By Penot Double Glazing
It is the question Perth homeowners ask more than any other before committing to double glazing: does double glazing Perth summer heat performance actually keep a home cooler? The short answer is yes. The longer answer is more useful, because the wrong spec of double glazing in a Perth home will deliver a fraction of the cooling benefit you paid for. Knowing which spec actually matters for Perth summer heat is the difference between a window upgrade that pays back and one that does not.
This article explains what the numbers mean, why Perth is a harder test for glazing than most of Europe or Tasmania, and how to read a glazing quote so you can tell whether you are being sold the right product for our climate.
How Perth’s Summer Heat Changes Double Glazing Choices
Most public information about double glazing was written for cool-climate or temperate countries where the priority is keeping heat in during winter. Perth’s priority is the opposite. We sit in Building Code of Australia Climate Zone 5, “warm temperate”. The Bureau of Meteorology records a long-term average of 26 days a year above 35 degrees at Perth Metro, with five days above 40 degrees and a record run of six consecutive 40-plus days in January 2022. Summer UV index averages an Extreme reading of 12.
For a glazing specifier, this changes the conversation. A window that performs well in a cold climate may not perform well in Perth. The thermal questions are different, and so are the spec choices.
The Two Ways Heat Enters Through Glass
Heat moves through a window in two main ways. Understanding the difference is the foundation of every other decision.
Conduction and convection describe heat transferred through the glass and frame from the hot air outside to the cooler air inside. This is what U-value measures. The lower the U-value, the slower this transfer.
Radiation describes the sun’s energy passing straight through the glass as visible light and infrared, then being absorbed by the floor, walls and furniture inside, where it converts to heat. This is what solar heat gain coefficient (SHGC) measures. SHGC runs from 0 to 1. A clear single pane sits around 0.80. A solar-control low-e double glazed unit can sit as low as 0.20 to 0.40.
In a Perth summer, radiation is the larger problem on the west and on unshaded north windows. On a 40-degree afternoon, a west-facing single pane of clear glass can transmit hundreds of watts per square metre of radiant solar energy straight into the room. The room behind it heats up not because the air outside is hot, but because the sun is being delivered inside as light, then converted to heat.
This is why SHGC matters more than U-value for most Perth homes. The Your Home guidance from the Australian Government’s Your Home program is clear on this: in cooling-dominant climates, reducing solar gain is the priority, particularly for east and west orientations. The Department of Climate Change, Energy, the Environment and Water’s NCC compliance guidance for Climate Zone 5 follows the same principle. NCC 2025 DTS provisions set maximum solar admittance targets for west-facing glass in Climate Zone 5; a NatHERS assessor confirms the applicable figure for a specific project.
What Double Glazing Actually Does to These Numbers
A standard double glazed unit reduces both U-value and SHGC compared to a single pane, but the size of each reduction depends on the glass spec.
| Glazing | Typical Uw (whole window) | Typical SHGC |
|---|---|---|
| Single standard 4mm | 5.5 to 6.6 | ~0.80 |
| Standard double, aluminium frame, no thermal break | 3.5 to 4.5 | 0.70 to 0.75 |
| Double with hard-coat low-e, argon, thermal break | 1.8 to 2.5 | 0.45 to 0.60 |
| Double with soft-coat solar-control low-e, argon, thermal break | 1.8 to 2.5 | 0.20 to 0.40 |
| Triple, best spec | 1.0 to 1.4 | 0.30 to 0.50 |
These ranges are drawn from Australian manufacturer technical data, AGWA technical fact sheets and published product specifications. A given product will sit somewhere inside the range depending on the exact glass, coating, gas fill and frame. The full Penot Double Glazing windows range covers low-e, solar-control low-e and acoustic laminated options across every window type.
Two things jump out for Perth. First, going from single to standard double cuts U-value by roughly 40 per cent but only nudges SHGC. Second, adding a solar-control low-e coating to that same double glazed unit can halve SHGC again. For a Perth home, that second step is often the bigger win. For homes pushing past 7-star NatHERS or with west-facing rooms exposed to long afternoon sun, triple glazing layers a third pane on top of these gains.
How Much Cooler Does Double Glazing Make a Perth Room?
Homeowners want a number. “How much cooler is the room?” The honest answer is that no Perth-specific peer-reviewed dataset gives a clean single number, because the answer depends on orientation, shading, ceiling insulation, the air conditioner, and the run of hot days leading up to the measurement.
What the building physics does tell us, with cross-referenced support from manufacturer modelling and the WHO 2009 indoor environment guidance, is that an unshaded west-facing single pane on a 40-degree afternoon can leave the interior glass surface temperature within a few degrees of the outside air. Behind that glass, surfaces in direct line of sight absorb radiant heat for hours. A low-SHGC double glazed unit in the same position blocks more than half of that radiant load before it ever reaches the room.
For a typical Perth living room with a 3 by 2 metre unshaded west-facing window, the practical effect is that the air conditioner runs harder, longer, and starts losing the battle in the late afternoon. Replacing that single pane with a solar-control double glazed unit will not eliminate the afternoon heat, but it shifts the load enough that the room actually stabilises at the temperature you set on the wall. For openings larger than that, our guide to large glass openings in Perth homes covers what changes when an entire wall is glass.
How Low-E Coatings Work
The “low-e” in low-e glass stands for low emissivity. A microscopically thin metal oxide layer is applied to one face of the glass during manufacture. There are two main families.
Hard-coat (pyrolytic) low-e is applied during the float glass process and is durable enough to be handled and used as a single pane. It cuts U-value but its effect on SHGC is modest.
Soft-coat (sputtered) low-e is applied in a vacuum coating chamber and is more fragile, which is why it is sealed inside a double glazed unit where the coating faces the gas gap. Soft-coat coatings give the best performance, particularly on SHGC. Solar-control soft-coat low-e is what gives a Perth-specified double glazed unit its lowest SHGC numbers.
For Perth, soft-coat solar-control low-e is usually the right answer for west-facing rooms and unshaded north. Hard-coat is reasonable for shaded or south-facing windows where the SHGC priority is lower. A specifier who chooses the same low-e product for every orientation in a house is not specifying for Perth.
Why Window Orientation Matters as Much as Glass Spec in Perth
Climate Zone 5 glazing guidance from Your Home recommends different SHGC targets for different orientations. The reasoning is straightforward. North-facing windows in Perth catch high winter sun (helpful) and lower-angle summer sun that is largely blocked by a properly sized eave (manageable). West-facing windows catch low-angle afternoon sun in summer that no eave can block effectively. South-facing windows get the least direct sun all year.
A practical Perth approach:
- West and unshaded south-west: lowest SHGC available, typically 0.20 to 0.35 solar-control low-e double glazed
- East and unshaded south-east: similar treatment for morning summer sun, particularly in bedrooms where heat lingers
- Unshaded north: low SHGC for summer, but watch that you do not give up too much winter solar gain
- Shaded north (under deep eaves): SHGC less critical, choose for U-value and clarity
- South: SHGC matters least, prioritise U-value and acoustic if relevant
For exact target numbers tied to a specific home’s design, the ABCB Glazing Calculator or a NatHERS-accredited assessor is the right tool. The orientation logic above is consistent with Your Home and DCCEEW guidance but specific numbers should come from a project-specific calculation. Our deeper guide to U-values, SHGC and window energy ratings in WA covers what each number on a quote actually means.
What Double Glazing Does Not Do
This part matters because expectations get inflated in sales conversations.
Double glazing does not replace ceiling insulation. A Perth roof space exposed to direct sun radiates heat downward into the ceiling all afternoon. If your ceiling insulation is degraded or missing, no glazing upgrade will fix the problem.
Double glazing does not replace external shading on west-facing glass. The sun’s angle in mid to late afternoon is too low for an eave to be useful, which is why west-facing rooms benefit from awnings, external blinds or planting in addition to low-SHGC glass.
Double glazing does not replace a working air conditioner. It reduces the cooling load, which means your AC works less hard and a smaller unit can keep up. It does not eliminate the need for active cooling on a 40-degree day.
Double glazing does not seal an old house. If the doors, the floor and the ceiling are leaky, the cool air your AC is producing will find its way out. Glazing is part of the building envelope, not the whole envelope.
How to Read a Perth Glazing Quote
Two quotes can look identical and deliver wildly different summer performance. The Penot Double Glazing cost guide for Perth covers what drives price and how to compare apples with apples. Three checks worth running on any quote.
Check the SHGC, not just “double glazed”. A quote that says “low-e double glazed” without an SHGC number is incomplete. Ask for the SHGC for each window or by orientation group. For west-facing windows the number should be at the low end of the range.
Check the frame. A premium glass unit in a standard non-thermally-broken aluminium frame loses much of its energy benefit through the frame itself. Standard aluminium frames have a U-value around 6.6 W/m²K, where thermally broken aluminium drops to around 2.2 W/m²K. Australian manufacturer technical data and AGWA fact sheets confirm the order of magnitude. If your project is a serious thermal upgrade, the frame matters.
Check WERS or AGWA certification. The Window Energy Rating Scheme at agwa.com.au/WERS lists Australian glazing products with verified Uw and SHGC ratings. Products listed there have been tested rather than estimated. AGWA membership means the supplier has agreed to Australian Standards compliance and a Code of Conduct.
Where Penot Double Glazing Fits
Penot Double Glazing is a full AGWA member based at 92 Beechboro Rd S, Bayswater WA 6053. Every window and door supplied complies with AS 2047 and AS 1288, and glass selections are made for the orientation of each opening rather than a one-size spec across a house. Double glazing is the standard product. Triple glazing is available as part of the same range for projects where the extra step is justified, typically west-facing rooms in homes targeting NCC 2025 best-practice thermal performance (the requirement that now applies to new home builds in WA), or where acoustic and thermal priorities combine.
If you are weighing up a quote and want a second look at the spec choices, a free in-home consultation will work through orientation, glass spec and frame choices specific to your home. You can compare your quote against the wider double glazing Perth range, book a free in-home assessment, call 1300 121 603, or visit the Bayswater showroom at 92 Beechboro Rd S.
Will Double Glazing Pay for Itself on Energy Bills Alone in Perth?
In Perth, the cooling-load reduction is real but the payback period depends on tariff, usage and the spec you choose. Australian industry modelling on energy savings varies by climate zone and home configuration. Perth is cooling-dominant so the honest framing is that the saving is meaningful but should not be the sole reason for upgrading. The comfort, acoustic, condensation and resale benefits combine to make the case.
Does Double Glazing Help with the Fremantle Doctor in the Afternoon?
The Doctor cools the outside air, which lowers the conducted heat load through the glass. It does not change the radiant solar load on the west, which is the bigger problem in a Perth summer. Low-SHGC glass handles the radiant load. The Doctor handles the rest.
Do You Have to Replace the Window Frame to Get Double Glazing?
For most Perth aluminium frames built in the 1980s or later, yes. The frames were not designed for a 24 mm IGU and the rebate depth will not accept the new glass. The standard approach is to replace the entire window unit rather than swap the glass in an existing frame. Secondary glazing (a separate inner pane installed inside the existing window) is a different product available in some heritage and apartment cases, with more modest performance than a full double glazed window.
Is Triple Glazing Worth It for Perth Homes?
For most Perth homes, well-specified solar-control double glazing in a thermally broken frame is the sweet spot. Triple glazing makes sense for west-facing rooms targeting the highest comfort and acoustic standards, or for homes pushing past 7-star NatHERS. The decision is project-specific. A separate article on triple glazing pricing in Perth covers what drives the premium and where it pays back.
