A double glazed unit can only perform as well as the frame holding it. Aluminium conducts heat hundreds of times more readily than timber or uPVC, so a standard aluminium frame acts as a bypass around the insulating glazing, dragging down the performance of the whole window. Thermal breaks address this directly. But in Sydney’s climate the more consequential number is often solar heat gain rather than conductive loss, and that is decided almost entirely by the glass.
What the frame actually does thermally
A window loses and gains heat through two paths that behave differently.
Through the glazing. In a sealed unit, two panes separated by a gas filled cavity slow conduction and convection, and a low emissivity coating reflects radiant heat. This is the part most people are buying when they buy double glazing.
Through the frame. The frame surrounds the glazing and connects the outside of the building to the inside. If the frame material conducts readily, heat simply goes around the insulated glazing rather than through it.
That second path is called thermal bridging, and it is why a well specified glass unit in a poor frame underperforms a modest unit in a good one.
The frame typically occupies 20 to 30 percent of a window’s area. Its share of the window’s total heat transfer is frequently much larger than that, because conduction through aluminium is so much faster than through an insulated glass cavity.
Understanding U-value and SHGC
Australian windows are rated on two numbers, and confusing them is the most common error in specification.
U-value
U-value measures how readily heat passes through the window by conduction, convection and radiation. Lower is better. It matters most when there is a temperature difference between inside and outside, so it drives winter heat loss and summer heat gain through the fabric of the window.
Whole window U-value is not the same as centre of glass U-value. Glass manufacturers publish figures for the glazing alone. Those numbers are always better than the finished window, because the frame and the edge of the glass both perform worse than the middle of the pane.
This is the single most useful thing to know when comparing quotes. A supplier quoting a centre of glass figure is quoting the flattering number, and the gap between it and the whole window figure is largely the frame.
SHGC
Solar Heat Gain Coefficient measures how much solar radiation passes through the window, expressed between 0 and 1. Lower means less heat admitted.
SHGC is determined mostly by the glass, particularly by tinting and low emissivity coatings, with the frame contributing only through the proportion of the opening it covers.
Why the distinction matters in Sydney
Much of the material written about window thermal performance originates in northern Europe and North America, where heating dominates and U-value is the number that counts.
Sydney sits in a different position. Cooling loads matter at least as much as heating, and on north and west facing elevations, solar heat gain through the glass is frequently the larger problem. A window with an excellent U-value and a high SHGC can make a west facing room hotter, not cooler.
The practical consequence is that frame material, which affects U-value substantially and SHGC hardly at all, matters less in Sydney than the marketing around it suggests. It still matters for compliance and for condensation, both covered below, but the glass usually does more.
How the materials compare
Thermal conductivity is measured in watts per metre kelvin. Lower means better insulation.
| Material | Approximate conductivity |
|---|---|
| Aluminium | 160 to 200 |
| Steel | around 50 |
| Polyamide (thermal break) | around 0.3 |
| uPVC | around 0.17 |
| Timber | around 0.13 to 0.15 |
The gap between aluminium and the alternatives is not marginal. Aluminium conducts heat more than a thousand times faster than timber does.
Timber insulates best of the common frame materials, and its cellular structure is genuinely effective. Its drawbacks in Australia are dimensional movement, maintenance and coastal durability rather than thermal performance.
uPVC uses multi chamber hollow sections, and the trapped air in those chambers adds to the material’s own low conductivity. It performs well thermally. Its limitations are strength, requiring bulkier sections and often internal steel reinforcement, ultraviolet exposure, and bushfire performance under AS 3959.
Standard aluminium performs worst. A single piece aluminium section is a direct metal path from outside to inside.
Thermally broken aluminium is the resolution, and it is covered next.
How a thermal break works
A thermally broken frame is not one aluminium section. It is two, an outer and an inner, joined by insulating polyamide struts that are mechanically crimped into both halves.
Because polyamide conducts heat at roughly one five hundredth the rate of aluminium, interrupting the metal path with a polyamide bridge dramatically reduces transfer. The frame keeps aluminium’s strength, stiffness, dimensional stability and coastal durability, while losing most of its thermal disadvantage.
The improvement in whole window U-value is substantial rather than marginal, which is why thermally broken systems have become common in Australia since NCC 2022 raised residential energy requirements.
The same geometry also interrupts the path that vibration takes along the frame, which is a separate benefit covered in our article on the impact of window frame material on sound proofing properties.
The spacer bar, the other thermal bridge
This one rarely gets mentioned and it sits inside the glazing rather than in the frame.
The two panes of a sealed unit are held apart by a spacer bar running around the perimeter. Traditionally that spacer was aluminium, which means a metal strip connecting the outer pane to the inner one around the entire edge of the unit.
The insulating cavity is bridged at its perimeter, and the edge of the glass runs colder in winter and warmer in summer than the centre does. This is why condensation appears first around the edges of a double glazed pane rather than in the middle.
Warm edge spacers use lower conductivity materials such as stainless steel, structural foam or composites. They improve whole window U-value and, more noticeably to an occupant, reduce edge condensation.
If you are specifying thermally broken frames, asking about the spacer is a reasonable next question. There is limited point interrupting the frame bridge while leaving the edge bridge intact.
Condensation
Condensation on the inside of a window is a thermal problem, and the frame is frequently where it appears first.
Moist internal air condenses when it meets a surface below its dew point. A standard aluminium frame runs close to outside temperature because it conducts so readily, so on a cold morning the frame is the coldest surface in the room and water forms on it.
Thermally broken frames run considerably warmer on the internal face, which is often the change people notice most, ahead of any difference in their energy bills.
Two clarifications worth making.
Condensation on the frame or the room side of the glass is a thermal and humidity issue, not a fault. It indicates the surface is cold and the air is moist, and it is addressed with better frames, better glazing and ventilation.
Condensation between the panes of a sealed unit is a failure. The seal has been compromised and moisture has entered the cavity. It cannot be repaired and the unit is replaced. See our double glazed windows for how these units are constructed.
WERS and NCC 2022
WERS, the Window Energy Rating Scheme, rates whole windows for heating and cooling performance in Australian conditions. Crucially it rates the complete window including the frame, not the glass in isolation, which is what makes it a fairer basis for comparison than a centre of glass figure.
NCC 2022 raised the energy requirement for new residential buildings to 7 stars under NatHERS, alongside a whole of home energy budget. Windows are one of the larger contributors to a home’s thermal performance, and the tightening is much of why thermally broken aluminium has moved from a specialist product to a common one.
Ask for WERS rated figures for the specific window system rather than glass data, and confirm the numbers relate to the configuration you are actually ordering.
Requirements are periodically revised, so confirm current obligations for your project.
What to specify
Get whole window figures, not centre of glass. This single question separates suppliers who know their product from those reading off a glass brochure.
Specify thermally broken framing where thermal performance matters, particularly on elevations that get cold in winter or where frame condensation is a concern.
Prioritise SHGC on north and west elevations, since in Sydney solar gain through the glass frequently outweighs conductive loss through the frame. This is where low emissivity and tinted glazing earn their cost.
Ask about the spacer. A warm edge spacer improves both the U-value and the edge condensation behaviour.
Do not neglect the seals. Air leakage carries heat as well as sound, and infiltration is not captured in a U-value at all. A well specified window that does not seal properly underperforms its ratings.
Remember the whole opening. Doors are typically the largest glazed elements in a home and the same considerations apply. See our double glazed doors.
Aluminium’s thermal conductivity is a genuine weakness, and it is one of the reasons the material is worth understanding rather than simply choosing. We set out the full picture in our article on the advantages and disadvantages of aluminium doors.
Frequently asked questions
Does the window frame really affect thermal performance?
Substantially. The frame is typically 20 to 30 percent of the window area but often a larger share of its total heat transfer, because aluminium conducts heat hundreds of times faster than an insulated glass cavity allows it to pass.
What is the difference between U-value and SHGC?
U-value measures heat passing through the window by conduction, convection and radiation. SHGC measures how much solar radiation the window admits. Both are lower is better, but they address different problems, and in Sydney SHGC often matters more on north and west elevations.
Is centre of glass U-value the same as the window’s U-value?
No, and this is the most common source of confusion. Centre of glass figures exclude the frame and the edge of the glass, both of which perform worse. Always compare whole window figures.
Are thermally broken aluminium windows worth it?
Where thermal performance or frame condensation matters, yes. The polyamide strut interrupts the metal path through the frame and improves whole window U-value substantially, while keeping aluminium’s strength and durability.
Why does condensation form around the edges of my double glazed window?
Usually the spacer bar. A traditional aluminium spacer bridges the cavity at the perimeter, so the edge of the glass runs colder than the centre. Warm edge spacers reduce this.
Is uPVC better than aluminium for insulation?
On material conductivity alone, yes. But uPVC needs bulkier sections and often steel reinforcement, which reduces the glazed area, and thermally broken aluminium closes most of the gap while performing better on strength, ultraviolet resistance and bushfire requirements.
What is WERS?
The Window Energy Rating Scheme, which rates complete windows for heating and cooling performance in Australian conditions. It rates the whole window including the frame rather than the glass alone, which makes it a fairer basis for comparison.
