Double glazing gets talked about like a miracle and sold like a luxury. It's neither. It's a sensible upgrade that does two specific things very well, and a few things people expect it to do that it doesn't. Here's the plain version.
The short answer
A double-glazed unit is two panes of glass with a sealed gap between them, usually filled with argon. That trapped layer slows heat moving through the window. On a single-glazed villa, the windows are almost always the coldest surface in the room, which is why you get the draught feeling even when the air is warm.
Swap to double glazing and the inner pane sits much closer to room temperature. The room feels even, the cold-radiating effect by the window disappears, and condensation has far fewer cold surfaces to form on.
The biggest change most people notice isn't the power bill. It's that they stop avoiding the seat by the window in winter.
What you actually feel
Comfort is the headline. With single glazing, your body loses heat to the cold glass by radiation, so a room can be 20°C and still feel chilly near the windows. Double glazing raises that inner-pane temperature and the cold spot goes away. Noise drops too: a sealed double unit knocks back road and neighbour noise noticeably, though it isn't full acoustic glazing.
The power-bill maths
Windows are a small share of total heat loss compared to an un-insulated roof, but they're the surface you feel most. Expect a real, modest reduction in heating cost, not a halving. The stronger case is comfort and condensation, with the energy saving as a bonus that compounds over the life of the joinery.
When to spec it
If you're replacing joinery anyway, double glazing is almost always worth it: the marginal cost over single glazing is small relative to the whole job, and you only do it once. If budget is tight, a thermally broken frame with double glazing on living and bedroom spaces gives you most of the benefit where you spend your evenings.
We'll walk through exactly this on a free site measure, room by room, what's worth doing and what isn't.
