Aluminium Window Condensation and Drainage: Separating Surface Moisture from Water Ingress

Condensation on an aluminium window is one of the most frequently reported defects in residential and light commercial buildings, and one of the most frequently misattributed. A homeowner sees water on the glass, on the frame, or pooled on the sill and concludes that the window leaks. A builder inspects the same window, finds no obvious opening in the weather seal, and concludes that the occupant is simply not ventilating. Both parties are describing something real, but neither has yet separated the two mechanisms that produce water at a window: moisture that condenses out of the indoor air onto a cold surface, and water that enters from outside through a failure of the building envelope. The remedies are almost opposite in character. Treating surface condensation as a leak leads to unnecessary re-sealing, which can trap moisture and make the problem worse. Treating a genuine ingress path as condensation allows water to remain inside the wall, the sill cavity and the frame for years. This article sets out a disciplined way to tell the two apart, how drained and ventilated window systems are supposed to work, where they commonly fail, and how to diagnose and maintain them without guesswork.

Why window condensation is so often misdiagnosed

Windows are the most visible part of the building envelope. They are also the coldest visible surface in most rooms during cold weather, particularly where the frame is uninsulated aluminium. Water therefore appears at the window first, even when the underlying condition is a whole-building issue of moisture load, ventilation or air leakage. The window is a symptom location, not necessarily the cause.

Several factors conspire to confuse the diagnosis. Condensation frequently appears at the bottom of the glass and runs onto the sill, which mimics water tracking through a sill joint. It often appears after cold nights and disappears during the day, which mimics an intermittent leak driven by wind-driven rain. It may stain plasterboard reveals and skirting, which mimics a failed flashing. And because modern buildings are tighter and better insulated than older ones, the moisture produced by cooking, showering, laundry and breathing has fewer paths to escape, so the same window that performed acceptably in a draughty house may now show heavy condensation in a well-sealed one.

The correct starting point is therefore not the window at all, but the question of where the water came from. Was it delivered as liquid water from outside, or was it delivered as water vapour from inside and then converted to liquid on a cold surface? Everything that follows depends on answering that question honestly.

Surface condensation and interstitial condensation

Surface condensation

Surface condensation occurs when indoor air, carrying water vapour, comes into contact with a surface whose temperature is at or below the dew point temperature of that air. The dew point is not a fixed number; it rises as the indoor humidity rises and as the indoor air temperature rises. This is why the same window can be dry for most of the winter and streaming during a cold snap, a period of prolonged rain, or a week when the household is drying laundry indoors.

The visible signs are characteristic. Water beads on the room-side face of the glass, most heavily near the bottom edge and in corners where air movement is weakest. The frame may be wet on its internal faces, particularly on uninsulated aluminium profiles, which conduct heat outward quickly and stay close to outdoor temperature. Water may sit in the reveal or on the sill, and in severe cases run down the wall below. Critically, surface condensation is always on the warm side of the assembly. It is a room-side phenomenon, even when it appears to originate at the window.

Interstitial condensation

Interstitial condensation occurs within the thickness of the wall, roof or window assembly rather than on a visible surface. Water vapour diffuses or is carried by air movement into the construction and reaches a point in the build-up that is cold enough for the vapour to condense. Because that point is hidden, the evidence arrives late and indirectly: damp patches on internal linings, blistering paint, musty odour, mould at the junction of reveal and wall, corrosion of fixings, or deterioration of timber framing that is only discovered during renovation.

Interstitial condensation is governed by the sequence of layers in the assembly, not by the surface temperature of the glass. It depends on where the insulation sits, where the vapour control layer sits, how airtight the construction is, and how quickly the assembly can dry in either direction. A wall that is airtight but has a vapour control layer in the wrong place, or a window installed with a continuous bead of sealant on both faces of the frame, can accumulate moisture internally while showing almost nothing on the room-side surface.

Why the two demand different responses

Surface condensation is managed by raising surface temperatures, reducing indoor moisture production, and improving ventilation and air distribution. Interstitial condensation is managed by correcting the order and continuity of the control layers: air barrier, vapour control, insulation and weather seal. Confusing the two leads to predictable failures. Applying additional internal sealant to stop surface condensation achieves nothing and may reduce drying. Increasing ventilation to cure interstitial condensation may help, but will not repair a misplaced vapour control layer.

The underlying physics, without the numbers

Three variables govern whether condensation forms. The first is the amount of water vapour in the air, usually expressed as relative humidity because that is what instruments measure. The second is the temperature of the air. The third is the temperature of the surface or layer where that air arrives. Condensation occurs at the intersection of the three: warm, humid air meeting a cold surface.

For aluminium windows, the third variable is the difficult one. Aluminium conducts heat roughly three orders of magnitude better than typical insulation materials, so an unbroken aluminium profile acts as a fin connecting the warm interior to the cold exterior. Its internal surface tracks outdoor temperature closely, which is why older aluminium windows in cold climates produce heavy condensation on the frame long before the glass does. The glass, by contrast, is a poor conductor, and a sealed insulating glass unit improves the internal surface temperature further by interposing a still air or gas layer between the panes.

This is also why the bottom corners of a window are usually the wettest. Air movement at the corner is restricted, so the surface there is both colder and less able to be dried by room air. The corner is where the frame, the glass edge, the spacer and the installation junction all meet, and it is where the geometry is least favourable.

Air barriers, vapour control and where windows meet walls

Air movement matters more than diffusion

In most buildings, far more water vapour is transported into a construction by air leakage than by diffusion through materials. A small gap in an air barrier, driven by stack effect or wind pressure, can carry a large moisture load into a cavity in a way that diffusion calculations never predict. This is why air barrier continuity, not just vapour control layer selection, is central to avoiding interstitial condensation. It is also why window perimeter sealing is a moisture-control detail, not merely a draught-control detail.

Vapour control and the direction of drying

A vapour control layer is intended to limit the movement of vapour from the warm, humid side into the colder parts of the assembly. Its position must suit the climate and the construction. In heating-dominated climates the control layer generally sits on the warm side; in cooling-dominated climates the logic changes and the risk of trapping moisture on the wrong face increases. The layer must be continuous, sealed at penetrations and junctions, and compatible with the need of the assembly to dry. A vapour control layer that is punctured, lapped the wrong way, or terminated short of the window frame is a pathway rather than a barrier.

The window perimeter detail

At the window perimeter, four functions must coexist: structural connection, weather sealing, air sealing and thermal continuity. These are not the same function, and they do not belong on the same bead of sealant. A common and damaging simplification is to run a continuous sealant bead around the frame on both the internal and external faces, which closes the joint from both sides. The result is a sealed cavity with no drying path and no pressure equalisation. Any moisture that does enter, whether by leakage or by diffusion, has nowhere to go. The correct approach separates the weather seal from the air seal and allows the concealed zone to drain and ventilate.

Thermal breaks and frame behaviour

What a thermal break does

A thermally broken aluminium frame interrupts the conductive path between the inner and outer aluminium profiles with an insulating element, usually a polyamide or polyurethane component. The purpose is straightforward: reduce heat flow through the frame so that the internal surface stays warmer, which reduces the likelihood of surface condensation on the frame and reduces the heating load. The break must be continuous around the frame, including at corners, mullion and transom junctions, and at any coupling where two frames meet, otherwise the conduction path simply bypasses the break.

Design and installation faults that defeat the break

Thermal breaks are defeated in practice more often than they are omitted in design. A fixing screw driven through the break, a metal packer bridging the inner and outer profiles, a head or sill flashing that contacts the inner profile directly, or a frame that is set too far forward in the wall so that it sits outside the insulation line, all undermine the intended performance. The last point is particularly consequential: a thermally broken frame installed in a position where the insulation does not wrap the frame will still run cold, because the break only addresses conduction through the aluminium, not the thermal environment around it.

Spacers, warm edges and the glass perimeter

Within an insulating glass unit, the edge seal and the spacer that separates the panes form the weakest thermal path. Conventional aluminium box spacers conduct readily, producing a cold band around the perimeter of the glass and a corresponding increase in condensation risk at the sight line. Warm-edge spacer systems reduce this effect by using low-conductivity materials or a hybrid construction. The practical consequence is that two units with identical centre-of-glass performance can behave quite differently at the edge, and it is the edge where condensation first appears.

Glazing behaviour and unit integrity

Sealed insulating glass units are designed to keep the cavity dry. A desiccant within the spacer absorbs residual moisture after manufacture, and the primary seal limits vapour transmission into the cavity. When the primary seal fails, or when the unit was poorly manufactured or mechanically damaged during installation, the desiccant becomes saturated and the cavity begins to show internal condensation: fogging or a milky film between the panes that does not clear when the weather changes and cannot be wiped away.

Internal condensation is a definitive finding. It indicates a unit that has lost its integrity and must be replaced; no amount of room-side drying, ventilation or sealant will correct it. It should not be confused with condensation on the room-side surface, which clears when the glass warms, nor with condensation on the external surface, which can form on high-performance glazing in humid outdoor conditions and is a normal physical result rather than a defect.

Glass selection also affects condensation indirectly. Coatings that reduce radiative heat loss keep the internal glass surface warmer, which lowers condensation risk, while tinted or body-tinted glass absorbs solar radiation and can run warmer or cooler depending on orientation and time of day. None of this changes the fundamental rule: the room-side surface temperature relative to the indoor dew point determines whether water appears.

Sill drainage and weeps: the drained and ventilated principle

The principle

Most proprietary aluminium window systems are designed on a drained and ventilated principle. The system accepts that a small amount of water will penetrate the outer weather line, and provides a concealed path for that water to reach the exterior: a sloped sill chamber, drainage slots or channels, and weep openings that discharge to the outside. Ventilation openings allow air to enter and leave the chamber so that pressure can equalise and the chamber can dry. This is an intentional design philosophy, not a defect, and it depends entirely on the drainage and ventilation openings remaining open and correctly positioned.

Weep openings and baffles

Weep openings must be at the lowest point of the drained chamber, sized and spaced to discharge the water that reaches them, and protected against wind-driven rain and insect entry, usually by a baffle, flap or hood. They must not be painted over, blocked by sealant, covered by a sill trim, obstructed by a packer, or buried below the external sill level. A weep that discharges into a closed cavity, or that sits behind a bead of external sealant, is functionally absent even though it is physically present.

Baffles and hoods deserve particular attention. A baffle that is fitted backwards, omitted, or replaced with a solid plug during servicing will either admit driven rain or prevent discharge. Because these components are small and easily lost, they are a frequent cause of unexplained water at the sill after maintenance work.

Installation errors that defeat drainage

Sealant across the drainage path. A continuous bead applied along the internal face of the sill, or around the full perimeter of the frame, closes the chamber and traps water.
Lost fall or reversed fall. A sill that is not set level and slightly outward, or that is distorted by packers, will pond water rather than discharge it.
Blocked or undersized openings. Debris, mortar, foam, tape or paint in the drainage slots reduces capacity below what the system requires.
Projecting sills without an under-sill drip. Water that runs back under the sill can re-enter the frame or stain the wall below.
Head flashings that discharge onto the window. Water should be directed past the opening, not onto the frame or into the head cavity.
Frames set out of plane. Twisting during installation can open joints on one side and close drainage paths on the other.

Ventilation and the internal moisture load

Because surface condensation is driven by indoor humidity, ventilation is a genuine control measure rather than a distraction. Moisture is generated continuously by occupants, cooking, showers, baths, indoor laundry drying, unflued gas appliances, pot plants, aquariums and damp building materials. In a well-sealed dwelling, that moisture accumulates unless it is removed at source or diluted by outdoor air.

Local exhaust at kitchens and bathrooms removes moisture before it disperses, and is more effective than opening a window afterwards. Background ventilation, whether provided by purpose-designed openings, mechanical supply or extract systems, or a heat recovery system, manages the residual load. The critical point is that ventilation must be continuous enough to control average humidity, not merely occasional. Opening a window for ten minutes after a shower helps, but it does not compensate for a dwelling that is closed up for the rest of the day with a persistent moisture source.

Air distribution within the room matters too. Furniture pushed against a window, heavy curtains closed across the frame, and sealed pelmets all reduce the warm air movement that would otherwise keep the glass and frame above the dew point. Where condensation is consistently worse behind a curtain, the cause is often restricted air circulation rather than a window defect.

Sealants: what they can and cannot do

Sealants are essential at the window perimeter, but they are frequently asked to do work they cannot perform. A sealant joint is a movement-accommodating weather seal, not a structural connection and not a substitute for a flashing. It must be compatible with the substrates, applied to clean and dry surfaces, correctly proportioned for the expected movement, and continuous where continuity is required. It must also be discontinuous where drainage and ventilation are required.

Common failures include adhesion loss at the frame or substrate, cohesive tearing from over-extension, application over damp or dusty surfaces, incompatible sealant over previously applied product, and premature failure from ultraviolet exposure where the joint was not intended for exposed service. Each of these produces a water path that looks like a leak and is often blamed on the window rather than the joint.

The diagnostic implication is important. Adding sealant to a joint that is already wet traps moisture and reduces the chance of the assembly drying. Before any re-sealing, the joint should be assessed for the reason it failed, and the concealed zone should be allowed to dry. Re-sealing a wet detail is one of the most reliable ways to convert a minor leak into a durability problem.

A diagnostic sequence that separates the two mechanisms

The following sequence is intended to be worked in order. Each step is designed to eliminate a hypothesis rather than to confirm a preferred one, and the sequence deliberately starts with the least invasive observations.

Step 1: Establish the timing and the weather conditions

Record when water appears. Condensation typically appears during cold nights and early mornings, clears as the day warms, and correlates with indoor activities that produce moisture. Ingress correlates with rain, particularly wind-driven rain on a specific elevation, and may persist or reappear after the rain has stopped as water stored in the wall or sill drains. A diary of observations over two to four weeks is more valuable than a single inspection.

Step 2: Identify the location of the water

Condensation on the room-side glass surface, the frame’s internal face, or the internal reveal is characteristic. Ingress usually appears at joints, corners, fixings, the head, the sill-to-frame junction, or the perimeter seal, and often carries dust or staining from the cavity. Water that appears above the window, on the wall or ceiling, is unlikely to originate from the window itself.

Step 3: Check the glass and the frame for internal moisture

Fogging between panes, a milky film, or water droplets inside the sealed unit indicates a failed unit and is not a condensation-control problem. Water within the frame profile, visible through drainage openings or at joints, indicates that water is entering the system and not discharging.

Step 4: Inspect the drainage and ventilation paths

With a torch and a probe, confirm that weep openings are open, correctly positioned at the low point, fitted with their baffles, and discharging freely to the exterior. Check that no sealant, paint, trim, packer or debris obstructs them. Confirm that the sill has fall toward the openings and that water placed gently at the low point of the chamber exits outside.

Step 5: Inspect the perimeter seals and flashings

Check the external sealant joint for adhesion, continuity and correct termination at the drainage openings. Check head, jamb and sill flashings for correct lapping, end dams and discharge away from the opening. Check that the air seal on the internal side is continuous and that the joint is not sealed on both faces with no drying path.

Step 6: Assess the thermal detail

Determine whether the frame is thermally broken and whether the break is continuous. Check whether fixings, packers or flashings bridge the break. Confirm that the frame sits in the intended position relative to the insulation line, and that insulation is not compressed, omitted or displaced at the junction. Thermal imaging, used by a competent operator under suitable conditions, can reveal cold zones and air leakage paths that are not visible to the eye.

Step 7: Measure indoor conditions

Measure relative humidity and temperature in the room over time, including during the periods when water appears. Persistent high humidity with a cold surface is a strong indicator of surface condensation. Low humidity with water appearing only during rain points toward ingress. Measurements taken at a single moment are rarely conclusive.

Step 8: Test selectively, and only after the above

If the evidence still points to ingress, a controlled water test at the suspected detail, performed from the exterior with attention to the drainage openings and the internal observations, can localise the path. Testing should not be used to replace the earlier steps, because indiscriminate water testing can introduce water into areas that were previously dry and complicate the diagnosis.

Maintenance that keeps the system working

Aluminium windows with drained and ventilated sills require routine maintenance, and most of the work is simple. The objective is to keep water moving outward and air moving through the concealed zones.

Clear weep openings, drainage slots and ventilation openings of dust, insect nests, spider webs, pollen and debris at least twice a year, and more often in exposed or dusty locations.
Wash frames and glass with mild detergent and clean water; avoid abrasive pads and aggressive solvents that damage coatings or seals.
Inspect sealant joints annually for adhesion, cracking, shrinkage and continuity, and repair failed sections rather than covering them.
Check that baffles and hoods are present, correctly oriented and undamaged, and replace missing components with the manufacturer’s parts.
Keep the sill and the ground or paving below it free of build-up so that discharge is unimpeded and water does not splash back.
Confirm that weep paths have not been painted, rendered, tiled or trimmed over during later works.
Maintain internal ventilation systems, clean filters, and keep exhaust paths unobstructed.
Record observations and repairs so that recurring patterns can be recognised.

Where a unit has failed internally, or where drainage components are damaged beyond repair, replacement of the affected components is the appropriate response. Patching a failed sealed unit or sealing over a blocked drainage path does not restore performance.

Standards and regulatory context

Window performance and condensation control sit within a framework of standards and building regulation, and it is worth being precise about what each element does and does not establish.

NCC and ABCB condensation context

The National Construction Code, maintained through the Australian Building Codes Board, addresses condensation risk through its performance requirements and referenced provisions, including the management of moisture and the contribution of building fabric to health and amenity. The NCC does not prescribe a single condensation solution for windows; it establishes outcomes that the design and construction must satisfy, and it relies on referenced standards, acceptable construction manuals and qualified design judgement for the means of compliance. The practical consequence for a window installation is that compliance is demonstrated by the whole detail, not by a product label. A window that meets a product standard can still fail in service if it is installed without drainage, without an air seal, or outside the insulation line.

AS 2047

AS 2047 is the Australian standard for windows and external glazed doors in buildings. It provides the framework for performance characteristics, classification and marking, and it is the reference point for establishing what a window system is claimed to do before it is installed. Its relevance to this discussion is that a system’s drainage and ventilation provisions, structural capacity and weather performance are part of the product’s defined characteristics, and installation must preserve them.

AS 1288

AS 1288 covers glass in buildings and the selection and installation of glass to suit its application, including structural and safety considerations. Glass thickness, type, support and glazing methods affect how a window behaves under load and how its edges and seals perform, which in turn affects the durability of the sealed unit and the integrity of the weather line.

AS 4284 and AS 4666

AS 4284 and AS 4666 are also nominated in many project specifications and in the technical literature surrounding window and facade work. Because editions, titles and scopes are revised over time, the applicable edition and its exact application should be confirmed against the project specification and the current published standard rather than assumed. Where a specification calls up a standard, the obligation is to the edition cited, and any substitution should be documented and agreed.

New Zealand references

In New Zealand, the Building Code addresses external moisture and internal moisture through its performance clauses, supported by acceptable solutions and referenced standards for weathertightness, glazing and framing. The weathertightness framework is particularly instructive because it formalises the same principles described in this article: deflection, drainage, drying, and the separation of the weather line from the air seal. New Zealand practice also emphasises the importance of the drained and ventilated cavity and the correct sequencing of the cladding and window installation, both of which bear directly on whether a sill drains or ponds.

Across both jurisdictions, the common thread is that standards define performance and method, while the installed detail determines outcome. A window system assessed against a standard and installed without regard to drainage, thermal continuity and air sealing will not deliver the performance the standard describes.

Frequently asked questions

Is condensation on a new aluminium window normal?

Some surface condensation can occur on any window when indoor humidity is high and outdoor temperatures are low. On a new thermally broken window with good glazing, however, heavy or persistent condensation usually indicates a moisture load or ventilation issue, or an installation detail that leaves the frame colder than intended. It should be investigated rather than accepted.

Does water on the sill always mean the window leaks?

No. Water that collects at the base of the glass and runs onto the sill is very often surface condensation, particularly if it appears on cold mornings and clears later. A leak is more likely when water appears during or after rain, at joints and fixings, or with staining from the cavity.

Should I seal the weep holes to stop draughts?

No. Weep openings are part of the drainage and pressure equalisation system. Blocking them traps water inside the frame and increases the risk of corrosion, mould and internal damage. If draughts are a concern, the air seal and the frame’s weather seals should be assessed instead.

Can I stop condensation by adding more sealant?

No. Sealant does not change the temperature of the glass or the humidity of the air. Adding sealant around a condensation-affected window may reduce drying and can trap moisture in the frame or wall.

What does fogging between the panes mean?

It indicates that the sealed unit has lost integrity and moisture has entered the cavity. The unit should be replaced. It is a product failure, not a ventilation problem.

Why is the condensation worse in the corners?

Corners have restricted air movement, more thermal bridging, and a geometry where frame, spacer and glazing edge converge. They are the coldest and least ventilated part of the window, so they reach the dew point first.

Does opening a window cure condensation?

It lowers indoor humidity temporarily and can help, but it is not a substitute for controlling moisture at source and providing adequate continuous ventilation. In cold weather, brief opening may also cool surfaces and produce a short-term increase in condensation.

How do I know whether the problem is interstitial?

Interstitial condensation is indicated by dampness, staining, mould or deterioration that appears away from the window surface, often on internal linings or within the frame cavity, without visible room-side water. Confirming it usually requires inspection of the assembly and, in some cases, moisture measurement or thermal imaging.

Conclusion

Water at an aluminium window is evidence, not a diagnosis. The task is to determine whether it was delivered as vapour from the inside and condensed on a cold surface, or as liquid from the outside and failed to drain. Surface condensation responds to warmer surfaces, lower moisture production and better ventilation. Interstitial condensation responds to correct layer sequence, continuity and drying capacity. Ingress responds to drainage, flashings, seals and installation geometry. Each has a distinct diagnostic signature, and each is confirmed by observation over time rather than by a single inspection.

The drained and ventilated window is a designed system with a defined sequence: accept limited water at the weather line, drain it to the exterior, ventilate the concealed zone, and keep the air seal separate from the weather seal. Every part of that sequence is vulnerable to a bead of sealant, a blocked weep, a missing baffle, a bridged thermal break or a frame set outside the insulation. Maintenance is therefore not cosmetic; it is the periodic restoration of the system’s function. Where the evidence points to a failed unit, a damaged drainage component or a defective installation detail, repair or replacement is the correct response, and sealing over the symptom is not.