Windows, Heat Loss, and Heat Gain: Why Glazing Decisions Matter More Than You Think
Up to 40% of a home's heating energy can be lost through windows. In summer, up to 87% of unwanted heat gain enters through glass. Windows are the most thermally active element of your building envelope, and the most commonly specified without any reference to their performance.
Windows lose and gain heat in three ways, conduction through the glass, radiation of solar energy, and convection from air movement across the surface. Understanding each mechanism helps explain why window specification matters so much for both comfort and energy performance.
The performance ratings explained
• U-Value - measures how much heat is conducted through the window assembly. Lower is better. Single-pane glass: approximately 5.8 W/m²K. Standard double glazing: approximately 2.7 W/m²K. Thermally broken double glazing: below 2.0 W/m²K.
• Solar Heat Gain Coefficient (SHGC) - measures how much solar radiation passes through the glass as heat. Values range from 0 to 1. High SHGC (0.6+) lets in more solar warmth, useful on north-facing windows to capture winter sun. Low SHGC (below 0.35) limits solar gain, important on east and west-facing windows.
• Solar Absorptance (SA) - relevant to the frame as well as the glass; darker frames absorb more heat and transfer it to the interior.
It's worth noting that WERS (Window Energy Rating Scheme) whole-window ratings assess the combined performance of glass and frame together, a more accurate indication of real-world performance than glass-only ratings. When comparing window products, always compare on a like-for-like rating basis, glass specifications alone don't tell the full story.
Frame materials and thermal performance
Standard aluminium frames conduct heat readily, their thermal performance is significantly worse than the glass they hold. Thermally broken aluminium frames (with a non-conductive break separating the inner and outer sections) perform substantially better. uPVC frames have even lower thermal conductivity and are increasingly used in high-performance builds.
In the Southwest context, the frame material choice also intersects with corrosion requirements. In coastal corrosivity zones C3 and C4, standard aluminium finishes may not be adequate, marine-grade or special coastal specifications are required.
A practical glazing strategy for WA
• North-facing windows - specify higher SHGC glass to capture winter solar gain. Manage summer gain with eave shading rather than low-SHGC glass, which would also reduce winter benefit.
• East-facing windows - moderate size; SHGC of 0.35–0.50 is reasonable. East sun is low and relatively gentle.
• West-facing windows - minimise size wherever possible; specify low SHGC (below 0.35). West sun in WA's summer is intense and very difficult to shade.
• South-facing windows - no solar gain benefit; specify low U-value to minimise winter heat loss. South windows are best used for diffuse daylighting in rooms that benefit from consistent light without heat.
Double glazing: when is it worth it?
Double glazing adds approximately $200–$400 per window over single glazing, depending on size and specification. On a full four-bedroom home, the premium is typically $6,000–$15,000. In WA's climate (compared to colder southern states where heating loads are higher), the payback period is longer than in Melbourne or Canberra, typically 10–15 years in energy savings alone.
However, the comfort benefit is immediate and real. Double-glazed windows eliminate condensation, reduce radiant chill near the glass in winter, and significantly reduce noise, benefits that go beyond energy cost.
A targeted approach, double glazing on the most thermally active window areas (large west and south-facing windows) while maintaining single glazing on smaller, better-positioned openings , can deliver most of the comfort benefit at a lower overall premium.
Frequently Asked Questions
What is a WERS rating and how does it differ from a U-Value?
WERS (Window Energy Rating Scheme) provides a whole-window rating that combines glass and frame performance into a single star rating for heating and cooling. U-Value measures only the thermal conductance of the glass unit. WERS ratings are more useful for comparing complete window products; U-Value is useful for specifying glass performance in building documentation.
Is double glazing required for 7-star energy compliance in WA?
Not always. Double glazing is one of many tools available to achieve 7-star NatHERS compliance. Orientation, shading, and insulation can substitute for double glazing in many cases. However, on homes with large window areas or challenging orientation, double glazing may be the most cost-effective way to achieve compliance.
Can I specify different glazing for different parts of the house?
Yes, and this is generally the most cost-effective approach. Your building designer and energy assessor can model the performance impact of different glazing specifications on each facade and recommend a specification that achieves compliance efficiently.
Let's chat about energy-smart window design for your project, the right glazing specification can make a real difference to both compliance and comfort. Contact us at projects@fastlanedesign.com.au
