Three seals, three failure modes
The gasket
The rubber or EPDM profile the sash compresses against when it closes. This is the seal that does the daily work — it stops air, sound and driven rain when the window is shut.
It fails by hardening. Years of heat and UV take the elasticity out of it, and once it no longer springs back, the sash closes onto a rigid strip instead of a compressible one. Symptoms: draughts, rattling in wind, road noise getting worse, and dust on the sill.
Fix: replace with the correct profile for that system. Packing it out with foam tape is a common shortcut that closes the gap and jams the sash.
The perimeter seal
The line of sealant between the outside of the frame and the wall. It is what stops water getting into the gap around the window.
It fails by shrinking, cracking or lifting, usually at the corners first, and usually because it was applied to a dusty or damp surface. Symptoms: damp patches at the jambs or above the window, particularly after wind-driven rain.
Fix: cut out the failed sealant entirely, prepare the surface, and reapply. Smearing new sealant over old is the single most common reason a leak returns.
The drainage path
Not a seal at all, but it is where most “sealing” jobs go wrong. Every properly made window expects some water to reach the rebate and drains it back out through weep holes at the sill.
It fails by silting up with dust. Symptoms: water pooling on the sill inside, or seeping from the bottom of the frame, typically in the first heavy rain of the season.
Fix: clear and flow-test the slots. Never seal them.
Diagnosing it yourself
Where the water appears tells you a great deal:
- On the inside sill, in a puddle → blocked drainage
- Running down the reveal from above or the sides → perimeter seal
- No water, but draughts and noise → gasket
- Between the panes, as fog → failed sealed unit, which is glass replacement
- Damp in the wall well away from the frame → not the window at all
What sealing properly gets you
Comfort and energy. A sealed window stops the conditioned air leaking out, which is what the air conditioning has been fighting.
Quiet. Air paths are sound paths. Restoring gasket compression is often the cheapest acoustic improvement on an existing window. If you need more than that, acoustic inserts are the next step.
Less dust. The same gap that carries noise carries Mumbai’s road dust onto the sill.
Frame protection. Water trapped in a frame is what rots timber and corrodes steel. Sealing and drainage done properly is the cheapest form of frame repair — the kind you do before it is needed.
When sealing is not the answer
If the frame has moved out of square, no gasket will seal against it, because the gap varies along the length of the sash. If the frame has rotted or corroded through, sealant is covering a structural problem. In both cases the honest answer is repair or replacement, and we will tell you that rather than applying a cosmetic fix.
Most of this is checkable yourself in half an hour — see the pre-monsoon window checklist. Tell us where the water appears and when, and we can usually name the failing seal before we arrive.