?
Because Enduro models window performance as part of a whole energy-efficient home, not in isolation. Sometimes a window costs a little more but saves far more in comfort, thermal stability and long-term efficiency. It is not about picking the most expensive product, but the one that actually works for your home.
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