Triple glazing is better at one thing and not automatically better at the others, which is where most of the confusion and a good deal of the sales pressure comes from. It has a third pane and two cavities, so it loses less heat. It is heavier, it costs more, and for noise it is frequently no improvement at all and occasionally worse. Whether it is worth the difference depends on how the house is heated, how well the rest of the envelope performs and what you are actually trying to fix.
Heat: the one it genuinely wins
Two cavities lose less heat than one, so a triple unit has a better U value than an equivalent double one and the inside face of the glass sits warmer. The warmer inner pane is the part people notice, because it reduces the cold draught feeling near a window and makes the space next to it usable. The saving on the bill is real and modest, and it is largest in a house that is already well insulated elsewhere, because in a leaky house the windows are not the thing losing the heat.
Noise: usually not the answer
This is the assumption that costs people money. Sound reduction comes from asymmetric pane thicknesses, a laminated pane and a wide cavity, and standard triple glazing typically has three similar panes in two narrow cavities, which is close to the worst arrangement for resonance. A well designed acoustic double unit will normally outperform standard triple glazing on noise, and secondary glazing will usually beat both. If noise is the complaint, that is the specification to ask for, not a third pane.
Weight, frames and the hinges
A third pane is a third of the glass weight again, and that has to be carried. Frames must be specified for it, hinges and mechanisms take more load, and a large opening casement in triple glazing is a heavy thing to operate every day. On a replacement into existing openings the frame depth can also be a constraint. None of this is a reason not to do it; it is a reason the quote should say what the frames and the hardware are rated for rather than only what the glass is.
When it is worth paying for
In a new build or a deep retrofit where the rest of the envelope is already good, triple glazing is a sensible part of the specification and the comfort difference is noticeable. In a house with solid walls, an uninsulated loft or draughts elsewhere, the same money spent on those will do more for both the bill and the comfort. And if the problem you are solving is noise rather than heat, the honest answer is that a third pane is not what fixes it.
Other questions people ask
Is triple glazing worth the extra cost?
In a house whose walls and loft are already well insulated, yes for comfort and a modest saving. In a house losing heat through solid walls or a cold loft, the same money spent there does more. The windows are rarely the weakest part of a British house.
Is triple glazing better for noise than double?
Usually not. Three similar panes in two narrow cavities is close to the worst arrangement for sound. Asymmetric pane thicknesses, a laminated pane and a wider cavity are what reduce noise, so a well designed acoustic double unit typically beats standard triple glazing.
Can I put triple glazing into my existing frames?
Sometimes, and it depends on whether the frame depth takes the thicker unit and whether the frame and hinges are rated for the extra weight. It is a question for the installer to answer in writing about your frames rather than in general.
Does triple glazing cause more condensation?
On the inside, less, because the inner pane sits warmer and is less likely to reach the temperature at which room air gives up its moisture. On the outside face it can show more, which is a sign the unit is working rather than a fault.