Aluminium Thermal-Break Windows and Condensation: Frames, Glass and Indoor Humidity

Condensation on an aluminium window is a building-physics symptom, not automatically proof that the window is defective. Water appears when moist indoor air meets a surface that is colder than the air’s dew point. That surface may be the glass, an aluminium frame, a glazing bead, a spacer at the edge of the glass, a metal fastener, or a small section of the reveal around the opening. A modern window can reduce the likelihood of condensation, but it cannot remove moisture from a room or guarantee a dry surface under every combination of weather, occupancy, heating, ventilation and construction detail.

This article explains the general relationship between thermal-break window frames, glazing and indoor humidity. It also identifies the information needed for a project-specific diagnosis. The discussion uses Australian references where relevant, including the National Construction Code 2025 (NCC 2025), AS 2047, AS 1288 and AS 4666, and then notes the different regulatory context in New Zealand. The standards are source labels, not substitutes for reading the current standard, checking its scope and obtaining advice from a suitably qualified designer, building surveyor, glazier or building-science professional.

What condensation means

Air can hold water vapour. The warmer the air, the greater its potential moisture capacity. When that air is cooled, its relative humidity rises. If a surface is cold enough for the local air to reach saturation, vapour changes into liquid water on the surface. The temperature at which that change begins is called the dew-point temperature.

The important comparison is therefore between the temperature of the relevant window surface and the dew point of the air immediately next to it. A room can feel comfortable while a narrow aluminium edge, corner or spacer remains cold enough for condensation. Conversely, a cold night may produce no visible water when indoor moisture is low and air movement keeps the interior surface above the dew point.

General principle: condensation risk increases when indoor moisture increases, when the window surface temperature decreases, or when air movement at the surface is restricted.

Project-specific boundary: a reliable dew-point or surface-temperature assessment requires measured or defensible inputs, such as indoor air temperature, indoor relative humidity, outdoor conditions, window orientation, glazing configuration, frame sections, spacer type, installation geometry and local thermal bridges. A generic statement such as “double glazing will stop condensation” is not a project calculation and should not be presented as a guarantee.

Why thermal breaks matter in aluminium frames

Aluminium conducts heat readily. In an unbroken aluminium frame, the interior and exterior portions can be connected by a continuous metal path. In cold conditions, heat can move outward through that path and the inner frame surface can become relatively cold. A thermal-break frame places a lower-conductivity component between the principal interior and exterior aluminium sections. This interrupts the most direct conductive path and can raise the interior frame temperature compared with a similar non-thermal-break frame.

A thermal break is not a blanket of insulation around every part of the window. Screws, pressure plates, reinforcement, corner cleats, drainage paths, glazing beads and other components can still conduct heat. The frame’s performance depends on the exact section, the length and continuity of the break, corner construction, hardware, seals, glazing system and installation. The thermal break can reduce condensation risk at the frame, but it cannot control condensation on a poorly performing pane, a cold spacer, a metal sill, or an adjacent wall junction.

A thermal break is part of a system

Window descriptions often use broad terms such as “thermally improved” or “thermally broken.” Those descriptions do not by themselves identify the tested or calculated performance of a particular size and configuration. A project specification should identify the system, frame section, opening type, glazing, spacer and relevant evidence of performance. The same series may perform differently as a fixed light, awning sash, sliding panel, corner window or heavily divided unit.

The continuity of the thermal break also matters at corners and junctions. A break that is effective in a straight jamb may be interrupted or bypassed at a corner, transom, mullion, hinge, lock or sill. The installation can create another bridge if the frame is fixed through a conductive packer, if the reveal is very narrow, or if a metal lining connects the internal and external sides.

Frame temperature, glass temperature and edge temperature

“The window temperature” is not one temperature. A window has several thermal zones. The centre of a glazed pane often behaves differently from its perimeter. The inner surface of a frame may behave differently from the inner surface of the glass. The bottom edge may be cooler than the top because of frame geometry, drainage details, blinds, curtains or limited warm-air circulation.

Glass performance depends on the number and type of panes, coatings, gas fill where specified, cavity width, pane thickness, orientation and boundary conditions. A higher-performing centre of glass does not necessarily mean that the glass edge will be equally warm. Heat can leave the pane through the spacer and into the frame, producing a colder perimeter. This is why edge condensation can occur while the centre of the glass remains clear.

Spacers and the perimeter seal

The spacer is the component that separates the panes around the edge of an insulating glass unit. Its material and geometry influence edge temperatures and the thermal bridge between glass and frame. A lower-conductivity spacer can reduce the severity of the edge bridge compared with a highly conductive spacer, but spacer selection must also satisfy compatibility, durability, gas retention and manufacturer requirements.

The insulating glass unit also relies on primary and secondary seals. A failed or damaged edge seal can permit moisture entry into the cavity, which is different from ordinary room-side surface condensation. Persistent fogging between panes, droplets inside the sealed unit, or a visible deterioration of the perimeter seal should be assessed as a glazing-unit issue. Do not confuse that condition with water forming on the room-facing surface.

Reveals and surrounding construction

The reveal is the return surface between the wall face and the window frame. Its depth, material, insulation, lining and finish influence the local surface temperature. A deep, cold masonry reveal can be more condensation-prone than the centre of the window. A steel angle, uninsulated lintel, metal flashing or poorly insulated sill can create a narrow cold strip next to an otherwise good frame.

Interior finishes can also hide the symptom. Paint, plasterboard, trim and sealant may conceal dampness until staining, peeling or mould appears. A curtain or blind set close to the glass can reduce mixing of room air at the window, allowing the surface to cool relative to the room. It can also trap moisture in a small air volume. The window may therefore be sound while the furnishing arrangement makes the local microclimate worse.

Indoor humidity is often the decisive variable

Indoor moisture comes from people, cooking, bathing, drying clothes, plants, aquariums, combustion appliances, wet building materials, unvented appliances and moisture entering from the building fabric. A closed room with several occupants can accumulate vapour quickly, especially when doors are shut and extraction is weak. Air conditioning or heating can change air temperature without removing all of the moisture generated by occupants and activities.

Relative humidity is temperature-dependent. The same quantity of water vapour can produce a different relative humidity when the air temperature changes. A room that is warmed can show a lower relative-humidity reading without having removed the moisture. When the air later cools near a window, the local relative humidity can rise sharply.

Humidity measurements should be taken with a suitable instrument and interpreted over time rather than based on a single reading. Place the instrument away from a heater, direct sunlight, an open window, a bathroom exhaust outlet and the immediate cold surface. Record indoor temperature and relative humidity during the conditions when condensation occurs. A project assessment should also consider outdoor temperature, wind, rain, heating operation and whether blinds or curtains were closed.

Ventilation and moisture control

Ventilation removes moisture-laden indoor air and replaces it with drier outdoor air when outdoor conditions permit. Kitchen and bathroom exhaust should discharge to the outside through correctly installed ductwork rather than into a roof space, wall cavity or enclosed balcony. Run extraction during moisture-producing activities and for an appropriate period afterwards. Closing an internal door while exhausting a wet room can help prevent vapour spreading, provided replacement air can enter safely.

Natural ventilation depends on weather, opening size, wind pressure, occupant behaviour and security. Mechanical ventilation depends on design, commissioning, filters, controls, duct routes and maintenance. Opening a window may reduce humidity in one climate and increase it in another. Ventilation advice must therefore be climate- and project-specific rather than a universal instruction.

Moisture control is not achieved by permanently leaving windows open if that causes security, weather, energy or indoor-air-quality problems. Nor should ventilation be used to conceal water ingress, a plumbing leak, an unsealed shower, a failed roof junction or rising damp. Diagnose the moisture source first, then select a safe and suitable control strategy.

Drainage, seals and water that is not condensation

Aluminium windows are designed with drainage paths that allow incidental water within the frame system to leave through weep holes or other openings. Those openings must remain clear. Sealant, paint, debris, insect screens, dust and later renovation work can block drainage. Blocked drainage can cause water to remain in the sill, overflow internally, or enter the wall around the opening.

External perimeter seals control water and air movement at the interface between the frame and the wall. Internal air seals and suitable back-up seals help prevent humid indoor air from reaching cold concealed surfaces. A joint can look sealed on the face while remaining open behind the trim. Sealant should not be applied over designed drainage openings or used to close a path that the system requires for pressure equalisation and drainage.

Water on a sill after wind-driven rain, water that follows a crack or joint, and water that appears only when a hose is directed at an opening are not automatically condensation. Staining, salt deposits, swollen linings or localised wetness after rain point toward an ingress investigation. Plumbing and roof leaks should also be excluded.

A practical condensation diagnosis

Start by recording exactly where and when the water appears. Note whether it is on the room-side glass, the frame, the spacer edge, the reveal, the outside face, or between panes. Record whether it follows cold clear nights, rain, showers, cooking, laundry, closed curtains or a change in heating. Photographs taken before wiping are useful because they show patterns and location.

Identify the surface. Use observation and, where suitable, a non-destructive moisture and temperature survey. Do not drill or remove sealed units simply to test a theory.
Check the indoor conditions. Record room temperature and relative humidity at several times, especially during the event. Compare the room with a nearby room that does not show condensation.
Inspect airflow. Look for closed blinds, curtains touching the glass, furniture blocking a convector or supply outlet, and rooms with little air movement.
Inspect ventilation. Confirm that kitchen and bathroom exhaust operates, discharges outdoors and is not obstructed. Consider whether moisture-producing activities are adequately contained.
Inspect the window system. Check frame joints, seals, glazing beads, drainage openings, hardware, damaged gaskets and signs of movement. Do not remove system components outside the manufacturer’s instructions.
Check the surrounding junction. Examine the sill, jambs, head, trim and reveal for cold spots, gaps, cracking, staining or missing insulation.
Separate condensation from ingress. Compare the timing with rainfall and plumbing use. Trace water paths rather than assuming that every wet mark began on the glass.

Thermal imaging can help locate patterns, but it does not directly measure dew point and can be misleading when emissivity, reflections, solar exposure, wind or surface moisture are not controlled. A competent assessment should combine images with measured temperatures, humidity, drawings and physical inspection.

General principles versus project-specific calculations

The general principles in this article explain why a colder surface and higher indoor humidity increase risk. They do not establish compliance, a window’s U-value, a whole-window energy rating, a dew point, a minimum surface temperature or a guaranteed condensation outcome.

A project-specific calculation may need the exact frame and glazing data, spacer properties, window dimensions, orientation, shading, indoor and outdoor design conditions, wind exposure, adjoining construction, fixings, seals and installation geometry. The assessor may use two-dimensional or three-dimensional thermal-bridge analysis, manufacturer data, laboratory evidence, a recognised rating method, or a combination appropriate to the question. Any reported result should state its assumptions, boundary conditions, uncertainty and scope.

Do not copy a centre-of-glass value into a whole-window specification. Do not treat a product brochure’s generic performance as evidence for every size. Do not infer compliance from a single infrared image. Do not promise that a thermal break, a second pane, a new spacer or a seal replacement will eliminate condensation without checking the complete assembly and the indoor moisture load.

Retrofit options and their limits

Retrofit measures can include improving exhaust ventilation, reducing indoor moisture production, adjusting curtains or blinds, repairing drainage, renewing compatible gaskets, correcting perimeter seals, replacing a failed insulating glass unit, adding an appropriate secondary glazing system, or replacing the complete window. Each option addresses a different mechanism.

Replacing glass alone may improve the glass centre while leaving a cold frame, spacer, sill or reveal. Adding a thermal-break frame may not correct a wet wall junction or excessive humidity. Re-sealing an external joint may stop rain entry but will not warm a cold interior surface. A secondary glazing layer may alter airflow and moisture paths and must be detailed so that concealed condensation does not develop between layers.

Existing openings can impose limits on frame depth, drainage, sill level, reveals, flashing, fire and acoustic requirements, accessible operation, heritage controls and the structural support available. Removal can disturb membranes and finishes. New fixings can bridge insulation or damage waterproofing. Before committing to a retrofit, obtain a condition survey and confirm the window manufacturer’s installation requirements, the building approval pathway and any required evidence.

Australian references and NCC 2025 boundaries

NCC 2025

The National Construction Code 2025 (NCC 2025) is an Australian performance-based code published by the Australian Building Codes Board. Its application depends on building classification, building work, climate zone, state or territory adoption, variations, exclusions and the relevant provisions in force for the project. It is not a universal product label and it is not a condensation guarantee.

For a window project, the relevant code pathway may involve energy efficiency, building envelope performance, ventilation, health and amenity, weatherproofing, safety glazing, fire or other provisions. Existing buildings, alterations, additions and repair work can have different triggers and concessions from new construction. The certifier, building surveyor or relevant authority should confirm which NCC provisions apply. This article does not declare compliance with NCC 2025 and should not be used as a substitute for the adopted code or project documentation.

AS 2047

AS 2047, Windows and external glazed doors in buildings, addresses performance requirements and testing or classification matters for windows and external glazed doors within its scope. Depending on the product and project, relevant performance topics can include resistance to water penetration, air infiltration, structural strength and operating forces. AS 2047 evidence should be read for the actual product configuration and exposure, not assumed from a similar-looking frame.

AS 1288

AS 1288, Glass in buildings – Selection and installation, provides requirements and guidance for selecting and installing glass in buildings. It is particularly relevant to safety glazing, human impact, glass type, thickness, support and installation details. It is not, by itself, a complete method for predicting indoor condensation or proving the thermal performance of a window assembly.

AS 4666

AS 4666, The prevention of mould in air-conditioning systems, is a source concerning mould prevention in air-conditioning systems and associated management considerations within its scope. It should not be mislabelled as a general window-condensation standard or treated as a complete mould-remediation specification for every building. If mould is present, address the moisture source, protect occupants and use appropriately qualified advice for cleaning, material removal and system hygiene.

New Zealand guidance

New Zealand projects are governed through the New Zealand Building Code and its compliance documents, rather than by assuming that Australian code provisions transfer automatically. For moisture and thermal matters, check the current versions of the relevant H1 Energy efficiency and E3 Internal moisture provisions, including the applicable Acceptable Solutions or Verification Methods. The correct pathway depends on the building, work type, climate, materials and date of application.

New Zealand guidance from the Ministry of Business, Innovation and Employment, BRANZ and industry bodies can help explain ventilation, internal moisture, window installation and weathertightness. Guidance is not the same as a product certification or a project approval. Window suppliers and designers should coordinate glass selection, thermal performance, drainage, flashings, air seals and the surrounding wall system. A New Zealand Building Code reference should therefore be checked against the current official documents and local consent requirements.

When professional assessment is warranted

Seek professional assessment when condensation is persistent, widespread, associated with mould, damaging finishes, occurring in a newly completed building, or accompanied by water after rain. It is also important when the window is part of a high-rise, coastal, bushfire, heritage, acoustic or high-wind project, or when a proposed retrofit changes the wall, flashing or drainage system.

Ask for a written scope. It should say whether the inspection covers indoor humidity, ventilation, window performance, glazing-unit failure, drainage, weatherproofing, thermal bridges, mould, code compliance or only one of those subjects. Ask the assessor to distinguish measured observations from assumptions and to identify which documents or standards were used. This avoids turning a limited inspection into an unsupported promise about the whole building.

Frequently asked questions

Does a thermal-break aluminium window eliminate condensation?

No. It can reduce heat flow through the frame and may raise interior frame temperatures, but condensation can still occur on glass, spacers, hardware, reveals or adjacent construction when indoor humidity is high or another thermal bridge is cold. The result depends on the complete installed assembly and actual conditions.

Why is water appearing only at the bottom edge of the glass?

The lower edge can be colder because of the spacer, frame geometry, sill detail and limited air circulation. Curtains, blinds and dust can further restrict warm room air. Check indoor humidity, the spacer and sealed-unit condition, drainage, the sill and the wall junction before deciding that the whole window has failed.

Is water between the panes ordinary condensation?

No. Water or persistent fogging between panes usually indicates a problem with the insulating glass unit or its edge seal. Room-side condensation is on the accessible interior surface. A glazier or window supplier should assess the unit and confirm whether replacement is required.

Will opening the window solve the problem?

Sometimes ventilation lowers indoor moisture, but the result depends on outdoor humidity, weather, security, heating and the building’s ventilation design. Opening a window cannot repair rain ingress, blocked drainage, a failed edge seal or a cold thermal bridge. Use a suitable ventilation strategy for the particular climate and building.

Can I compare two windows using their advertised U-values?

Only if the values are defined on the same basis and apply to comparable configurations. Centre-of-glass, frame, edge and whole-window values are different quantities. Confirm the size, frame, glazing, spacer, test or calculation method and applicable rating scheme. A generic value should not be used to predict condensation without project-specific boundary conditions.

Does AS 2047 prove that a window will not condense?

No. AS 2047 concerns performance matters within its scope, including relevant weather, air and structural performance testing or classification. It is not a promise that indoor humidity and surface temperature will never reach condensation conditions.

Does AS 1288 calculate window thermal performance?

AS 1288 is primarily concerned with the selection and installation of glass in buildings, including safety-related requirements within its scope. It does not replace a thermal assessment or establish a whole-window condensation result.

What information should be collected before replacing a window?

Collect the window system and section details, glass build-up, spacer and seal information, opening dimensions, installation drawings, wall and reveal construction, orientation, shading, drainage paths, indoor temperature and humidity records, ventilation details and the timing of the problem. Also record rain exposure and any previous repairs. This information helps distinguish moisture control, glass, frame, junction and installation issues.

Conclusion

Condensation is governed by the relationship between moisture in the indoor air and the temperature of the coldest relevant surface. Thermal-break aluminium frames are an important part of a better-performing window, but they work as one element in a system that includes glass, spacers, seals, drainage, reveals, wall junctions, ventilation and occupant moisture loads. Good diagnosis starts by locating the water, recording the conditions and separating surface condensation from rain or plumbing ingress.

Use NCC 2025, AS 2047, AS 1288, AS 4666 and the applicable New Zealand Building Code documents as accurately labelled sources within their respective scopes. Confirm current editions, adoption and project applicability before relying on them. Keep general principles separate from project-specific calculations, and require evidence that matches the exact installed assembly. No window description alone can provide a universal dew point, U-value, saving, rating or guarantee.

Aluminium Thermal-Break Windows and Condensation: Frames, Glass and Indoor Humidity