Aluminium Window Corner Seals: Junctions, Gaskets, Drainage and Long-Term Care

Open a window schedule and the frame is described by its series, its finish and its glazing. Almost nothing on the sheet describes the small moulded pieces at the four corners of every sash, or the sealant line where a mullion meets a transom, or the way a gasket changes section at a corner window where two frames meet at an angle. Those details are where aluminium windows leak, whistle, stain and eventually corrode. They are also the details that nobody inspects until water appears on an internal sill.

A window is a product in a factory and a junction on a building. Corner seals are the place where those two identities meet. A frame that performed well in a laboratory rig can still fail in service because the corner was assembled on site, because the gasket was stretched around a radius rather than moulded to it, because a silicone bead was applied over a contaminated surface, or because the drainage path that the designer intended was blocked by a setting block and never revisited.

This article examines corner windows, gasket and seal junctions, mullion and transom intersections, silicone compatibility, pressure equalisation, drainage, glazing interfaces, movement, corrosion, inspection, replacement and testing. It refers to the Australian and New Zealand compliance framework without asserting product-specific performance. It deliberately contains no dimensions, costs, thermal or acoustic values, or performance ratings, because those are properties of a particular tested system and a particular project, and a general guide that invents them is worse than useless.

Why Corner Seals Deserve Their Own Conversation

Aluminium is a good material for window frames but an unforgiving one for joints. It conducts heat readily, it expands and contracts measurably over daily and seasonal cycles, and its mill finish, anodised finish and powder-coated finish each behave differently under a bead of sealant. A corner is where two extruded profiles, two machined ends, one or more gaskets and often a mechanical fastener all have to act as a single continuous barrier.

Three separate functions are being asked of the same small area. The corner must be structurally sound, because a sash corner carries the dead load of glass and the wind load transferred through the frame. It must be watertight, because it faces the weather on two sides at once. And it must be vapour-tight enough to keep the interior dry, because a corner that leaks air will also leak moisture-laden air into the wall cavity, where it condenses on the first cold surface it meets.

When a corner fails, the failure is rarely dramatic. Water tracks along the inside of the extrusion, emerges at a joint in the reveal, stains plaster, lifts paint and feeds mould in a corner of a room that nobody looks at. By the time the source is identified, the sealant has usually been cut away more than once and the true cause has been obscured.

The Difference Between a Sealed Corner and a Drained Corner

Many aluminium systems are designed on the assumption that some water will enter the frame. The outer gasket is the first line of defence, the frame’s internal chambers are the second, and the drainage path to the exterior is the third. A corner seal in such a system is not intended to make the frame a sealed box. It is intended to keep water inside a defined channel and keep it moving outward.

This distinction explains a common and expensive misunderstanding. An installer who seals every internal corner with silicone, believing that more sealant means more protection, can block the drainage path and turn the frame into a container. The frame then holds water against the glazing edge, against the fasteners and against the base of the extrusion, and the resulting corrosion and glass-edge damage appear years later. A drained system must be assembled the way it was designed, with the drainage path preserved.

Corner Windows: Geometry, Structure and the Awkward Post

A corner window removes the wall that would normally separate two elevations. That single decision changes the structural behaviour of the opening, the thermal behaviour of the junction and the way water is shed at the most exposed point of the building.

Structural Continuity at the Corner

In a conventional pair of windows meeting a solid corner, each frame has a jamb that can be fixed back to structure and braced independently. In a corner window, one or both frames lose that jamb. The two units are joined to each other, and the joint must transfer wind load from one frame to the other and back to the structure. That is a load path question, not a sealing question, and it must be resolved before the seal is detailed.

Where the corner is formed by a dedicated corner post, the post takes the load and the seals follow it. Where the corner is formed by two frames mitred and joined directly, the joint itself is structural and the seal sits within it. Where a glazed corner is achieved with a structural silicone joint between two panes, the corner is no longer a frame joint at all and the sealing strategy changes completely. Each approach has different movement behaviour, different installation tolerance and a different inspection regime.

The Corner Post and the Sightline

Corner posts are the visible compromise of corner glazing. A post that is generous enough to fix, seal and insulate properly will interrupt the view; a post that is slim enough to nearly disappear leaves little room for gaskets, drainage and a mechanical connection. This is a genuine engineering trade-off, not a marketing one, and the resolution belongs to the system designer rather than the installer.

What matters at the junction is continuity. The corner post must present a continuous external face, a continuous internal face, and a continuous gasket line to both frames. Any break in that gasket line, however small, becomes an air path. Air paths at corners are audible in wind, visible as condensation in cold weather and measurable in any air permeability test.

Thermal Breaks and Cold Corners

Aluminium frames rely on a thermal break to separate the inner and outer extrusion. At a corner, the break has to turn through an angle, and the turning point is the hardest place to maintain. If the break is interrupted by a fastener, a bracket or a continuous metal path, the corner becomes a cold bridge. Cold bridges do not necessarily leak, but they do produce condensation on the interior face in cold weather, and persistent condensation on an aluminium surface eventually damages finishes and encourages mould on adjacent linings.

The practical consequence for specifiers is that a corner window should be treated as one assembly with a declared thermal strategy, not as two windows that happen to meet. The same logic applies to corner seals: they are part of the thermal envelope, not just the weather envelope.

Gaskets and Seal Junctions

Gaskets do most of the sealing work in a modern aluminium window, and they do it invisibly. A gasket is expected to compress enough to close a gap, recover enough to keep closing it after years of cycling, resist UV and ozone, resist the plasticiser migration that softens some rubbers, and stay in its groove while the frame moves.

Materials and What They Are Asked to Do

Ethylene propylene diene monomer rubber, usually abbreviated EPDM, is common in external window gaskets because it tolerates UV and weathering well. Thermoplastic elastomers are also widely used and offer good colour stability and recyclability, with different compression behaviour. Silicone gaskets appear where temperature extremes or chemical exposure are the governing issue. Each material has a characteristic compression set, a characteristic hardness and a characteristic compatibility with the sealants used around it, and mixing materials without checking compatibility is a common source of long-term failure.

The gasket’s function is not to be airtight in the manner of a glued joint. It is to remain elastic. A gasket that has taken a permanent set, hardened, shrunk or been stretched during installation no longer presses against the surfaces it is meant to seal, and the resulting gap is often too small to see and large enough to leak.

Corner Details: Moulded, Mitred and Welded

The corners of a gasket run are the weakest points in the gasket system. Three approaches are common. A moulded corner is produced as a single piece with the corner formed in the mould, giving continuous material and no joint. A mitred corner cuts two ends at an angle and brings them together, relying on the joint geometry and often on adhesive. A welded or vulcanised corner joins the ends permanently by heat and pressure.

From a sealing standpoint the order of preference is straightforward: continuous moulded or vulcanised corners outperform mitred corners, and mitred corners depend heavily on the skill of the person who made them. A mitred gasket corner that has been cut slightly short leaves a gap exactly where two weather directions meet. It is one of the most common hidden defects found during a close inspection of an installed window.

The corner must also be supported. A gasket that turns a sharp radius without a corresponding radius in the extrusion will lift out of its groove on the outside of the bend. This is why the profile and the gasket are designed as a pair, and why substituting a gasket from another system is not a neutral act.

Sealant at the Frame-to-Gasket Interface

Where a gasket terminates against a sealant bead, the two materials must be compatible and the transition must be deliberate. Sealant applied over a gasket rather than against it creates a three-material junction that moves differently under thermal cycling. The sealant cracks, the gasket continues to move, and a capillary gap opens at the boundary.

A better detail terminates the gasket in a groove or a rebate and seals the remaining joint to a rigid surface. The principle is to avoid relying on adhesion between a flexible material and a semi-flexible one in a location that will be cycled thousands of times over the life of the building.

Mullion and Transom Intersections

A mullion is a vertical member; a transom is a horizontal member. Where they cross, four frame sections meet, and the intersection has to resolve structure, drainage, sealing and often the fixing of both members to each other.

Continuity of the Seal at a T-Junction

The classic defect at a mullion and transom intersection is a discontinuity in the gasket or the weather seal on the upslope side. Water running down the mullion arrives at the transom and must be directed outward, not inward. If the transom gasket runs continuously past the mullion, the water is stopped but the mullion’s own drainage is interrupted. If the mullion gasket runs continuously past the transom, the transom’s drainage is interrupted instead. The correct detail usually involves a purpose-made connector or a carefully sequenced seal that maintains both paths.

This is why proprietary intersection connectors exist. They are not cosmetic. A connector that maintains the internal chamber, closes the external face and preserves the drainage outlet is doing structural and hydraulic work at the same time.

Movement and Fixing at Intersections

Mullions and transoms are usually fixed to each other with cleats, brackets or screw-fixed shear blocks. The fixing must transfer load without locking the two members so rigidly that thermal movement has nowhere to go. A rigidly locked intersection in a long run accumulates movement along the frame, and that movement is eventually expressed at the weakest seal in the assembly.

Long runs of framing also need deliberate provision for expansion. Where that provision is omitted, the frame effectively expands against its own fixings, and the resulting distortion shows up as gasket compression loss, sealant cracking and, in severe cases, glass edge contact.

The Intersection as a Water Shed

Every horizontal member is a ledge. Water lands on it, dust accumulates on it, and debris collects in the angle where it meets a vertical member. The intersection therefore needs both a drainage path and a shape that does not trap material. A detail that relies on a small weep path at a location that collects leaf litter will block within a season in a bushland setting and within a few years in most urban ones.

Silicone Compatibility

Silicone is the default sealant at window junctions, and it is also the material most often blamed for failures that were caused by preparation rather than chemistry. Understanding what silicone will and will not adhere to is fundamental to corner seal durability.

Neutral Cure and Acetoxy Systems

Acetoxy silicones cure by releasing acetic acid. They adhere well to many substrates, but the acid they release can attack metals, can corrode unprotected surfaces and is aggressive towards some substrates and some adjacent materials. Neutral cure silicones release an alcohol or an oxime and are generally the appropriate choice around metal frames, coated surfaces and sensitive adjacent materials.

The choice is not a matter of preference. Using the wrong cure chemistry against a coated aluminium surface or a cementitious surround can produce staining, adhesion loss or corrosion that appears months after the bead was tooled and looks like a product defect.

Adhesion to Anodised and Powder-Coated Aluminium

Anodised aluminium presents a hard, stable, inorganic surface. Powder coating presents a polymer surface with its own surface energy and its own contamination risk from handling, release agents and site dust. Neither surface is automatically bondable. Both require the correct preparation, which usually means cleaning to remove contamination and, for some sealants, a primer or an adhesion promoter specified by the sealant manufacturer.

The failure mode when adhesion is poor is not always obvious. The bead may remain visually intact while water passes behind it through a capillary gap. This is why adhesion should be checked by a peel test on a sample or a small area, not by looking at the outside of the joint.

Compatibility With Gaskets, Spacers and Glazing Accessories

Silicone in contact with a gasket, a glazing tape, a setting block or an insulating glass unit edge seal can cause problems that have nothing to do with adhesion. Plasticiser migration can soften or swell some elastomers. Solvents in a curing sealant can affect some spacers. An incompatible sealant in contact with an insulating glass unit’s perimeter seal can compromise the unit itself.

Compatibility is therefore a system question answered by the manufacturers of the frame, the glass and the sealant together. Where that information is not available, contact between dissimilar materials should be avoided by design rather than tested on the building.

Structural and Weather Sealing Are Different Duties

A weather seal keeps water out. A structural silicone joint transfers load between glass and frame. The two require different products, different joint geometry and different quality control. Using a weather sealant in a structural application, or a structural silicone as a general gap filler, is a category error that can have consequences well beyond a leak.

Pressure Equalisation

Pressure equalisation is the principle that makes modern drained window systems work. Rather than trying to keep water out with an impermeable barrier, the system allows the air pressure in an internal chamber to approach the external pressure, so that the driving force pushing water inward is reduced and gravity can do its job.

Why Compartmentation Matters

Equalisation only works within a defined volume. If a chamber is connected to a larger continuous cavity, pressure differences across the building envelope are transmitted along the cavity and equalisation is lost. This is why drained and ventilated systems are compartmentalised at intervals, with baffles or dividers that limit the volume being equalised.

Corner seals are part of that compartmentation. A corner that leaks air into the adjacent chamber, or into the wall cavity, defeats the compartment boundary. The symptom is a window that performs acceptably in moderate weather and admits water in wind-driven rain, which is exactly when the pressure differential is greatest and exactly when the failure matters most.

Baffles, Weeps and the Order of Operations

An equalised system typically includes an outer baffle that breaks the direct path of wind-driven water, an internal chamber, and a weep path that drains water while allowing air to move. The order matters. Installing the weep before the baffle, sealing the weep during painting, or omitting the baffle because it was not in the delivered package all change the behaviour of the assembly.

Because these components are small and hidden, they are frequently lost on site. A commissioning walk-through that confirms every weep is open and every baffle is present is worth more than a great deal of later remedial work.

Drainage

Drainage is the last defence and the one that determines whether a small ingress event becomes a building defect. Water that enters a frame must be able to leave it, and it must leave toward the exterior.

Path, Fall and Outlet

A drainage path has three parts: a route from where the water enters to a collection point, a fall along that route, and an outlet at the exterior face. Any one of the three can be defeated. The route can be blocked by a setting block or a bead of sealant. The fall can be reversed by frame distortion during fixing. The outlet can be blocked by debris, paint, render or a poorly positioned flashing.

Frame distortion is a particularly common cause because it is invisible after installation. A frame that is fixed out of wind, or packed so that a sill bows, changes the fall of its own drainage chamber. The window then drains correctly in the workshop and incorrectly on the wall.

Corner Drainage in Corner Windows

In a corner window, the drainage of two frames converges at the most exposed part of the assembly. Water shed from one elevation arrives at the corner and must be handed to the other frame’s drainage or directly to the exterior. A joint that simply abuts two frames leaves the water with no defined route and relies on the external sealant bead, which is exactly the dependency a drained system is meant to avoid.

The correct detail provides a continuous collection path around the corner, or a dedicated outlet at the corner itself. It also provides a backing detail that prevents water from tracking behind the corner post into the wall.

Glazing Interfaces

Corner seals and glazing seals are separate systems that meet at the same small area, and each can undermine the other.

Setting Blocks and Edge Clearance

Glass is supported on setting blocks that transfer its weight to the frame. The blocks must be positioned so that the glass bears on them properly and so that they do not obstruct drainage. A block placed across a weep path is a routine site error. Edge clearance and edge cover must also be maintained so that the glass does not bear directly on aluminium, which is hard enough to damage a glass edge and create a starting point for breakage.

Selection and installation of glass, including the requirements for safety glazing and for structural performance of the glass itself, are governed by AS 1288 in Australia, with AS 4666 applying to insulating glass units. New Zealand projects work to NZS 4223 for glazing. These standards define the selection framework; they do not remove the need to follow the frame manufacturer’s installation requirements.

Gaskets, Tapes and Wet Glazing

Glazing may be retained by dry gaskets, by tape, by wet sealant, or by a combination. Each approach has a different tolerance for frame movement and a different maintenance profile. Dry gaskets can be replaced; wet seals must be cut out. A drained glazing rebate must remain able to drain even after the glazing seal is installed, which means the seal must not be tooled into the drainage path.

Insulating glass units add a further consideration. The unit’s perimeter seal is a manufactured component with its own durability, and it must not be exposed to standing water inside the rebate. A rebate that holds water against the unit edge shortens the life of the unit, and the resulting failure appears as internal condensation between the panes rather than as a leak.

Movement

Windows move. Frames expand and contract with temperature, buildings deflect under load, slabs and walls move differentially, and in New Zealand seismic movement is a design condition rather than an exception. Corner seals live at the points where that movement concentrates.

Thermal Movement

Aluminium has a relatively high coefficient of thermal expansion for a framing material. A long dark-coloured frame on a west-facing elevation will change length between a cold morning and a hot afternoon. That movement must be accommodated by the frame’s own joints, by the fixings and by the sealant, in a defined order. If the frame is over-constrained, the movement is absorbed by distortion; if the sealant is expected to absorb all of it, the sealant will fail first.

Building and Seismic Movement

At the interface between a window and structure, relative movement is normal. Deflection of a slab edge, shrinkage of a timber frame, or inter-storey drift in a seismic event all impose movement on the window perimeter. A perimeter seal that has no capacity to accommodate that movement will either tear or transfer load into the frame.

In New Zealand, the Building Code and its verification methods treat these movements as design inputs, and NZS 4211 provides the framework for window performance classification. The practical implication for corner seals is that the seal detail must be designed as part of a movement strategy, not selected at the end of the project as a finishing item.

Corrosion

Aluminium corrosion at window corners is usually a moisture-retention problem rather than a material problem. Aluminium forms a protective oxide layer, and in most atmospheres that layer is stable. When water is trapped in a joint with no access to oxygen or with a continuous supply of chlorides, the protection breaks down.

Galvanic Coupling

Where aluminium is fixed with steel fasteners, or brought into contact with other metals, a galvanic couple can form. The severity depends on the metals involved, the electrolyte available and the ratio of the areas in contact. Coastal sites, where airborne salt provides a persistent electrolyte, are where this becomes visible first. Correct fastener specification, isolation washers where required and drainage that prevents water pooling at the fixing are the standard controls.

Coastal Exposure and Finish Performance

Salt deposits on a frame hold moisture against the surface. Corners, ledges and the tops of transoms collect deposits that rain does not fully remove. Powder coating and anodising both provide protection, but both can be compromised by edge damage, by cutting or drilling after coating, and by sealant that traps moisture against a discontinuity in the coating.

This is why touch-up of cut edges and damaged coating is a specification item rather than a cosmetic courtesy, and why the choice of sealant matters in coastal locations. An acidic cure sealant in a salt-laden environment is a poor combination.

Standards and the Compliance Framework

In Australia, windows and external glazed doors are specified against AS 2047, which sets out requirements for materials, construction and performance, and which is called up by the National Construction Code (NCC), the code maintained and administered by the Australian Building Codes Board (ABCB). AS 4284 provides the laboratory test method for building facades, which is the basis on which water and air performance claims are commonly demonstrated. AS 1288 governs the selection and installation of glass, AS 4666 covers insulating glass units, and safety glazing materials are addressed by AS 2208.

In New Zealand, NZS 4211 specifies performance requirements for windows and provides the classification framework used in that market, while NZS 4223 addresses glazing in buildings. Both sit under the New Zealand Building Code, with external moisture performance addressed through Clause E2 and its associated verification methods and acceptable solutions.

What these documents do not do is certify a particular installation. A compliant product installed with a discontinuous gasket, a blocked weep or an incompatible sealant is a non-compliant installation. Compliance at the corner is demonstrated by the assembly record: the system as designed, the components as supplied, the sequence as followed, and the inspection as documented.

Inspection

Corner seals are inspected properly only when someone is looking for them specifically. A general walk-through at handover will not find a mitred gasket that is short by a sliver, or a weep that was painted over.

What to Look For

Inspection should cover the continuity of the gasket line around every corner, the presence and correct seating of corner connectors, the condition of sealant at all frame-to-frame and frame-to-wall junctions, the openness of every drainage outlet, the presence of baffles, the flatness of sills, and the condition of the coating at cut edges and fixings. On corner windows, inspection should also confirm that the corner post is fixed as designed and that the internal seal line is continuous with the wall’s weather barrier.

Timing matters as much as method. Many defects are created by trades that follow the window installer, and an inspection that happens only at the end will find the symptom without being able to attribute the cause. Staged inspection, before cladding, before lining and at practical completion, is far more informative.

Documentation

A photographic record of each corner before the sealant is applied, and of each drainage outlet after it is, is a small effort that resolves most later disputes. It also gives the maintenance regime a baseline: what the detail looked like when it was correct.

Replacement and Repair

Corner seals are repairable, but repair is only durable when the cause has been identified. Re-sealing a corner that leaks because its drainage is blocked simply moves the water somewhere else.

Gasket Replacement

Gaskets are replaceable in most systems, and replacement is often the correct response to a gasket that has hardened, shrunk or taken a permanent set. The replacement must match the original profile and material, must be installed without stretching, and must have its corners formed correctly rather than mitred on site unless the system permits it. Gasket replacement is also an opportunity to confirm that the groove is clean and undamaged, because a gasket seated in a contaminated groove will not seal.

Sealant Replacement

Sealant replacement requires full removal of the old bead, not a bead applied over the top. The joint faces must be cleaned and, where specified, primed, and the new sealant must be tooled to a shape that accommodates movement. Applying new sealant over failed sealant is the single most common reason a leak returns within a season.

Component Replacement and Frame Repair

Where a corner connector is missing, corroded or broken, the frame may need partial disassembly. This is a specialist task. Attempting to force a connector into a distorted corner, or to close a gap with sealant, usually produces a joint that is neither structurally nor hydraulically sound. Where corrosion has affected the extrusion itself, replacement of the affected member is generally more reliable than repair.

Testing

Testing provides evidence, and evidence is what a dispute or a warranty claim turns on.

Laboratory Testing

Laboratory testing of a window system to AS 2047 and AS 4284 evaluates structural performance, air permeability and water penetration under controlled conditions. It establishes what the system can do when it is correctly manufactured and correctly installed in a test rig. It does not establish what a particular installation on a particular building will do.

Site Testing

Site testing closes that gap. A water test applied to an installed window, using a calibrated spray rack and a controlled pressure differential, can reveal defects that laboratory testing cannot, because it tests the actual junctions, the actual sealant and the actual workmanship. Air testing similarly identifies leakage paths that are invisible to the eye.

Site testing is most valuable when it is planned rather than reactive. Testing a representative sample early in the installation programme identifies a systematic defect while it is still cheap to correct. Testing only after a complaint has been received identifies the same defect at the worst possible moment.

Diagnostic Testing After a Leak

When a leak is reported, the diagnostic question is where the water enters, not where it appears. Water commonly travels laterally along a frame or a cavity before emerging. A staged test, isolating sections of the assembly in sequence, is more reliable than a single test of the whole window. This is where a documented assembly record earns its keep, because it narrows the list of plausible entry points.

Frequently Asked Questions

Why do corner seals fail before the rest of the window?

Because a corner concentrates more functions into a smaller area than any other part of the frame. It carries load, it faces weather from two directions, it contains the termination of at least two gasket runs, and it is assembled by hand rather than by machine. It is also the hardest place to inspect and the easiest place to leave a small gap.

Can a leaking corner be fixed with more sealant?

Usually not, and often the attempt makes things worse. If the frame is designed to drain, sealing internal corners can block the drainage path and trap water inside the extrusion. The correct approach is to identify whether the leak is a seal failure, a drainage failure or a pressure equalisation failure, and to repair the actual cause.

Is a mitred gasket corner acceptable?

It can be, in systems that are designed for it and where the joint is executed accurately. It is more dependent on workmanship than a moulded or vulcanised corner, and it is more likely to open over time. Where a choice is available, a continuous or permanently joined corner is the more robust detail.

Does a corner window need a different seal strategy from a standard window?

Yes. A corner window changes the structural load path, the thermal continuity and the drainage geometry at the same time. It should be treated as a single assembly with a defined strategy for each, and the interface between the two frames should be designed rather than improvised on site.

How does coastal exposure change the maintenance requirement?

Airborne salt keeps surfaces wet for longer and provides the electrolyte that drives galvanic corrosion. It also accumulates in the ledges and corners that rain does not flush. Coastal installations therefore benefit from more frequent cleaning of drainage paths and ledges, more frequent inspection of sealant and fixings, and stricter attention to coating damage and dissimilar metal contact.

What is the most useful thing a specifier can write into the documents?

A requirement that the window system be installed as a complete system, with the manufacturer’s corner connectors, gaskets and drainage components, in the manufacturer’s sequence, and with staged inspection of corners and drainage outlets recorded before concealment. That single clause prevents more defects than any amount of post-completion testing.

Do standards guarantee that a corner will not leak?

No. Standards define requirements, test methods and classification frameworks. They allow a product’s performance to be demonstrated and compared, and they set the basis on which compliance is assessed. They cannot guarantee the workmanship of a mitred gasket, the cleanliness of a surface before sealing, or the diligence of a site supervisor. Those are delivered by process, inspection and care.

Conclusion

Corner seals are the smallest components in an aluminium window and the ones most likely to determine whether the building performs. The reasons are structural, hydraulic and thermal at once, and they are compounded by the fact that corners are hand-assembled, rarely inspected closely and almost never revisited until something goes wrong.

The principles that produce durable corners are not complicated. Treat the corner as a system rather than a gap. Preserve the drainage path even when it seems easier to seal it. Use the components the system was designed around, including moulded or properly joined gasket corners and purpose-made connectors. Match the sealant chemistry to the substrate and check adhesion rather than assuming it. Keep pressure equalisation volumes intact. Design for movement instead of resisting it. Detail for corrosion, especially near the coast. Inspect in stages and record what you saw.

Corner windows raise the stakes because they concentrate all of these issues at the most exposed point of the envelope, and because the structural and thermal strategies must be resolved before the seal is even considered. A corner window that is designed as one assembly, installed as one assembly and inspected as one assembly will outperform a pair of windows that meet at an angle.

The compliance framework in Australia and New Zealand provides the vocabulary and the test methods for all of this, from AS 2047 and AS 4284 through AS 1288 and AS 4666, and from NZS 4211 and NZS 4223 through the New Zealand Building Code. What the framework cannot supply is the discipline at the corner. That comes from the people who assemble, seal, inspect and maintain the window, and from specifications that make those steps explicit rather than assumed.

Aluminium Window Corner Seals: Junctions, Gaskets, Drainage and Long-Term Care