What a sealed unit actually is
Two panes of glass separated by a spacer bar, with the cavity sealed and filled with dry air or an inert gas. The seal is what makes it work and what eventually fails — the cavity has to stay dry, which is why a failed unit fogs.
The spacer contains a desiccant that absorbs residual moisture. When people talk about a “warm edge” spacer, they mean one made of a material that conducts less heat around the edge of the unit, which reduces condensation at the perimeter.
What it does in this climate
Heat
The important word is coating. An uncoated sealed unit reduces conducted heat but does little about radiant solar gain, which is what actually makes a west-facing Mumbai room unbearable at four in the afternoon.
A low-E or solar-control coating on the outer pane reflects infrared while passing visible light. That is where the real performance is. If you take one thing from this page: specify the coating, not just the second pane.
Noise
This is where double glazing earns its place in most Mumbai flats, and where specification matters just as much.
Two panes of the same thickness resonate together at a shared frequency, and traffic noise contains plenty of it. Two panes of different thickness, with the inner one laminated, break that coupling and damp what gets through. The difference between the two builds is easily audible.
For rooms where noise is the primary problem, soundproof windows take this further.
Condensation
A warmer inner pane means less condensation forming on the glass during humid months, which in turn means less water running onto the sill and into the reveal.
Double or triple?
Triple glazing adds a third pane. In cold climates it is a clear thermal win. In Mumbai the additional benefit over a well-specified coated double unit is modest, while the weight increase is significant — and weight drives the frame, reinforcement and hinge specification.
For most residential work here, a properly specified double unit is the better value. Triple makes sense where acoustic requirements are unusually demanding.
The frame has to carry it
A sealed unit is typically 20–28 mm overall and several times the weight of a single pane. That means:
- adequate rebate depth and a drained, ventilated glazing channel
- steel reinforcement in the uPVC profile appropriate to the span
- hinges and rollers rated for the sash weight
Fitting a heavy unit into a frame not designed for it is how sashes drop and how sliders flat-spot their rollers within a year. See uPVC window service for what that looks like afterwards.
Specifying yours
Tell us the elevation, the floor, and whether heat or noise is the bigger problem. Those three answers determine the build. See glass options for coatings and laminates, or glass replacement if the frames are sound and only the glazing needs upgrading.