Glazing Spacers and Edge Seals in Aluminium Windows: Condensation, Durability and Specification

A window is judged by what a person can see: the frame profile, the glass, the finish, the way the sash closes. The components that decide whether that window still performs after a decade of weather are almost all invisible. The spacer and the edge seal sit inside the perimeter of an insulating glass unit, buried behind a bead, a gasket and a cover cap. They are never part of a showroom conversation, and they are rarely named in a building specification. Yet they determine whether the cavity between the panes stays dry, whether the unit stays square, whether the edge of the glass is protected from the frame, and whether the first cold morning of winter produces a band of fog around the perimeter that never goes away.

This guide sets out how spacers and edge seals work in aluminium windows and glazed doors, why the boundary of an insulating glass unit deserves as much specification attention as the glass itself, how the frame and rebate interact with that boundary, and where condensation, movement, handling, replacement and maintenance fit into the picture. It also sets out the standards and compliance framework that applies in Australia and New Zealand, and the checks that should be recorded before a unit is accepted. It deliberately does not quote performance figures or nominate proprietary components, because those belong to a tested assembly and a specific project rather than to a general article.

Why the Spacer Is Not a Minor Component

The spacer is the element that holds two or more glass panes apart at a fixed distance and, together with the sealants applied around it, closes the cavity between them. In an insulating glass unit the spacer runs the full perimeter, turns at every corner, and connects back to itself. It is a continuous structural and environmental element, not a packing piece. Its geometry, its stiffness and the way it is bonded to the glass all influence how the unit behaves in service.

The Geometry the Spacer Defines

The most obvious function of the spacer is dimensional. It sets the width of the cavity between the panes, and that width is a design decision recorded by the unit manufacturer for a particular build-up. The spacer also controls the parallelism of the panes. If the spacer is not uniform in section, or if it is compressed or distorted during assembly, the panes will not sit parallel to one another. A cavity that is wider at one edge than another changes the optical behaviour of the unit and can produce visible distortion when the glass is viewed at an angle near the edge.

The spacer also defines the sight line. The width of the spacer, combined with the depth of the frame rebate and the position of the bead, determines how much of the unit perimeter is hidden and how much of the edge seal is exposed to view. That is a visual decision as much as a technical one, and it has consequences. A deep rebate that conceals the spacer also conceals early signs of seal deterioration, which is convenient at handover and unhelpful at the ten-year inspection.

Load Path and Pane Separation

Wind pressure, human load, the weight of the glass itself and the movement of the building all produce forces that the unit must carry. The spacer contributes to the ability of the unit to transfer those forces between the panes and into the frame. When two panes are structurally connected around their perimeter, they share load; when that connection degrades, each pane behaves more independently than the design assumed.

This is why the spacer cannot be considered separately from the sealants that bond it. The spacer provides the section and the stiffness; the sealant provides the continuous attachment to the glass. Together they form the structural edge of the unit. A spacer that is adequate in section but poorly bonded is a weak edge, and a strong bond around a spacer that is too flexible is also a weak edge. The two elements must be designed and assembled as a system.

The Cavity as a Controlled Space

The cavity between the panes is not empty air in the sense that a room is full of air. It is a closed volume whose moisture content is set at the moment of assembly and then managed by whatever drying agent the unit design includes. If moisture enters the cavity faster than it can be absorbed, the dew point inside the cavity rises, and the unit begins to condense on the inner faces of the glass. Once that happens, the visible result is permanent fogging, staining or a milky deposit that no amount of cleaning can reach.

The spacer is central to this because it is the carrier for the drying agent in most unit designs, and because it is the substrate against which the primary seal is formed. A spacer that is contaminated, damaged or incorrectly joined at a corner compromises both functions at once. The cavity stops being a controlled space and becomes a slow leak.

The Edge Seal: A Two-Stage Boundary

The edge seal is the collection of materials and bonds that close the perimeter of the unit. In most insulating glass constructions it is described in two stages: a primary seal at the inner boundary of the cavity, and a secondary seal at the outer edge, between the spacer and the outer faces of the glass. The division of labour between the two is the single most important idea in this part of the subject.

Primary and Secondary Sealing

The primary seal is the vapour barrier. Its job is to limit the rate at which water vapour passes from the outside atmosphere into the cavity. It sits at the cavity line, immediately behind the spacer, and it must be continuous around the entire perimeter, including the corners, where it is formed by the junction of two runs of material. Corners are where continuity is hardest to achieve and where most seal failures begin.

The secondary seal is the structural and environmental backstop. It bonds the spacer to the glass, carries a large share of the structural demand at the edge, and protects the primary seal from direct exposure to water, ultraviolet radiation and mechanical damage. It is the part of the unit most likely to be seen if the rebate is shallow, and the part most likely to be damaged during handling if the unit is carried without edge protection.

The logic of the two-stage arrangement is redundancy. The primary seal addresses vapour transmission; the secondary seal addresses structure and protection. A unit with a good primary seal and a poor secondary seal will stay dry for a while and then fail structurally at the edge. A unit with a strong secondary seal and a poor primary seal will look sound and fog from the inside within a few seasons. Neither defect is visible at the point of delivery.

Adhesion, Cohesion and Compatibility

Seal performance depends on two distinct properties. Adhesion is the strength of the bond between the sealant and the surfaces it touches: the glass, the spacer, and in some designs the frame interface. Cohesion is the internal strength of the sealant itself. A seal can fail by pulling cleanly away from the glass, which is an adhesion failure, or by tearing within its own body, which is a cohesion failure. The two look different on inspection and point to different causes.

Compatibility is the third requirement and the least visible. Sealants, gaskets, setting blocks, glazing tapes and frame finishes all come into contact with one another. If any combination is incompatible, the result may be softening, hardening, discolouration, loss of adhesion or chemical attack on the spacer. Compatibility is the manufacturer’s responsibility to establish for the components supplied as a system, and the specifier’s responsibility to preserve by not substituting one component for another on site. A gasket swapped for a different grade during installation can undo a compatibility assessment that was never written down where the installer could see it.

Vapour Transmission and the Long Game

No seal is perfectly impermeable. The question is not whether water vapour will cross the edge seal, but how quickly, and whether the drying agent inside the unit can absorb what arrives over the intended service life. That is why edge seal performance is a rate question rather than a pass or fail question, and why it cannot be assessed by looking at a new unit.

The practical consequences are important for specification. A unit that is adequate for a mild, well-ventilated situation may be marginal in a hot, humid or coastal environment where the vapour pressure difference across the seal is sustained. A unit that performs well in a fixed window may be stressed differently in an opening sash, where the whole assembly flexes every time it is used. These are not reasons to demand a particular product; they are reasons to describe the exposure and the duty in the specification, and to require the supplier to demonstrate that the proposed build-up is appropriate to it.

Insulated Glazing Boundaries and What “Sealed” Really Means

An insulating glass unit has two boundaries, and confusing them is the source of much of the misunderstanding in the market. The face boundary is the glass surface itself, which is exposed to the room on one side and to the weather on the other. The edge boundary is the perimeter, where the spacer, the seals and the outer faces of the glass meet. The face boundary is easy to inspect and easy to clean. The edge boundary is where the unit lives or dies.

Perimeter Versus Face

When a unit is described as “sealed”, the description usually refers to the face boundary being intact: no cracks, no chips, no visible damage to the glass. That tells you very little. The perimeter seal is a manufactured joint that has been formed in a factory, cured, packed, shipped and handled before it ever reaches a building site. Its integrity is established by process control and by testing, not by visual inspection at the point of installation.

The distinction matters when a unit is rejected or accepted. A chipped edge on the glass is a face-and-edge defect that can be seen. A poorly bonded secondary seal at a corner is invisible until the unit fogs or the edge opens. This is why the specification should require evidence of the manufacturing process and the seal quality, rather than relying on a site inspection to catch what cannot be seen.

The Weakest Path

Every unit has a weakest path for vapour ingress, and it is almost always a corner, a joint in the spacer, or a point where the seal has been interrupted by a fixing, a setting block or a handling mark. Water vapour does not need a large opening; it needs a continuous path. The practical lesson is that perimeter continuity is more important than the nominal properties of any single material. A modest seal applied continuously will outperform an excellent seal applied with a break in it.

This is also why the edge of the unit should never be treated as a handling surface. Lifting a unit by its edge, resting it on a corner, or dragging it across a stillage all concentrate force exactly where the seal is thinnest and least tolerant of damage. The damage may not be visible, and it may not show up for years.

Edge Effects and Visual Quality

At the perimeter of a unit, light passes through the glass, the spacer and the seal in a way that differs from the centre of the panel. This produces the slight change in appearance that most people recognise as the edge zone of a double-glazed window. It is normal. What is not normal is a localised change in appearance: a dark patch, a milky crescent, a rainbow pattern that appears and disappears with temperature, or a line of discolouration along one edge.

These localised symptoms are diagnostic. A patch of fogging that grows from one corner inward suggests a breach near that corner. A pattern that appears on cold mornings and clears by midday suggests a cavity that is close to saturation rather than one that has failed completely. A discolouration along a fixing line suggests a thermal or chemical interaction at that point. None of these should be dismissed as cosmetic, because each of them narrows the search for a cause.

Frame and Rebate Compatibility

A spacer and edge seal that are correct in isolation can still fail in service if the frame does not accept them properly. The frame is not a passive holder; it is the environment in which the unit must sit, drain, move and be held. Compatibility is therefore a geometric, mechanical and environmental question, and all three have to be answered before a unit is ordered.

Rebate Depth and Glazing Clearances

The rebate is the recess in the frame that receives the unit. Its depth determines how much of the unit edge is covered, how the bead or cover cap sits, and whether the unit can be bedded and sealed correctly. If the rebate is too shallow, the unit sits proud, the bead cannot engage properly, and the edge seal is left exposed to weather and to view. If the rebate is too deep relative to the unit and the bead, the unit may sit too far back, reducing the effective seal line and making drainage difficult.

Clearances around the unit are equally important. The unit must not be a tight fit in the rebate. It needs room for setting blocks, for movement, for the glazing gasket or tape, and for the bedding material. A unit forced into a rebate with no clearance is preloaded before any wind load or thermal movement is applied, and the resulting stress concentrates at the corners of the unit and at the fixing points. The correct clearance is a function of the frame system and the unit size, and it should come from the system documentation rather than from site judgement.

Setting Blocks, Location Blocks and Rotation

The unit should not bear directly on the frame. Setting blocks carry the dead weight of the glass and transfer it into the frame at defined positions. Their location along the bottom edge, their hardness and their dimensions all affect how the unit is stressed. Blocks placed too close to the corners concentrate load where the seal is most vulnerable; blocks that are too soft allow the unit to settle and the edge seal to be worked.

Location blocks, where used, control lateral and rotational movement rather than weight. In opening sashes, the unit also has to resist the racking that occurs each time the sash is operated, and the glazing system has to accommodate that movement without transferring it into the edge seal as a peeling force. This is a system design question, not a component question, and it is one of the reasons a frame system should be specified as a system rather than assembled from separately sourced parts.

Drainage, Pressure Equalisation and the Thermal Break

Aluminium window frames are designed to drain water that enters the rebate and to equalise pressure across the weatherseal line so that water is not driven inward. The glazing detail sits inside that strategy. If the drainage path is blocked by bedding compound, sealant or debris, water stands in the rebate and the edge of the unit sits in a reservoir. If the pressure equalisation path is sealed off by an over-enthusiastic bead of sealant, the frame loses the mechanism that keeps water out.

The thermal break in an aluminium frame introduces a further consideration. The break separates the inner and outer parts of the profile to reduce heat flow, and the glazing rebate crosses that break. The way the unit is bedded and sealed at that crossing determines whether the break is effective or bypassed. A continuous bead of sealant bridging the break, or a gasket that does not align with it, can compromise the thermal performance of the frame independently of the glass.

Condensation Risk

Condensation is the most common complaint about glazing and the most commonly misdiagnosed. It is a physics problem before it is a product problem. Air contains water vapour, and the amount it can hold depends on its temperature. When a surface is colder than the air in contact with it, and colder than the point at which that air becomes saturated, water condenses on the surface. The only variables that matter are the surface temperature, the air temperature and the moisture content of the air.

Why the Edge Is the First Place to Fog

The edge of a glazed unit is usually the coldest part of the visible glass, because it is closest to the frame and furthest from the centre of the cavity. The frame itself is a path for heat loss, and the spacer and seal form a thermal bridge between the panes at the perimeter. The result is a band around the edge that runs colder than the middle of the panel. When conditions are marginal, that band is the first place to reach dew point.

This is why edge condensation is often a frame and installation issue rather than a glass issue. Poorly installed insulation around the frame, a rebate that is not sealed to the building envelope, a cold bridge at a fixing, or an Internal corner where air does not circulate can all lower the edge temperature without any defect in the unit. Replacing the glass will not fix a cold frame.

Interior, Exterior and Cavity Condensation

Condensation on the room-side surface is an interior humidity and ventilation question. It indicates that moisture produced inside the building is not being removed, and that the glazing is the coldest surface available for it to condense on. Condensation on the outside surface, which typically appears on mild humid mornings and clears as the day warms, is a different phenomenon and generally indicates that the outer pane is behaving as a cold surface relative to the outdoor air. Neither of these is a failure of the edge seal.

Condensation inside the cavity is the one that matters for the unit. It cannot be wiped away, it does not clear with ventilation, and it indicates that the cavity has gained moisture. It may appear as a general haze, as a band along the bottom edge, or as droplets on the inner face of the outer pane. Once present, it is evidence that the unit’s moisture balance has been lost, whether through the edge seal, through a manufacturing issue, or through a mechanical breach.

Ventilation, Humidity and Building Use

Buildings produce moisture continuously: cooking, bathing, laundry, breathing, plants, and the drying of construction materials. In a well-sealed modern building, that moisture has nowhere to go unless it is mechanically removed or unless the building is ventilated. A glazed area that is cold, poorly ventilated and in a room with high moisture production will condense, regardless of the quality of the window.

The correct response is to treat condensation as a diagnostic signal rather than a defect to be argued about. Where does it appear, when does it appear, how quickly does it clear, and does it appear on every unit in the room or only on some? A pattern that tracks a particular elevation, a particular room or a particular time of day points to the cause. A pattern that tracks a single unit points to that unit.

Movement: Thermal, Structural and Building

A window is never still. Glass expands and contracts with temperature, aluminium moves more than glass for the same change, the building frame deflects under load, and the structure settles over time. The glazing detail has to accommodate all of this without transferring it into the edge seal as a peeling or shearing force. Movement is not an exception to be designed out; it is a normal service condition to be designed for.

Differential Movement

Glass and aluminium respond to temperature at different rates, and a unit that is restrained rigidly at its perimeter will develop stress at the interface. The glazing system manages this through clearances, through the compressibility of gaskets and tapes, and through the way the bead or cover cap is fixed. Where a unit is bedded in a hard compound with no allowance for movement, the differential has to be absorbed by the seal itself, and the seal becomes the sacrificial element.

The same principle applies between the unit and the frame at the setting blocks. If the blocks are positioned so that the unit cannot rotate or translate slightly as the frame deflects, the unit becomes part of the load path and the edge seal takes the strain. Correct block placement is therefore not a minor installation detail; it is a movement allowance.

Wind and Serviceability

Wind load deflects the glass and the frame. Under a positive pressure the panes bow inward, and under suction they bow outward. The spacer resists the relative movement of the panes at the perimeter, and the secondary seal resists the peeling action that develops at the edge. Repeated cycling under wind is one of the most demanding conditions for an edge seal, because it works the bond in a way that static load does not.

Serviceability limits for deflection are set by the frame system and by the project’s structural requirements. Exceeding them is not just an aesthetic issue. Large deflections increase the movement at the glazing rebate, which increases the demand on the seal and on the bead fixing. This is why the frame system and the glass build-up must be selected together, and why a heavier or stiffer unit cannot simply be substituted into a frame without checking the consequences.

Settlement and Frame Distortion

Buildings settle. Slabs deflect, columns shorten, and openings that were square at frame installation may be slightly out of square years later. In load-bearing and framed construction, this movement is transferred to the window frame unless a movement joint is provided. A frame that is distorted in plane will distort the unit it holds, and a unit that is forced out of square concentrates stress at the corners, which is exactly where the edge seal is least able to tolerate it.

The practical defence is to check the opening before the frame is fixed, to fix the frame without introducing twist, and to ensure that any movement joint between the frame and the structure is actually functioning. A packer that has been left in place after fixing, or a fixing that has been over-tightened, can lock a frame into a distorted shape permanently.

Handling, Storage and Installation

Most edge seal failures that are not manufacturing defects begin before the unit is glazed. Handling damage is cumulative and largely invisible, and it is the reason suppliers place such emphasis on packaging and site practice. The unit is a manufactured product with a protected perimeter, and it should be treated as such from the moment it leaves the factory.

Transport and Site Storage

Units should be transported in a rack or stillage that supports them across their face, not on their edges. Edge contact against a hard surface, vibration against a metal frame, and point loads from strapping all concentrate force at the perimeter. Units should never be stacked flat without separation, and they should never be stored where they can be walked on or used as a work surface.

On site, the storage position matters as much as the storage method. A unit stored in direct sun will heat unevenly, and a unit stored in a damp or dusty location will accumulate contamination on the surfaces that later have to be sealed or cleaned. Storage at an angle against a wall, with the bottom edge on the ground, is a common and damaging practice: it puts the full weight of the unit onto the edge seal and invites water into the perimeter.

Lifting and Carrying

Units should be lifted with suction lifters or with clean gloves and a firm grip on the face, never by the edges or by the spacer. Carrying a unit vertically between two people, with hands on the face near the edges, is acceptable if the hands are clean and the unit is not squeezed. Carrying it horizontally with hands under the edge is not. The weight of a large unit on a narrow edge contact is enough to deform the seal locally.

Personal protective equipment is not optional. Glass handling causes serious cuts, and the risk increases when a unit is damaged or when the edge is exposed. Cut-resistant gloves, safety footwear and eye protection should be standard for anyone moving or installing glazed units, and the manual handling assessment should reflect the actual size and weight of the panels being handled.

Setting and Packing

Before a unit is set, the rebate should be clean, dry and free of debris, mortar and sealant residues. The setting blocks should be in the positions defined by the system documentation, with the correct hardness and dimensions. The unit should be lowered into the rebate without sliding, because sliding abrades the seal and can roll a gasket out of position.

Packing should be inserted so that the unit is held firmly but not distorted. Over-packing at one point and under-packing at another is worse than uniform packing, because it introduces a local bending moment. The bead or cover cap should then be fitted in accordance with the system, with the gasket compressed to its designed extent rather than crushed.

Sealing Practice and Protection After Installation

Where sealant is used at the frame interface, it should be applied to a prepared surface, in a continuous bead, with the correct profile and with compatibility confirmed against the frame finish and the gaskets. Sealant applied over dust, moisture or release agent will not adhere, and a bead that looks continuous may be a bridge over a gap rather than a seal.

After installation, protection should remain in place until the surrounding trades have finished. Render, mortar and paint all contain materials that can etch glass or stain seals, and abrasive cleaning of construction residue is one of the most common causes of surface damage on new glazing. Protective film should be removed in accordance with the manufacturer’s instructions, because film left in place through hot weather can transfer adhesive to the glass.

Replacement and Retrofit

Sooner or later a unit will need to be replaced, whether because of breakage, because the cavity has fogged, or because the performance requirements of the building have changed. Replacement is a controlled operation, and the way it is carried out determines whether the new unit performs as intended.

When a Unit Must Be Replaced

A unit with a broken pane must be replaced, because the structural and sealing integrity of the assembly has been compromised and because broken glass is a safety hazard. A unit with persistent cavity condensation must be replaced, because the moisture cannot be removed and the visible defect will not resolve. A unit with a failed edge seal that is not yet visible as condensation may still warrant replacement if inspection shows separation, cracking or displacement of the seal.

What does not warrant replacement is surface condensation on the room side, which is a ventilation and humidity issue, or dirt on the accessible surfaces, which is a cleaning issue. Distinguishing between these before ordering a replacement avoids unnecessary cost and, more importantly, avoids replacing a unit and finding the complaint unchanged.

Matching the Original Build-Up

A replacement unit should match the original in thickness, build-up, edge detail and appearance. A unit of a different overall thickness may not fit the rebate, may not engage the bead correctly, and may change the clearances that the frame was designed around. A unit with a different edge seal or spacer may look identical and behave differently.

The build-up should be confirmed from the project documentation or from the marking on the unit, not from memory or from a sample. Where the original is no longer available, the replacement should be specified to the same performance requirements and confirmed as compatible with the frame by the system supplier.

Replacing Without Damaging the Frame

Replacement requires the bead or cover cap to be removed without damaging the frame finish, the gaskets to be released without tearing, and the unit to be lifted out without levering against the rebate. Gaskets and tapes are usually replaced at the same time, because they deform permanently in service and will not reseal reliably after removal. Sealant is cut out and renewed rather than topped up.

The new unit should be set on new blocks in the correct positions, checked for square and alignment, and sealed in the same sequence as the original installation. The frame should be inspected while it is open: drainage slots clear, thermal break intact, fixings sound, and no evidence of water tracking that would affect the new unit.

Retrofit into Existing Frames

Upgrading the glazing in an existing aluminium frame is a common request, and it is constrained by the frame that is already there. The rebate depth, the bead profile and the clearances limit the thickness of unit that can be accommodated, and the frame’s structural capacity limits the weight that can be carried. A heavier unit in an unreinforced frame may deflect the sash, bind the hardware and load the edge seal beyond its design.

Retrofit also raises the question of the interface between new glazing and an older frame. Corrosion, failed gaskets, blocked drainage and distorted profiles are common in older installations, and a new unit installed into a deteriorated frame will not deliver the expected performance. The honest approach is to assess the frame before committing to the glazing, and to be prepared to replace the whole assembly when the frame cannot support the upgrade.

Quality Checks

Quality in glazing is established by process, not by inspection, but inspection is how the process is verified. The checks below are arranged in the order in which they become possible, so that a fault is caught at the stage where it can still be corrected cheaply.

At Order

Before a unit is manufactured, the order should record the frame system and profile, the opening size and the unit size, the glass build-up, the overall thickness, the edge detail, the spacer type and cavity width as nominated by the supplier, the required performance, and the exposure conditions the unit must withstand. It should also record the intended glazing method: gasket, tape, wet sealant, or a combination. Ambiguity at this stage is the most expensive defect in the whole process, because it is discovered only when a finished unit does not fit.

On Delivery

On arrival, units should be checked against the order and inspected for transit damage. The check should include the overall dimensions, the thickness at several points around the perimeter, the glass type and any marking, the edge seal condition, and the presence of any contamination, scratches or chips. Labels and traceability markings should be recorded, because they are the only link between the installed unit and its manufacturing record.

Where the unit has a visible spacer, the perimeter should be examined for uniformity. A spacer that steps, bulges or shows a gap at a corner is a manufacturing defect. Where the spacer is concealed, the visible edge seal and the glass-to-spacer line should be checked for separation, bubbles or discolouration. Any unit showing these signs should be quarantined rather than installed.

During Installation

Installation checks cover the frame and the unit together: rebate clean and dry, drainage clear, blocks in the correct positions, unit square in the opening, clearances even, gaskets correctly seated, bead engaged and fixed, sealant continuous and tooled. Each of these is quick to check and expensive to correct later. A photograph of the glazing detail before the bead is fitted is worth more than a paragraph of description at handover.

After Completion

After completion, the checks shift to performance and appearance. Operate every opening sash and confirm it moves freely without loading the glass. Confirm that drainage works by observing the frame during rain or by testing in accordance with the project’s requirements. Confirm that there are no visible signs of condensation inside the cavity. Record any defects with their location, so that patterns can be identified rather than treated as isolated complaints.

Documentation

The documentation set should include the manufacturer’s product data, the certificate or test evidence supporting the claimed performance, the installation instructions, the maintenance requirements, and the warranty terms including what is and is not covered. It should also record the as-installed build-up. This is the difference between a defect that can be resolved and a dispute that cannot, because a unit that cannot be identified cannot be assessed against its specification.

Cleaning and Maintenance

Glazing requires very little maintenance, and most of the maintenance it does require is aimed at protecting the frame, the seals and the drainage rather than the glass itself. The common failures in service are caused not by cleaning too rarely but by cleaning incorrectly.

Routine Cleaning

Glass should be cleaned with a mild detergent solution and a soft cloth or a clean squeegee, working from the top down and rinsing with clean water. The frame should be wiped at the same time, including the drainage slots and the rebate line, because debris that collects there holds water against the glazing detail. Frequency depends on exposure: coastal sites, sites near construction activity and sites with high airborne dust need more frequent attention than sheltered suburban sites.

What to Avoid

Abrasive pads, scrapers, blades and strong alkaline or acidic cleaners should not be used on coated glass, on frame finishes or on seals. They remove coatings, dull finishes and attack the seal surface. High-pressure water should not be directed into the glazing rebate or at the drainage slots, because it can drive water past the weatherseal line. Cleaning should never be carried out on hot glass in direct sun, because the cleaning solution dries before it can be rinsed and leaves deposits.

Solvent-based cleaners and adhesive removers should be used only where the glass and frame manufacturers permit them, and only on the surfaces they are intended for. A solvent that is harmless to glass may be harmful to a gasket or to a powder-coated finish, and the damage may not appear until months later.

Seal and Drainage Checks

At each service interval, the visible seals should be checked for cracking, hardening, shrinkage, separation and displacement. Gaskets should be checked for compression set and for correct seating in their grooves. Drainage slots and weep paths should be confirmed clear. Beads and cover caps should be checked for security and for any gap that would allow water behind them.

Long-Term Inspection

Over the life of the building, inspection should also look for changes rather than defects: a seal that was continuous and is now visibly interrupted, a drainage path that was clear and is now staining, a frame that was true and is now distorted. Recording these observations at each inspection builds a history that makes it possible to distinguish normal ageing from a developing problem.

Standards, the NCC and Evidence

In Australia and New Zealand, glazing compliance rests on a small number of documents applied consistently. The purpose of the framework is not to make life difficult for suppliers; it is to ensure that a product installed in a building has been shown to be fit for the duty it will face.

The NCC and the ABCB

The National Construction Code is the primary building regulation in Australia. It is administered by the Australian Building Codes Board and given legal effect through state and territory legislation. The NCC sets out performance requirements for buildings, including those that touch on glazing, weatherproofing, structural adequacy, energy efficiency and safety. Because the NCC is performance-based, it allows a number of pathways to demonstrate compliance, including deemed-to-satisfy provisions, verification methods and performance solutions supported by evidence.

The practical consequence for a project is that compliance is not a single certificate. It is a chain of evidence: the product is tested or assessed to the relevant standard, the installation follows a documented method, and the completed assembly is inspected. A gap anywhere in that chain is a gap in the compliance position, regardless of how good the product is.

The Australian Standards

The standards most often cited for windows and glazing in Australia are AS 2047 for windows and external glazed doors, AS 4284 for the performance testing that supports those claims, AS 1288 for glass in buildings, AS 2208 for safety glazing materials, and AS 4666, which addresses the insulating glass unit as a manufactured product together with the conditions of its installation. Each of these covers a different part of the problem, and none of them can substitute for another.

AS 2047 is the product performance standard for windows and external glazed doors. It deals with the assembly as a whole: structural performance, water penetration resistance, air infiltration and operating forces. AS 4284 provides the test basis against which window performance is demonstrated, which is why a performance claim should always be traceable to a test report rather than to a brochure. AS 1288 deals with the selection and installation of glass in buildings, including the determination of when safety glazing is required and how glass must be supported. AS 2208 sets the requirements for safety glazing materials themselves, which is a separate question from where they must be used. AS 4666 deals with insulating glass units, and therefore with the spacer, the seal and the cavity as a manufactured assembly.

The division of responsibility matters. A safety glazing material that complies with AS 2208 is not automatically in the right location; AS 1288 determines that. A window that passes AS 4284 testing is not automatically suitable for every building; AS 2047 and the project requirements determine that. A unit made to AS 4666 is not automatically correctly installed; the installation method and the frame determine that.

The New Zealand Documents

For New Zealand, the relevant documents in this knowledge base are SNZ TS 4211:2022 and SNZ 4223. SNZ TS 4211:2022 addresses the performance of windows and external glazed doors in the New Zealand context, and SNZ 4223 addresses the associated installation requirements. As with the Australian suite, the two work together: a product demonstrated to perform, and a method for installing it so that the demonstrated performance is realised in the building.

Products supplied across both markets must satisfy both frameworks, which is a more demanding position than satisfying either alone. The requirements are not identical, and a product certified for Australia is not automatically certified for New Zealand. This is why a supplier that operates in both markets should be able to present separate evidence for each.

What Certification Covers and What It Does Not

Certification is evidence about a product, not a guarantee about a building. It confirms that a defined product, tested or assessed in a defined way, met a defined requirement. It does not confirm that the product was installed correctly, that the frame is adequate, that the building is ventilated, or that the unit will remain sealed for its service life. Those depend on design, installation, use and maintenance.

CodeMark is the highest level of certification available in Australia, and it provides a strong pathway to compliance because it is accepted as evidence of conformity with the NCC. A supplier pursuing CodeMark is signalling an intent to be assessed at that level, but pursuing is not the same as holding. Specifications and marketing should distinguish clearly between certification held, certification in progress, and testing completed.

The Supplier Evidence File

For a project in Australia or New Zealand, the supplier should be able to provide: the product series and its certified scope, the test reports supporting the performance claims, the glass build-up and its evidence, the installation instructions, the maintenance requirements and the warranty terms. For context, the knowledge base for this article records a manufacturer with 18 to 19 years of industry experience, dedicated involvement in the Australian and New Zealand markets since 2017, a 20,000 square metre manufacturing facility, 43 product series certified in Australia and 13 product lines certified in New Zealand, with water tightness demonstrated up to 960 Pa under AS 4284 testing, and CodeMark certification actively in progress. Those are the kinds of facts a specifier should be able to verify against certificates and reports, rather than accept from a website.

Verification in Practice

Verification is a documentary exercise with a physical check at the end. Read the certificate: what product does it cover, what standard was it assessed against, who issued it, and is it current? Read the test report: what was tested, at what size, and does it match the product being supplied? Read the installation instructions: do they describe what is actually happening on site? Then look at the installed unit and confirm it matches the documentation. Where the documentation and the installation disagree, the installation is the problem, and no amount of paperwork will fix it.

Specification Checklist

The following checklist is intended to be copied into a project specification and completed for each glazing type. It is deliberately written in plain language so that it can be used by a builder, a certifier and a supplier without translation.

Frame system identified by manufacturer, series and profile, with the glazing rebate detail referenced.
Unit build-up recorded: glass types, overall thickness, cavity arrangement and edge detail, as nominated by the unit manufacturer.
Spacer and edge seal nominated by the unit manufacturer as part of a tested assembly, not selected independently on site.
Rebate depth, glazing clearances and bead engagement confirmed against the system documentation for the actual unit thickness.
Setting block and location block positions, dimensions and hardness specified.
Glazing method specified: gasket, tape, wet sealant or combination, with compatibility between all contact materials confirmed.
Drainage and pressure equalisation paths identified on the drawing and confirmed clear at installation.
Thermal break continuity confirmed at the glazing rebate, with no sealant bridging.
Performance requirements stated by the project, with the supporting test evidence named rather than assumed.
Condensation management addressed at the building level: ventilation, humidity control and insulation of the frame-to-wall junction.
Handling, storage and protection requirements included in the installation method statement.
Inspection and test plan covering delivery inspection, pre-bead inspection, completion inspection and documentation.
Maintenance schedule and warranty terms issued at handover, including what is excluded.

Frequently Asked Questions

What does the spacer actually do?

It holds the panes at a fixed distance, defines the cavity, contributes to the structural connection between the panes, and in most designs carries the drying agent that manages moisture inside the cavity. It is part of the sealed assembly, not a packing accessory.

Why does condensation appear around the edge first?

Because the perimeter is usually the coldest part of the visible glass. The frame, the spacer and the seal form a path for heat loss at the edge, so that band reaches dew point before the centre of the panel. Edge condensation is often a frame, insulation or ventilation issue rather than a failure of the unit.

How can I tell cavity condensation from surface condensation?

Surface condensation can be wiped away and usually clears when the room is ventilated or warmed. Cavity condensation is between the panes, cannot be reached, and persists. If it is inside the cavity, the unit has gained moisture and will normally need to be replaced.

Can a fogged unit be repaired on site?

No. The cavity is a sealed, factory-controlled space. Drilling, venting or injecting material into it destroys the remaining seal and does not restore the drying agent. Replacement of the unit is the only reliable remedy.

Does a thicker unit always perform better?

Not necessarily, and it may not fit. A thicker unit changes the weight, the rebate demand, the bead engagement and the clearances. Performance is a property of the tested assembly and the project conditions, not of thickness alone.

Is edge condensation covered by warranty?

That depends on the cause and on the terms. Cavity condensation from a failed seal is usually a warranty matter. Surface condensation caused by building humidity and ventilation is usually excluded, because it is a building performance issue rather than a product defect. The warranty terms should be read before the complaint is characterised.

How often should glazing be inspected?

At handover, at the end of the defects liability period, and then at intervals appropriate to the exposure and the building’s maintenance regime. Coastal, high-rise and high-use buildings warrant more frequent inspection than sheltered low-rise residential buildings.

Does the frame matter if the glass is correct?

Yes. The frame provides the rebate geometry, the drainage, the thermal break and the structural support. A correct unit in an inadequate frame will not deliver its performance, and the edge seal will carry loads it was never designed for.

Conclusion

The spacer and the edge seal are the least visible parts of a window and among the most consequential. They set the cavity geometry, they close the unit against moisture, they carry load at the perimeter, and they determine whether the glass still looks and performs as it did on the day it was installed. Almost every long-term glazing complaint, from a fogged corner to a stained edge, can be traced back to a decision made about that boundary: how it was designed, how it was assembled, how it was handled, and how it was installed into the frame.

For specifiers in Australia and New Zealand, the practical position is straightforward. Specify the unit as part of a system, confirm the frame and rebate can accept it, require evidence that matches the product being supplied, and treat the installation method as part of the performance claim rather than an afterthought. Check the easy things at delivery, photograph the detail before it is covered, and document the build-up so that a future replacement can match it. Address condensation at the building level, because no glazing product can compensate for a cold frame and a humid room.

Do this, and the invisible parts of the window will do their job quietly for the life of the building. Skip it, and the first sign of trouble will be a band of fog that cannot be cleaned, in a unit that cannot be repaired, behind a bead that has to be cut away to reach it.

Glazing Spacers and Edge Seals in Aluminium Windows: Condensation, Durability and Specification