What is a double glazed window?

Double glazing and thermal breaks are routinely treated as two options for the same upgrade, and that confusion costs money. They are not alternatives. Heat escapes a window along two separate paths, one through the glass and one through the frame, and each technology fixes one of them. Double glazing upgrades the glass unit. A thermal break upgrades the frame. Understanding which path each one closes is the difference between a specification that works and one that quietly leaks heat around a perfectly good window.

Double glazing and thermal breaks solve different problems

A window is an assembly of two very different materials. The glazing is the large transparent surface in the middle. The frame is the metal or plastic structure that carries it. Heat flows through both, but at wildly different rates, and the fixes target each material separately.

The industry has a shorthand for this. One Australian fabricator states it as a slogan: double glazing is to glass what a thermal break is to aluminium windows. The two products sit on different parts of the assembly, which is why the question is never which one is better. It is whether your specification closes both paths.

This also explains why the two terms appear together so often. A high-performance window sold in Australia today is usually double glazed, thermally broken, or both. The parts are independent, but the best numbers come from combining them.

What double glazing does

A double glazed window uses an insulated glass unit, commonly written IGU: two panes of glass separated by a spacer and sealed at the factory, with the cavity between them filled with air or an inert gas such as argon. The trapped layer is the insulator. It slows conduction, because gas conducts far less heat than glass, and it creates a dead air space in which convection barely circulates. A low-emissivity, or Low-E, coating on one surface goes further by reflecting radiant heat back toward the room.

The Australian government's energy authority puts the headline effect at up to 30 per cent less heat loss through the window, and notes that in Australian conditions double glazing is usually sufficient, with triple glazing reserved for extremely cold climates because a third pane can cut useful winter solar gain from north-facing windows. Most of the heat that reaches a window never gets the chance to move in any direction: on hot days up to 87 per cent of heat gain in Australian homes comes through windows, and windows account for roughly 40 per cent of heat loss on cold days, so the pane upgrade is working on a large share of the building's weakest surface.

Double glazing also changes how the window feels and sounds. The inner pane stays closer to room temperature, which removes the cold radiance near glass in winter and the condensation that forms wherever a surface drops below the dew point. The two panes and the cavity muffle outside noise by a meaningful margin, which is why buyers on busy roads and near freeways ask for it first. All of these benefits live in the glass unit itself, and they are described by the glass performance numbers: the centre-of-glass U-value and the Solar Heat Gain Coefficient, or SHGC.

What a thermal break does

A thermal break is an insulating material, almost always a reinforced polyamide strip, inserted between the interior and exterior sections of an aluminium window frame. Its only job is to interrupt the path that heat takes through the metal. Aluminium is an excellent conductor, roughly 160 W/mK, while the polyamide used for breaks sits near 0.3 W/mK, a difference of more than 500 times. Without the break, an aluminium frame is a continuous metal bridge from inside to outside.

That bridge matters more than most buyers expect. Frames are a smaller area than the glass, but metal conducts so readily that an unbroken aluminium frame can undo much of what an expensive glazing unit achieves. Australian forums make the point bluntly: double glazing without a thermally broken frame is widely described as a wasted premium, because winter warmth simply conducts out through the metal while the glass does its job beside it.

The break also improves comfort in the same ways the glazing does. With the conductive path cut, the inside face of the frame stays warmer in winter, which reduces cold surfaces, condensation and the mould that follows it. On hot days the effect runs in reverse, with outdoor heat held out of the frame. A frame without a break has no such mechanism.

How the two combine: the whole-window number

The performance of a window is not the performance of its glass. Technical literature separates three values: Uw for the whole window, Uc or Ug at the centre of the glass, and Uf for the frame alone. The number that governs your energy report is Uw, and it is a blend of glass performance, frame performance, spacers and seals. Two windows with identical glass can differ by a full point on the whole-window scale if one frame conducts and the other is broken.

The ladder shows how the two technologies stack. A typical single glazed window in an aluminium frame lands around 5.5 W/m2K. Double glazing in a standard, unbroken aluminium frame improves that to roughly 3.0. Double glazing in a thermally broken aluminium frame reaches about 1.8 W/m2K. Published bands for thermally broken double glazed aluminium sit at 2.0 to 2.7 for the whole window, a range Australian suppliers describe as rivaling mid-range uPVC. Each step is the sum of a glazing upgrade and a frame upgrade, and skipping either step leaves the other one carrying the loss.

This is why a single phrase is never a specification. The term double glazed covers plain float glass and Low-E coated argon filled units with a fourfold spread in performance. The term thermally broken covers 12 mm polyamide strips and 24 mm structural struts. Ask for the whole-window figure from a named product, and compare that, rather than accepting a description.

When you need both: the 7-star context

The energy code changed the shape of this decision. New homes in most Australian states must now reach a 7-star NatHERS rating, and single glazing has effectively disappeared from compliant new builds. Window suppliers describe double glazing with Low-E coatings, appropriate SHGC and thermally broken frames as the baseline rather than the premium in this environment, and thermally broken aluminium with Low-E argon double glazing is typically enough for 7 stars with margin, with 8 stars achievable in many configurations.

The regulatory tools treat the two technologies as independent dimensions, which is the correct mental model. The BASIX default window tables classify every window by frame material, with aluminium, thermally broken aluminium, and timber or uPVC as separate entries, and by glazing system, with single glazed, double glazed air filled and double glazed argon filled as separate entries. The grid that results is the full menu of choices: you can spec each column independently.

Whether you need both still depends on climate and orientation. Double glazing is not mandatory everywhere: a well oriented home can reach 7 stars with limited single glazing in some positions. And the value of a thermal break is highest where winter heating dominates, because the break's main job is stopping conductive heat loss through the frame. A Perth forum discussion puts the counterpoint bluntly, arguing that over plain aluminium double glazing, the break adds little for solar heat gain or acoustics, and its real benefit is the winter U-value. In a warm climate with modest heating needs, that benefit shrinks.

Making the call: retrofit, replacement and cost

The decision logic changes completely depending on whether you are building, replacing, or retrofitting. In a new build, the choice is nearly free: the price difference between single and double glazing at that stage is small, and a thermally broken frame is a modest line item on a larger window budget. Homeowner discussions consistently describe double glazing in a new build as a no brainer, and the same logic pushes thermally broken frames.

In an existing home the two upgrades separate. Double glazing can be retrofitted into frames that are deep enough to take an insulated unit, often for a few hundred dollars per square metre of glass. A thermal break cannot be retrofitted at all: it is built into the frame at manufacture, so it arrives only with a full window replacement. If your frames are staying, you can buy glazing but not the break, which is why retrofit buyers are the ones most likely to be quoted unbroken aluminium with upgraded glass.

Cost compounds the same way the performance does. Thermally broken aluminium typically runs 15 to 30 per cent above standard aluminium frames, while uPVC undercuts thermally broken aluminium by 20 to 30 per cent. Against the whole-house number, the premium for double glazing on a new build is usually a low four figure sum, while a full replacement in an existing home runs into five figures. The honest payback on energy savings alone is long, often measured in decades, which is why most owners buy double glazing for comfort, noise and condensation first and let the savings compound in the background.

Frequently asked questions

What is the difference between double glazing and a thermal break?

Double glazing is an upgrade to the glass: two panes separated by a sealed gas cavity, which insulates the glazed area of the window. A thermal break is an upgrade to the frame: an insulating strip inside an aluminium frame that stops heat conducting through the metal. They solve different heat-loss paths and are combined in high-performance windows.

Can you have double glazing without a thermal break?

Yes, and it is common, particularly in retrofits where the existing aluminium frames are kept and only the glass is upgraded. The window performs better than single glazing, but the unbroken frame remains a thermal bridge, and the whole-window U-value stays well above what the glass alone suggests. It is the reason specifications should always quote the whole-window number, not the centre-of-glass figure.

Does a thermal break work without double glazing?

It does. A single glazed window in a thermally broken frame loses far less heat through the frame than the same glass in an unbroken aluminium frame, and the frame stays warmer in winter. That combination is rare in Australia because buyers who specify the frame upgrade usually take the glazing upgrade as well, but the frame performance exists independently of the glass.

Which is more important: double glazing or a thermal break?

For an aluminium window in a cold climate, the thermal break is the foundation, because an unbroken aluminium frame undermines whatever the glass achieves. In a mild climate, double glazing is usually the larger upgrade of the two, and government guidance notes Australian homes generally get sufficient performance from double glazing without reaching for premium frame systems. For the best numbers, and for 7-star compliance with margin, you take both.

How do I know if my window has a thermal break?

Look at the frame from the side or from a cut edge: a thermally broken frame shows a thin dark line of polyamide running along the profile between the interior and exterior aluminium sections. On a closed window it is often visible in the join where the sash meets the frame. The reliable way is the paperwork: a WERS certificate, a BASIX listing, or the supplier's specification will state the frame construction and the whole-window U-value.