Aluminium vs uPVC Windows for Mumbai Flats: Monsoon Leakage, Salt Air and What Actually Decides It

Aluminium vs uPVC windows in Mumbai is usually argued on material. Most leaks come from the track, the sill and the gap around the frame. Here is how to choose, and what to insist on.

The aluminium vs uPVC windows question comes up on almost every Mumbai flat we walk into, usually phrased as a verdict someone is hoping for: uPVC does not leak, aluminium looks cheap, uPVC turns yellow, aluminium rusts near the sea. All four claims have a grain of truth in them and none of them is the thing that decides whether your window will keep the monsoon out. In a city where the rain arrives sideways for three months, the window that stays dry is the one that was detailed and fixed properly, whatever it is made of.

This guide is written from the site side of the job rather than the showroom side. It covers what the two materials actually are, why windows in Mumbai flats leak, what salt air near the coast does to each, how sliding tracks drain or fail to drain, why the sill matters more than the frame, how a frame should be packed and fixed into the opening, what to expect on noise, and what drives the cost. By the end you should be able to read two quotations side by side and see which one describes a window and which one describes a sale.

Aluminium vs uPVC windows in Mumbai: the short answer

If you want the conclusion before the reasoning, here it is. Both materials can give you a dry, quiet, long-lasting window in a Mumbai flat. Both can give you a window that leaks every monsoon. The material choice moves the result a little. The system quality, the opening type, the track design, the sill and the installation move it a lot.

• Choose uPVC when noise and air-tightness matter most, when the window faces a busy road, a railway line or a metro viaduct, and when the openings are of moderate size.
• Choose aluminium when you need slim sightlines, large sliding panels, big spans or a specific colour and finish, and specify a proper system rather than a fabricator's generic section.
• Near the sea, either can work: uPVC does not corrode, while aluminium needs a good anodised or powder-coated finish and stainless hardware, and both need stainless steel fixings.
• Whatever you choose, the leak-proofing is in the track drainage, the sill slope, the drip and the packing of the gap between frame and wall. Spend your attention there.

The rest of this article explains why that is the case, and what to check so the window you pay for is the window you get.

What an aluminium window actually is

An aluminium window is a set of extruded aluminium sections cut, joined at the corners and fitted with glass, gaskets, rollers and locks. The word covers a very wide range of products. At one end are the traditional local sliding windows made from standard sections by a fabricator in a small workshop, joined with screws, finished in a basic powder coat, and fitted with brush seals that wear flat in a few seasons. At the other end are engineered window systems from established brands, with thermally designed profiles, proper corner cleats and sealant, EPDM gaskets, multi-point locks, and tested drainage paths in the track.

Both are aluminium windows. They perform very differently, and much of the reputation aluminium has in Mumbai for rattling, whistling and leaking comes from the first category, not from the material. A well-specified aluminium system in a Mumbai high-rise can be as weather-tight as uPVC. What it will never be, on its own, is as good a thermal insulator, because aluminium conducts heat readily. Some systems address this with a thermal break, a strip of insulating material separating the inside and outside halves of the section, but in a Mumbai flat the heat through the glass matters far more than the heat through the frame.

Aluminium's strengths are stiffness and slenderness. A thin aluminium section can carry a large, heavy glass panel without bending, which is why the big sliding doors on balconies and the floor-to-ceiling glazing in newer towers are almost always aluminium. It also takes a very wide range of finishes: anodised, powder-coated in almost any colour, wood-grain effects and so on.

What a uPVC window actually is

uPVC stands for unplasticised polyvinyl chloride. A uPVC window frame is made from extruded plastic profiles with several internal hollow chambers. For anything beyond a small opening, the profile carries a galvanised steel reinforcement section inside one of those chambers to give it the stiffness that plastic alone does not have. The corners are usually fusion-welded, which means the two profiles are heated and pressed together so the corner becomes one continuous piece. That welded corner is one of uPVC's real advantages: it has no joint for water to find.

The chambered profile and the compression gaskets give uPVC windows good air-tightness, which is the main reason they are quieter than a basic aluminium slider. uPVC does not corrode, does not need painting, and is a poor conductor of heat. Its weaknesses are the opposite of aluminium's: the profiles are bulkier, so sightlines are thicker, very large sashes need heavy reinforcement or are not practical at all, and the material expands and contracts with temperature more than aluminium does. Good-quality uPVC for Indian conditions is formulated with UV stabilisers so that it does not chalk, go brittle or yellow in strong sun. Cheap uPVC without that formulation is where the yellowing stories come from.

As with aluminium, the word uPVC covers a range. There are established system suppliers who control the profile, the reinforcement, the gaskets and the hardware as a tested set, and there are fabricators who buy profiles from one source, hardware from another and assemble them with whatever reinforcement is to hand. Ask which you are getting.

Why windows in Mumbai flats leak: it is rarely the glass

When a window leaks in a Mumbai flat, the owner sees water on the inside sill, a damp patch on the wall below the window, or paint bubbling at one lower corner of the opening. The instinct is to blame the window. In our experience the window itself, meaning the glass and the sash, is the source in a minority of cases. The usual culprits are, in rough order of how often we find them:

• The gap between the frame and the wall, left hollow or filled only with a surface bead of sealant, which lets water track straight into the masonry.
• The external sill, laid flat or even sloping inwards, with no drip underneath, so water sits against the bottom of the frame and runs back along the underside into the wall.
• The bottom track of a sliding window, where water collects and has nowhere to go except over the inner lip when the wind pushes it.
• Blocked or missing drainage slots in the track, which turn the track into a trough.
• The corners of the frame, particularly on screw-jointed aluminium where the corner sealant has dried out.
• Cracks in the external plaster around the opening, or a chajja above that throws water onto the window rather than clear of it.

Notice that almost none of these are about the material. They are about the detail around the window and the quality of installation. That is why replacing a leaking aluminium window with a uPVC window, without addressing the sill and the frame-to-wall gap, so often produces a new window that leaks in exactly the same place.

The other Mumbai-specific factor is wind pressure. Monsoon rain on the western suburbs arrives on strong south-westerly winds, and the higher the floor, the more pressure is pushing that water against the window. A detail that is adequate on the second floor can fail on the twentieth. Water is not falling on a Mumbai window; it is being driven into every gap in it.

Sliding-track drainage: the detail that decides most sliding-window leaks

Sliding windows dominate Mumbai flats because they do not swing into the room or out over the street, they take grills easily and they are economical. Their weak point is the bottom track. Every sliding window collects water in its bottom track during heavy rain. That is normal. The question is what happens to that water next.

A properly designed sliding system handles it in three ways. First, the inner part of the track sits higher than the outer part, so water has to climb a step before it can reach the room. Second, the track has drainage slots or weep holes on the external face, positioned so water drains outwards and down. Third, those slots are protected by small flaps or caps so wind cannot blow straight back in through them. A sliding window designed like this can handle heavy driven rain.

A basic sliding window has a flat track of equal height on both sides, drainage holes drilled wherever the fabricator felt like drilling them, or none at all. Under wind pressure, water in the track is pushed across and over the inner lip. You find it on the sill in the morning.

• Ask to see a cut sample of the bottom track. Check whether the inner leg is taller than the outer.
• Check where the drainage slots are and whether they drain outwards, not into the wall cavity or onto the sill.
• Ask whether the slots have covers or baffles against wind-blown return.
• On site, after installation, pour water into the track and watch where it goes. This takes a minute and it is the most useful test you can do.
• Keep the track clean. Dust, dead insects and grit from construction block drainage slots quickly, and a blocked slot is a leak in waiting.

uPVC sliding windows are subject to exactly the same physics. The material does not make a flat track drain. What uPVC sliders often do better is the sealing between the sashes and the frame, because the gaskets and profiles are designed for compression. But a uPVC slider with poor drainage will still leak at the track.

Sliding, casement or tilt-and-turn: the opening type matters more than the material

If weather-tightness and noise are the priority, the single biggest lever is not aluminium versus uPVC. It is sliding versus casement. A sliding window has to leave a small clearance for the sash to move, sealed with brush pile or a wiper gasket. A casement window closes against a compression gasket all the way round, and the multi-point lock pulls it tight. That compression seal is fundamentally better at keeping out air, water and sound.

• Sliding: space-saving, easy with grills, most economical, weakest seal. Good for utility areas, kitchens and rooms facing a quiet side.
• Casement (side-hung, opening outwards or inwards): best seal, good ventilation, needs clear swing space and a thought-through grill arrangement. Good for bedrooms on a noisy road.
• Tilt-and-turn: tilts inward for secure ventilation, swings fully open for cleaning. Excellent seal and very practical on high floors where the outside of the glass is otherwise unreachable.
• Fixed glass: the best-sealed panel of all, because it does not open. Combining fixed panes with one or two opening sashes is often the smartest arrangement for a large window.
• Lift-and-slide doors: a heavier, better-sealed sliding system for balconies, where the sash lifts off its gasket to slide and drops back onto it to seal.

Outward-opening casements on high floors need careful thought about cleaning and about wind catching an open sash. Inward-opening casements need space inside and have to clear curtains and furniture. A good window supplier will ask you how the room is used before recommending a type.

Salt air near the coast: what it does to aluminium and uPVC

A large part of the western suburbs is close enough to the sea for salt to matter: Versova and Seven Bungalows in Andheri West, Juhu, Madh and Marve beyond Malad West, Gorai and parts of Borivali West, and Bhayander along the creek. Salt-laden air is corrosive. It attacks exposed steel quickly, attacks aluminium more slowly where the protective finish is damaged, and attacks every screw, roller axle and lock component that is not made of a suitable grade of stainless steel.

uPVC itself does not corrode, which is a genuine advantage near the coast. But a uPVC window is not all uPVC. The steel reinforcement inside the profile should be galvanised and should be sealed inside the chamber, the hardware such as hinges, handles, locking points and rollers should be suitable for coastal exposure, and the screws that fix the frame into the wall should be stainless. We have seen uPVC windows in coastal buildings where the profile looked new and the hardware had seized within a few years.

Aluminium near the sea depends on its surface finish. A good anodised finish or a good-quality powder coat applied over properly pre-treated metal holds up well. A thin or poorly pre-treated coat will chalk, blister and start to show white corrosion at cut edges and scratches. The corners and cut ends are the most vulnerable points, because that is where the finish is interrupted during fabrication.

• Ask for stainless steel fasteners throughout, and ask which grade. Near the sea, a higher marine-suitable grade is worth specifying for anything exposed.
• Avoid mild steel in direct contact with aluminium. Dissimilar metals in a wet, salty environment corrode each other.
• For aluminium, ask about the finish type, the coating thickness the supplier works to, and whether cut ends are sealed.
• For uPVC, ask whether the profile is formulated for Indian UV conditions and what the reinforcement is.
• Rinse coastal windows with fresh water occasionally, especially after the monsoon. Salt deposits hold moisture against every surface they sit on.

Heat, sun and movement

A west-facing window in Mumbai takes hours of direct afternoon sun. Two things happen. The frame heats and expands, and heat radiates through the glass into the room. On the frame side, uPVC moves more with temperature than aluminium does. Good uPVC systems allow for this in how the frame is fixed and how the gap around it is sealed. Dark-coloured uPVC frames absorb more heat and move more, which is one reason darker foiled uPVC needs more care in specification.

On the glass side, the material of the frame makes little difference. If heat gain is the problem, it is the glass that needs attention: a solar-control or reflective glass, a double-glazed unit, or external shading. A frame choice made purely on heat grounds, without changing the glass, will not make a west-facing bedroom noticeably cooler.

The sill: where most window leaks in Mumbai are born

If we could change only one thing about how windows are fitted in Mumbai buildings, it would be the external sill. In a large share of the leaking windows we open up, the frame is fine and the sill is the problem.

A correct external sill has three features. It slopes outwards, away from the frame, so water runs off rather than pooling against the bottom of the window. It projects beyond the face of the wall, so the water falls clear. And it has a drip, a groove cut into the underside near the front edge, so water running around the front of the sill breaks off and drops instead of tracking back underneath and into the wall. Without that groove, surface tension carries water along the underside of the sill straight back to the wall face, where it soaks into the plaster below the window.

• Slope the external sill clearly outwards. Water poured on it should run off immediately, not sit.
• Project the sill beyond the finished wall face.
• Cut a continuous drip groove on the underside, running the full length and returning at the ends.
• Seat the frame on a sill that is flat where the frame sits and sloped beyond it, not on a sill that is sloped under the frame.
• Seal the junction between frame bottom and sill properly, and never leave the sill top joint as a mortar joint facing the weather.
• Where a granite or marble sill is used, bed it fully rather than on dabs, and keep the joints tight and sealed.

Internal sills matter less for weather but matter for condensation and for what happens if water does get in. An internal sill that is slightly lower than the frame bottom lets any water that does get past the track show itself on the sill where you can see it, rather than run into the wall where you cannot.

The sill is part of the job we treat as ours rather than the window supplier's. In many projects the supplier fixes the frame and leaves, and the sill, the grout and the external finish around the window fall into a gap between the window vendor and the civil contractor. That gap is where the leak lives.

Frame fixing and packing: the gap nobody sees

Every window opening is slightly larger than the frame that goes into it, by design. The frame has to be positioned, squared and levelled in the opening, and that needs room. What happens to the resulting gap between frame and wall is the second great source of leaks.

The frame should be fixed mechanically to the structure with anchors or screws at the spacing the system supplier specifies, with packers behind each fixing point so tightening the screw does not bow the frame. It should be checked square, plumb and level before fixing, not after. Then the gap has to be filled properly: packed solid with a suitable non-shrink grout or mortar in the traditional method, or filled with an expanding foam and then sealed on both faces in the method many system suppliers use, with a backer rod and a flexible sealant on the external joint.

What goes wrong is easy to describe. The frame is propped in the opening on wedges, fixed with a few screws, the gap is left hollow or stuffed with loose mortar at the visible edges only, and a bead of silicone is run along the outside to make it look finished. The silicone is holding the weather out on its own. It is exposed to sun, it is often applied to dusty or damp surfaces, and it fails, usually in two or three monsoons. Water then walks straight into the hollow gap and from there into the plaster below.

• Check the frame is square by measuring both diagonals. They should match.
• Check plumb on the sides and level on the sill with a spirit level, not by eye.
• Confirm fixings are at the specified spacing, into sound masonry or concrete, with packers.
• Look at the gap before it is covered. It should be filled through its depth, not just at the face.
• The external sealant joint should have a backer rod behind it and a consistent profile, not a smear.
• Operate every sash through its full travel after fixing. A frame bowed by over-tightened screws will bind.

In new construction there is a cleaner route: a sub-frame or a properly formed opening, with the granite sill and plaster reveals finished to exact dimensions before the window arrives, so the final frame goes into a prepared, true opening rather than a rough one. It asks for coordination between the civil team and the window vendor, and it is one of the reasons a builder benefits from the finishing contractor owning the opening rather than leaving it to whoever arrives with the frames.

Sealant is the last line of defence, not the first

A good window detail does not rely on sealant. It relies on geometry: overlaps, slopes, drips and drainage that move water away before it ever reaches a joint. Sealant is then a secondary barrier and a finish. When the whole weather-tightness of a window depends on one bead of silicone, the window has a service life of however long that bead lasts in full sun.

Where sealant is used, it should be the right type for the job and the substrates, applied to clean and dry surfaces, with a backer rod to control depth, and tooled to a proper profile. Neutral-cure sealants are commonly recommended on window frames and around stone, because some acetic-cure silicones can stain or react with certain substrates. Your window supplier should specify this.

Noise: roads, trains and the metro

Much of the western suburbs sits alongside a noise source: the Western Express Highway, S.V. Road, Link Road, the Western Railway lines, and now the elevated metro lines running along Link Road and the highway. If your flat faces one of these, noise may be a bigger daily factor than rain.

Sound gets through a window in two ways: through gaps, and through the glass. Gaps are the bigger problem in most Mumbai flats. A sliding window with worn brush seals can let through a surprising amount of noise even with thick glass. This is why uPVC casements, with continuous compression gaskets, are so often recommended for noise, and why a well-sealed aluminium casement can perform in a similar way.

• Close the gaps first: casement or tilt-and-turn rather than sliding, compression gaskets, multi-point locks.
• Then improve the glass: laminated glass, which has an interlayer that damps sound, or a double-glazed unit with panes of different thicknesses.
• Mind the rest of the wall: an air-conditioner hole, a gap around the frame or an exhaust opening can undo the benefit of an expensive window.
• Be realistic: good windows reduce noise considerably, they do not make a flat on the highway silent.

A common mistake is paying for double glazing in a sliding frame with a weak seal. The glass is doing its job and the noise is simply going round it.

Glass: thickness, safety and heat

Glass choice deserves as much attention as frame choice, particularly on higher floors. Larger panes and higher floors mean more wind load, which means thicker glass or toughened glass as the system supplier specifies. Toughened glass is stronger and breaks into small granules rather than sharp shards. Laminated glass holds together when it breaks, which is valuable for safety and security as well as for noise. Low sill heights and full-height glazing often call for safety glass as a matter of good practice, and your supplier should confirm what is appropriate for the height and position of each window.

For heat, look at solar-control and reflective glass, and at double glazing, which also helps with condensation when a room is air-conditioned. Reflective glass changes how the building looks from outside, and some societies have views on elevation changes, which brings us to the next point.

Grills, mesh and the society facade

Replacing windows in an existing flat in a housing society is not purely an internal matter. The window is part of the building's elevation. Many societies regulate the colour, pattern and projection of windows and grills, and some require a uniform frame colour. Check this before ordering, not after the frames arrive in a colour the committee then objects to.

Safety grills and mosquito mesh are almost universal in Mumbai flats. With sliding windows, a mesh shutter can run in its own track. With casements, mesh is usually a fixed or roller screen on the inside and grills are fixed outside, which has to be planned so the sash can still open. Box grills that project from the wall need their own attention: they create a ledge where water collects, and their fixings into the wall are another potential entry point for water.

What drives the cost of aluminium and uPVC windows

We are deliberately not quoting a rate per square foot here, for the same reason we do not publish one for renovation. A rate without a specification behind it compares nothing. What we can do is tell you what moves the number, so you can see why two quotations for the same opening differ.

• System versus fabricated section. An engineered system from an established brand costs more than a generic section put together locally, and in most cases it is worth it on anything above a low floor.
• Profile weight and series. Heavier aluminium sections and larger uPVC profiles, with more chambers and more reinforcement, cost more and perform better at size and height.
• Opening type. Casements and tilt-and-turn windows carry more hardware than sliders. Lift-and-slide doors are a step above both.
• Glass specification. Moving from plain glass to toughened, laminated or double-glazed units can change the total more than the frame choice does.
• Hardware brand and finish. Multi-point locks, friction stays and coastal-grade hardware cost more and last longer.
• Finish. Anodising, special powder-coat colours and wood-grain foils on uPVC carry a premium over standard white or standard coat.
• Mesh and grills. Often quoted separately and then forgotten in the comparison.
• Civil work around the window. Removal of old frames, repair of the opening, new granite sill with drip, packing, plaster and paint touch-up. This is frequently excluded from a window quotation and then turns up as a separate bill.
• Access. High floors with no external access may need a scaffold, a hanging platform or specialised installation, which is a real cost.

In broad terms, a basic local aluminium slider is usually the least expensive option on the table, a good uPVC system and a good aluminium system tend to sit in a broadly comparable band above it, and premium systems with high-performance glass sit above both. The variations within each category are larger than the difference between the categories, which is exactly why a like-for-like written specification is the only fair way to compare quotes.

When comparing, ask each supplier to state the system and series, profile or section weight, reinforcement for uPVC, glass type and thickness for each window, hardware make, finish, whether mesh and grills are included, whether removal of old frames and the civil work around the opening are included, and what warranty they offer on the profile, the finish and the installation separately.

Replacing windows in an occupied Mumbai flat

Window replacement is one of the messier renovation jobs for its size, because the old frame is usually grouted into the wall and has to be cut out. That breaks plaster inside and outside, disturbs the sill and produces dust and debris. On the outside, working on a high floor without scaffolding needs care and safety measures, and some work may have to be done from a hanging platform.

• Measure after the old frame is out, or at least check the opening size carefully, because old openings are rarely square.
• Plan to redo the sill, not just to reuse the old one. If the old sill was part of the leak, a new window on it will leak too.
• Protect floors and furniture near the window, and close off the room if possible.
• Schedule outside the monsoon. An opening left even partly unsealed in July is an invitation.
• Budget for plaster, putty and paint repair around each opening, inside and outside.
• Check your society's working hours and any rules about external work and debris before booking.

The best time to replace windows in Mumbai is the dry season, from after the monsoon through to about May, once the walls around the openings have dried out and the new sill mortar and sealants can cure properly. We cover the timing of post-monsoon work in more detail in our article on the repair window between September and November.

Windows in new construction: what builders should specify

For a builder or developer, window leakage is one of the most common complaints raised by occupants in the first years after possession, and it is one of the most avoidable. The specification should cover not just the window system but the opening it goes into.

• Window system, series and glass for each elevation and floor band, with higher floors specified for higher wind load.
• Opening sizes finished to a tolerance before frames are ordered, so the frame fits the opening rather than the other way round.
• Sill material, slope, projection and drip detail drawn and agreed, not left to the site.
• Fixing method, fixing spacing and packing or foam method stated, along with the external sealant type.
• A water test on a sample of windows on each elevation before the external finish is completed, so failures are found while scaffolding is still up.
• Clear responsibility for the junction between the window vendor's scope and the civil contractor's scope.

That last point matters most. On many sites the window vendor installs frames and the civil contractor finishes around them, and nobody owns the packing, the sill and the external joint. When one contractor is responsible for the opening from sill to sealant, the leak has an owner before it happens.

Looking after windows so they keep working

• Clean sliding tracks regularly and clear the drainage slots, especially before the monsoon.
• Check gaskets and brush seals once a year and replace worn ones. They are inexpensive and they do most of the sealing.
• Lubricate rollers and locking points as the supplier recommends.
• Inspect the external sealant joint after each monsoon for cracks or gaps, and have it redone before it fails rather than after.
• Near the sea, rinse frames and hardware with fresh water from time to time.
• Keep the drip groove under the sill clear; paint and plaster sometimes fill it during repainting.

A practical way to decide

If you are standing in a Mumbai flat trying to choose, this is roughly how we would think it through with you on site:

• Which way does the window face, and how high is it? Driven rain and wind pressure point towards better sealing and better drainage.
• What is outside? A road, railway or metro points towards casements or tilt-and-turn with good gaskets, and probably laminated or double-glazed glass.
• How big is the opening? Very large panels and big balcony sliders point towards a good aluminium system.
• How close is the sea? Either material, with stainless fixings and coastal-grade hardware, and a good finish on aluminium.
• What does the society allow on the elevation?
• What is happening to the sill, the gap and the external joint? If the answer is nothing, fix that before you spend on a better frame.

In practice many flats end up with a mix: uPVC or aluminium casements in bedrooms facing the road, a well-drained slider in the kitchen and utility area, and an aluminium system for the balcony door. There is no rule that the whole flat has to use one material.

How Chamunda Constructions approaches window work

Chamunda Constructions has handled window frame fixing, sill construction and weather sealing as part of complete finishing work for Mumbai builders since 2004, under the supervision of founder Bhadresh Chavda. On a building, that means openings prepared and checked before frames arrive, frames fixed square and plumb, the gap packed solid rather than hidden behind sealant, sills laid with a fall and a drip, and every sash operated through its full travel before the flat is closed up.

On an individual flat in Kandivali, Malad, Borivali, Andheri or elsewhere in Mumbai, the same approach applies to a replacement. We look at why the existing window leaks before recommending what to replace it with, because in a good number of cases the honest answer is that the frame was never the problem. If you are deciding between aluminium and uPVC, or trying to stop a window that leaks every monsoon, contact us for a site visit and we will walk the openings with you and tell you what we find.