Replacing windows is among the least rewarding energy measures

Windows lose heat quickly and there are not many of them

A window is a poor insulator compared with a wall, and single glazing is a very poor one, since it is a thin sheet of a material that conducts reasonably well with nothing to slow the transfer. Double glazing improves matters substantially by trapping a layer of gas between two panes, and modern units improve it further with coatings and better fills.

So the per-square-metre gain from replacing glazing is large. The area is where the argument weakens. Windows typically make up a modest fraction of the total external surface of a house, and heat lost through them is one term among several that includes walls, roof, floor and air leakage.

That is the whole of the case against, and it is quite a strong one. Multiplying a large improvement by a small area gives a moderate saving, and the cost of achieving it is high because replacing a window is joinery, making good and disruption, not simply the purchase of a better material.

What actually determines how a window performs

Four things do most of the work. The number of panes and the gap between them. The gas filling that gap, since heavier inert gases conduct less than air. A low-emissivity coating, which is a nearly invisible layer that reflects long-wave radiation back into the room, and which does more than most people realise. And the frame.

The edge of the glazing unit deserves particular attention, because the spacer holding the panes apart is a direct path between inside and outside. A conventional metal spacer conducts heat around the edge of the unit, which is why condensation appears at the bottom corners of otherwise good windows, and warm-edge spacers exist specifically to interrupt that path.

Then there’s the installation itself. A window fitted with gaps sealed by foam alone, without proper airtightness detailing at the reveal, can leak enough air to undo a good part of the improvement in the glass. In older housing, air leakage around frames is frequently a larger loss than conduction through the glazing.

Windows also let heat in, which cuts both ways

Glazing is unique among building elements in being a net energy source at some orientations. Sunlight passing through a window deposits heat inside the building, and over a heating season a large equator-facing window can gain more than it loses. That property is measured as a solar transmittance figure, and it works against the insulating figure.

The best glass for keeping heat in is often coated in ways that also reduce the heat coming through, so specifying purely for the insulation number can reduce a useful winter gain. Which matters more depends on latitude, on climate and on which way the window faces, and no single specification is correct everywhere.

In hot climates the calculation inverts entirely. There, unwanted solar gain through glazing is a dominant cooling load, and glass selected to reject solar radiation is worth more than glass selected to retain heat. This is one of the clearest cases in domestic energy where advice written for one climate is actively wrong in another.

Cheaper things that recover most of the benefit

Draught sealing existing frames costs very little and addresses the air leakage component, which as noted is often the larger loss in an old window. It is unglamorous, it can be done in an afternoon, and it doesn’t require anybody to remove anything from the building.

Secondary glazing adds an internal pane inside the existing window, creating an air gap without touching the original. It is markedly cheaper than replacement, it is often the only option on a historic building where the original joinery must stay, and it is excellent at reducing noise, which is frequently the real complaint.

Heavy curtains and internal shutters reduce night-time loss meaningfully, at almost no cost, and they carry one caveat worth stating. Closing a heavy curtain isolates the glass from room heat, which makes the glass colder and can increase condensation on it. That is a comfort and maintenance issue rather than an energy one, and it surprises people.

When replacement is the right decision

The honest case for new windows is usually not the energy. It is that the frames are rotten, the units have failed and misted, the sashes no longer close, the security is inadequate, or the noise is intolerable. Those are all good reasons, and once the decision is made for one of them, specifying the best available glazing costs relatively little extra.

That is the correct way to sequence it: replace when replacement is warranted, and buy performance at that moment. What rarely stands up is a project justified on energy alone while the walls remain uninsulated and the loft is thin.

The ranking that follows is fairly consistent across climates. Air leakage first, then the roof, then the walls, then floors, and glazing somewhere well down the list. There are exceptions, particularly in buildings with very large areas of single glazing, and the exceptions are visible from inside the building rather than hidden in a specification.