The pane count is the wrong starting point
When a Melbourne homeowner asks for double glazing, the request usually describes the glass: two panes separated by an insulating cavity. That is an insulated glass unit, or IGU, not a complete window. The finished assembly also includes the aluminum frame, spacer, seals, locks, mullions, sill, threshold, and the installation around the opening.
The costly mistake is specifying the IGU as if it controls the whole result. A Low-E, argon-filled unit can have an excellent center-of-glass rating while a continuous aluminum frame provides a direct path for heat. The glass may be sophisticated; the frame can still behave like a metal heat bridge.
That is why a serious specification for double-glazed windows in Melbourne starts with a question about the frame and the whole assembly, not with how many panes appear in the brochure. A high-performance window is a system. Its performance is governed by the weakest path through that system.
One window has three thermal ratings, not one
Window quotes often become confusing because suppliers use different U-values to describe different parts of the same product.
- Ug is the U-value at the center of the glass. It describes the insulated glass unit away from the frame and edge.
- Uf is the U-value of the frame. It shows how readily heat moves through the aluminum profile itself.
- Uw is the whole-window U-value. It combines the glass, frame, spacer, edge effects, and the proportions of that specific window.
Lower numbers indicate less heat transfer. For comfort and energy modeling, Uw is the figure that matters most. A quote that provides only Ug is describing the best-performing part of the window while leaving the frame out of the comparison.
A simple comparison
Consider a 1.2-by-1.5-meter window with 1.8 square meters of total area. Assume the frame occupies 0.35 square meters and the glass occupies 1.45 square meters. Give both versions the same Low-E, argon-filled double-glazed unit with a center-of-glass Ug of 1.6 W/m²K.
A simplified area-weighted calculation produces these results:
- Non-thermally broken frame with Uf of 5.0: (1.45 × 1.6 + 0.35 × 5.0) ÷ 1.8 ≈ 2.26 W/m²K before edge effects.
- Thermally broken frame with Uf of 1.8: (1.45 × 1.6 + 0.35 × 1.8) ÷ 1.8 ≈ 1.64 W/m²K before edge effects.
The glass has not changed. The cavity has not changed. The difference comes from the frame, and it is roughly 0.6 W/m²K in this simplified example before the spacer and perimeter are included. Actual certified Uw values depend on the profile, glazing pocket, spacer, size, and operating configuration, but the relationship remains.
Frame area also changes the result. A large fixed panel may be mostly glass, so the frame contributes a smaller percentage of the total heat flow. A small awning window, a heavily framed casement, or a sliding door with substantial meeting stiles and tracks has a greater frame proportion. The same IGU can therefore perform differently in different openings.
What a thermal break actually changes
Aluminum is strong, lightweight, and durable, but it is also highly conductive. An uninterrupted aluminum extrusion can carry heat from the exterior surface to the interior surface with very little resistance. The frame then becomes a thermal highway around the insulated glass unit.
A thermal break interrupts that path. Most thermally broken aluminum systems use reinforced polyamide strips mechanically joined between the interior and exterior aluminum sections. The two metal sections remain structurally connected, but the direct conductive route is substantially reduced.
The objective is not to make aluminum itself insulating. The objective is to stop the inside and outside sections from acting as one continuous piece of metal. A well-designed thermal break can reduce frame U-values from roughly 4.0 to 7.0 W/m²K in unbroken systems to approximately 1.0 to 2.5 W/m²K in many thermally broken systems. The exact result depends on the profile and testing, so the product data matters more than the label alone.
Many residential systems use thermal barriers around 20 millimeters wide, while premium profiles may use wider barriers. Width is useful information, but it is not a performance certificate. Continuity through the head, jambs, sill, corners, mullions, and door thresholds matters just as much as the nominal width. A tested Uw value is the best way to confirm that the geometry works as intended.
A thermal break is not a weather seal
These two features solve different problems.
A weather seal is usually a rubber or elastomeric component that limits air and water movement around an operable sash or panel. A thermal break is an insulating barrier inside the frame extrusion that limits conductive heat transfer through the metal.
A tight weather seal on a non-thermally broken frame can stop drafts while the frame remains cold. A thermally broken frame with poorly adjusted hardware can reduce conductive heat flow but still leak air around the sash. High performance requires both. Confusing the two is one reason a window can feel uncomfortable even when it has a premium IGU.
Condensation is where the weak link becomes visible
Condensation provides a practical test of whole-window performance. At an indoor temperature of 21°C and relative humidity of 50%, the dew point is approximately 10°C. If the room-facing surface of the frame, glass edge, or spacer falls below that temperature, moisture from the indoor air can condense on it.
On a cold Melbourne morning, the center of a Low-E double-glazed unit may remain comfortably above the dew point while the aluminum frame or perimeter edge becomes much colder. Occupants then see water on the frame and conclude that the double glazing has failed. The more precise diagnosis is often that the glass package is outperforming the frame.
Condensation locations reveal different problems:
- On the room-facing frame: the frame may be creating a thermal bridge, especially if it is not thermally broken.
- Along the glass perimeter: the spacer may be conducting heat away from the edge, or the glazing pocket may have a cold-bridge detail.
- Between the panes: the sealed IGU has likely lost its seal or absorbed moisture. Ventilation will not fix condensation inside the cavity; the IGU normally needs replacement.
Warm-edge spacers reduce the cold perimeter effect, but they do not replace a thermal break. Likewise, a thermal break cannot remove moisture generated by showers, cooking, drying clothes, or inadequate ventilation. It keeps interior surfaces warmer and reduces the chance of condensation; it does not change the humidity inside the home.
Moisture on the exterior face of the outer pane can occur under clear, calm conditions when an efficient window prevents indoor heat from reaching the exterior glass. That is different from moisture on the room side and can be a sign that the glazing is retaining heat effectively.
Premium glass cannot compensate for every weak link
Each part of the window controls a different heat path:
- Low-E coatings reduce radiant heat transfer through the glass and influence solar heat gain.
- Argon filling slows heat movement through the cavity between panes.
- Warm-edge spacers reduce heat loss around the perimeter of the IGU.
- Thermal breaks interrupt conduction through the aluminum frame.
- Compression seals and accurate installation control air leakage and water entry around moving panels and the wall opening.
These features are complementary, not interchangeable. An argon-filled unit in a conductive frame still loses heat through the frame. A thermally broken frame with clear, poorly specified glass still has weak center-of-glass performance. A warm-edge spacer can improve the edge while a gap around the installed frame allows drafts.
Compare two hypothetical quotes for the same opening. Quote A uses 6-millimeter glass, a 12-millimeter cavity, and 6-millimeter glass with Low-E and argon in a standard aluminum frame. Quote B uses the same glass package in a thermally broken frame with a warm-edge spacer. The center-of-glass Ug may be identical on both quotes, but the whole-window Uw, interior surface temperature, and condensation risk will not be.
That is why a quote that emphasizes thicker glass or argon while saying nothing about Uf and Uw is incomplete. The most expensive upgrade is not always the one with the most impressive glass description. Sometimes the better investment is the less visible component that removes the frame's thermal bridge.
A thermal break is also not an acoustic rating. Homes beside a busy road may need laminated or asymmetric glass and strong compression seals in addition to the thermal barrier. The broader principle is the same: each performance objective must be assigned to the component capable of delivering it.
The same glass behaves differently in different openings
A window's tested performance is specific to its size and configuration. It is risky to apply the rating of a fixed panel to a sliding door or assume that a small awning window will perform like a large picture window.
Small openings often contain a higher percentage of frame. Operable windows add sashes, hinges, locking hardware, and seals. Sliding systems add tracks, meeting stiles, rollers, and a sill or threshold that can become the coldest part of the assembly. Large doors may use the same IGU thickness as a window but have far more perimeter framing and moving joints.
The thermal break must therefore continue into the parts that matter for that configuration. For a sliding door, ask whether the sill and track are thermally improved or simply fitted with a thermally broken vertical frame. For a multi-panel system, ask for the whole-door rating rather than the rating of a single fixed panel.
This also explains why two suppliers can quote the same nominal glass thickness yet produce noticeably different comfort outcomes. The profile geometry, frame percentage, track design, spacer, and seals all change the final result.
Retrofit glass is not the same as a new window system
Replacing single glass with a sealed double-glazed unit can improve the center-of-glass performance of an existing window. It does not remove the thermal bridge in an old aluminum frame.
A retrofit IGU inserted into a sound existing frame can be sensible when the frame is structurally solid, the rebate can accommodate the thicker unit, and heritage or budget constraints limit the work. It may improve comfort and reduce noise, but the original frame U-value, seals, hardware, and sill details remain.
Secondary glazing creates another air space inside the existing window. It can deliver a meaningful improvement without removing the original frame, particularly in heritage properties. It still does not turn the original aluminum extrusion into a thermally broken frame, and its performance depends heavily on the separation, sealing, and installation quality.
Full replacement creates the opportunity to coordinate the frame, IGU, spacer, seals, hardware, and perimeter installation as one system. That is usually the stronger long-term option when existing frames are corroded, difficult to operate, poorly sealed, or consistently cold to the touch. The higher initial cost buys more than new glass; it removes limitations inherited from the old frame.
Victoria's 7-star requirement makes whole-window data important
Since May 1, 2024, new Victorian homes have been designed to a minimum 7-star NatHERS rating. The requirement does not simply state that every window must use a thermal break or double glazing. It evaluates the building's modeled performance, including insulation, orientation, shading, window area, U-values, and solar heat gain coefficients.
That distinction matters. A project can sometimes compensate for less efficient glazing with smaller openings, more insulation, external shading, or a different orientation. But the NatHERS assessment still needs accurate window data. A description such as double-glazed aluminum is not enough to model a product reliably.
The assessor needs the performance of the actual frame and glass combination, not a generic value for the glass alone. Uw describes heat transfer through the whole unit, while SHGC helps determine how much solar energy enters through the glass. A low U-value may reduce winter losses, but the appropriate SHGC still depends on orientation and shading, particularly on west-facing windows exposed to intense summer afternoon sun.
Thermally broken frames make it easier to achieve the required performance without shrinking every opening or compensating elsewhere in the design. They do not replace good orientation and shading, but they prevent the frame from undermining the energy model.
Questions that expose a weak specification
A supplier should be able to answer these questions with product-specific documentation:
- What is the whole-window Uw for the actual size and opening type being quoted?
- Is the published U-value Ug for the glass or Uw for the complete window?
- What is the frame Uf, and is the profile genuinely thermally broken?
- Does the thermal barrier continue through corners, mullions, sills, tracks, and thresholds?
- What spacer is used, and is it a warm-edge design?
- What are the Low-E coating, gas fill, glass thickness, and SHGC for each orientation?
- Does the WERS label or test report apply to the exact frame and glazing combination?
- How will the perimeter be air-sealed, flashed, drained, and adjusted during installation?
Use a supplier's whole-window specifications as the comparison baseline; a brochure that offers only a center-of-glass value does not provide enough information to compare complete products.
A cross-section drawing is useful as well. It can show whether the polyamide barrier is continuous, how the glass is supported, where the seals sit, and whether the sill or track has been designed as part of the thermal system rather than added as an afterthought.
Installation can bypass a good thermal design
A laboratory-rated window is not the same thing as an installed window. The certified frame may have excellent thermal performance, but poor installation can introduce drafts, water entry, operating problems, and cold spots around the perimeter.
The opening should be square and adequately supported. Packers should carry the frame without distorting it. Sashes need to close evenly so compression seals make consistent contact. The perimeter gap needs a continuous air seal, while flashing and sill drainage direct water away from the wall cavity.
These details do not usually change the laboratory Uw printed on the product data, but they change what occupants experience. A small unsealed gap can create a noticeable draft beside an otherwise insulated frame. A frame installed out of square can prevent locks and seals from working properly. A poorly detailed sill can admit water that damages insulation and surrounding finishes.
The installation scope should therefore identify who handles removal, opening repairs, flashing, perimeter sealing, hardware adjustment, and final testing. A thermal break is built into the profile; whole-window performance is delivered only when the profile, glass, seals, and opening are treated as one assembly.
The specification that holds up
Double glazing describes the insulated glass unit. A thermally broken, properly sealed, whole-window system determines whether that unit delivers comfort in a Melbourne home.
For a practical specification, prioritize the components in this order:
- A genuinely thermally broken aluminum frame with continuity through the relevant profile and threshold.
- A documented whole-window Uw for the exact opening type and size.
- Low-E glass, argon, and a spacer selected for the home's orientation and condensation risk.
- SHGC data that suits solar exposure rather than simply choosing the lowest available U-value.
- Compression seals, sound hardware, flashing, drainage, and a continuous perimeter air seal.
- Installation and compliance documentation that matches the product actually delivered.
If a quote cannot show how the frame performs, its glass specification is incomplete. Two panes in a conductive frame can be better than single glazing, but they are not equivalent to a thermally broken system. The frame is not a supporting detail behind the glass; it is one of the main thermal paths through the building envelope.