Windows and glazing: how to choose

Start with what the window is for

View. What will you see, and from where?
Daylight. How much light does the room need?
Air. Does the window need to open for ventilation or escape?
Privacy and security. Who can see in, and who can get in?
Sun and heat. How much sun comes in, and when?
Noise. Is the street loud?

The glass

Double glazing has two panes with a sealed gap between them. Triple glazing has three. More panes mean better insulation, but also more weight and cost, and less light and solar gain.
Low-emissivity (low-e) coatings reflect heat back into the room while letting light through.
Gas fills, such as argon, in the gap improve insulation.
Warm-edge spacers reduce heat loss at the edge of the glass.
Solar-control glass reduces the heat from the sun, which helps in hot climates and for large west-facing windows, but it also reduces light and can change colour.
Laminated and toughened glass for safety, security and sound, as the rules require.
Acoustic glazing has laminated glass and different thicknesses to reduce noise.

Compare the whole window, frame and glass, rather than the glass alone.
Windows and glazing: how to choose

Key numbers

Manufacturers publish figures for performance. The main ones are:

U-value: how much heat passes through; lower is better.
g-value (solar factor): how much solar heat passes through; the right value depends on the climate and the orientation.
Light transmission: how much daylight passes through.

Compare the whole window, frame and glass, rather than the glass alone.

The frame

Timber is warm and renewable, and needs maintenance unless it is protected or modified.
Aluminium is slender, strong and durable, and needs thermal breaks to be efficient.
uPVC is low cost and low maintenance, with thicker frames, and has end-of-life questions.
Composite frames combine materials, such as timber inside and aluminium outside.
Steel gives very slender frames, and is expensive and needs a thermal break to be efficient.

Position and installation

A good window can perform badly if it is badly installed. The position in the wall affects heat loss, rain protection and appearance. Sealing around the frame matters for airtightness and thermal bridges. See Thermal bridging and Airtightness and ventilation.

Shading

Glass needs shade where the sun is strong. Overhangs, louvres, blinds and trees can control it. Shading often matters more than the glass specification for avoiding overheating. See Designing for a hotter future.

Rules and installers

Building rules cover energy, safety glass, escape, ventilation and sometimes security and fire. In the UK, replacement windows in existing homes must meet the regulations, which are usually satisfied through a registered installer or building control. Listed buildings and conservation areas have extra rules. Always check what applies in your country and for your building.

In short

Choose windows for what they do: view, light, air, sun and sound. Compare the whole window, position it and install it well, and add shade where needed.

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