A double glazed window works by trapping a sealed layer of gas between two panes of glass, and that trapped layer is what slows heat escaping from your home. It is a simple idea doing a clever job. Here is how the parts of a double glazed window fit together, why the gap matters, and what the modern coatings and gas fills add on top.
A single pane of glass is a poor insulator. Heat passes straight through it, which is why old single-glazed windows feel cold to the touch and lose so much warmth. Double glazing puts two panes together with a sealed space between them, and it is that space, not the glass, that does most of the insulating.
The trapped layer between the panes slows down the movement of heat. Warmth from your room struggles to cross the gap, so more of it stays inside. The same barrier works the other way in summer, slowing heat coming in. That sealed unit of two panes plus a gap is called an insulated glass unit, and it is the heart of any double glazed window.
The space between the panes is not just empty by accident. It is sized carefully, usually somewhere around 12 to 20mm, because there is a sweet spot. Too narrow and the panes are too close to insulate well; too wide and the gas inside can start to circulate and carry heat across. The right gap keeps the trapped layer still, which is exactly what you want, since still gas is a much better insulator than moving gas.
The gap is also sealed airtight. That seal keeps the insulating layer in and moisture out, which is why a healthy unit stays clear inside.
Around the edge of the two panes sits a spacer bar, the strip that holds the panes apart and keeps the gap even all the way round. Older spacer bars were aluminium, which conducts heat and can feel cold at the edges. Modern "warm edge" spacer bars use materials that conduct far less heat, which cuts the cold bridge around the rim of the glass and helps reduce condensation forming on the edges.
The spacer bar usually contains a drying agent that soaks up any trace moisture in the gap, keeping the inside of the unit clear. The whole edge is then sealed to lock everything in.
Many modern units are not filled with plain air but with an inert gas, most commonly argon. Argon is denser than air and moves less, so it slows heat transfer even more. It is harmless, colourless and does the job quietly for years. This is one of the upgrades that has made today's double glazing so much better than the units fitted decades ago.
The other big improvement is a low-emissivity, or "low-E", coating. This is a microscopically thin, see-through layer on one of the panes. It lets sunlight and warmth in but reflects heat from inside your home back into the room instead of letting it escape through the glass. You cannot see it working, but it makes a real difference to how much warmth a window holds onto. Between the gas fill and the low-E coating, a modern double glazed window insulates far better than the plain air-filled units of years ago.
Put the parts together and a well-made double glazed window gives you:
In York's older housing stock, moving from single glazing or tired old units to modern double glazing often makes a noticeable difference to how a room feels, especially through a Yorkshire winter. Our uPVC windows and aluminium windows both come with modern insulated units as standard.
Double glazing is not forever. The most common sign a unit has failed is misting or condensation between the panes, where the edge seal has gone and moisture has crept into the gap. Once that happens the insulating layer is compromised and the unit looks foggy. The good news is that a misted unit can often be replaced on its own, without changing the whole window, as long as the frame is sound.
If you have a foggy or draughty window, it is worth getting it looked at. Our team handles misted and failed double glazing repairs across York, which is usually cheaper than assuming you need whole new windows.
The price of double glazing depends on the size, the frame material, the glass spec and how many windows you are doing, so there is no single figure that is honest to quote. Prices also change over time. The sensible approach is to get a proper measure and a quote for your own home rather than rely on a rough number.
It traps a sealed layer of gas between two panes of glass. That still layer slows heat moving across it, so more warmth stays in the room. Modern units add argon gas and a low-E coating to slow heat loss further.
Often an inert gas such as argon, which is denser than air and moves less, so it insulates better. Some units use plain air. Either way, the sealed, still layer is what does the insulating.
The edge seal has failed and moisture has got into the gap between the panes, causing misting. The unit no longer insulates as it should. A misted unit can usually be replaced on its own if the frame is in good order.
It is the strip holding the two panes apart, made from a material that conducts less heat than old aluminium spacers. It reduces the cold bridge around the edge of the glass and helps cut condensation forming at the edges.
Often, yes. Units fitted decades ago lack argon fills and low-E coatings, so modern glazing insulates much better and cuts condensation. If your frames are sound but the units are tired or misted, replacing the glass alone can be enough. Get a quote for your situation.
Want warmer, quieter rooms this year? Download our latest brochure to see the window range, or request a free quote and we will come and measure up.