Why Curtain Walls Leak in the Monsoon

A curtain wall that leaks is almost never a curtain wall that was designed wrong. It is a curtain wall that was built as though it were something else.

The misunderstanding

Most people, including many site teams, believe a curtain wall keeps water out by being sealed. Water hits the outside, silicone stops it, nothing gets in.

That is not how the system works, and a wall built on that assumption will leak, because every sealant joint on a building of any size will eventually fail somewhere.

A curtain wall is a drained and pressure-equalised system. It assumes water will get past the outer face. Its job is to collect that water and take it back outside through designed drainage paths, before it reaches the interior.

The outer seal is the first line. The drainage is the actual defence.

What drives water in

Indian monsoon rain arrives with wind behind it, and wind creates a pressure difference across the facade , higher outside, lower inside. That difference will push water through any opening it finds, including upward.

Pressure equalisation defeats this by deliberately venting the cavity behind the outer face to the outside, so the pressure inside the cavity matches the pressure outside. With no differential, there is nothing to drive water inward, and what does enter simply drains.

Which means the vents are not optional and they are not a defect. Site teams seal them with genuine good intentions, believing they are closing a gap. It is one of the most common causes of leakage on Indian curtain walling, and it converts a working system into a bucket.

Where it actually fails

Blocked or omitted weep holes. The drainage path from each cavity. Blocked by sealant, by mortar, by dust. Every one must be open and must stay open, which means they need to be checked, not just installed.

Missing or discontinuous gaskets at the transom-to-mullion junction. Water travels along the horizontal and needs a positive, continuous path to the drainage. A gasket cut short at a corner is a leak with a delay on it.

No end dams. Where a horizontal drainage channel terminates, it needs a dam and an outlet. Without one, water runs off the end of the channel and into the building.

Transitions. Where the curtain wall meets the roof, the slab edge, a parapet, another cladding system. Interfaces are where different trades meet and where responsibility gets vague, and they leak far more often than the wall itself.

Thermal movement not allowed for. Aluminium moves substantially between a 5°C January night and a 50°C surface temperature in June. A frame installed without that allowance transmits the movement into the sealant, which fails, usually within three years.

Substitution. A lower-grade sealant, a thinner gasket, a different fixing. Each individually small, each a leak.

What actually prevents it

1. Mock-up testing. A full-size sample of the wall, including a typical junction, water-tested under pressure before the main installation starts. This is the single highest-value item in a facade budget and it is the first one cut.
2. Insist the installer explains the drainage path. If the site team cannot show you where water enters, where it collects and where it leaves, they are building a sealed wall and it will fail.
3. Never seal the vents. Write it into the specification and repeat it on site, because someone will try.
4. Detail the interfaces explicitly, and name who is responsible for each. Roof to wall, wall to slab edge, wall to parapet.
5. Test in sections during installation, not at the end. A hose test on a completed section costs an hour. Finding the same defect after handover costs a facade access system.

The maintenance point

A drained system has to stay drained. Weep holes in Delhi will clog with dust. Gaskets harden with UV. Sealant at movement joints has a service life measured in years, not decades.

A facade with no maintenance access designed into it is a facade that will not be maintained, which is a decision taken at concept stage by someone who will not be there when the leak appears.