Double glazing

Our guide to double and secondary-glazing options.

12apr double glazing hero

Better window insulation can make your home comfier all year round and there are a variety of ways to achieve this. These options vary in cost and effectiveness – and the most expensive may not be best for you.

First steps

Touch a windowpane on a cold day and it’ll feel cold. It’s likely to be as cold as the outside air. When the warm (inside) air hits the windowpane, it cools and the heat escapes outside.

There are 2 ways of dealing with this: using curtains to keep the warm inside air away from the windows or improving the heat loss from the windows through some form of double-glazing.

New homes or major renovations

The building code now lays down minimum requirements for new windows – that's almost certain to mean double-glazing.

Existing homes

Make sure you’ve blocked draughts – and that you’ve insulated your ceilings, under the floor, and maybe the walls too. Once you’ve done this, the windows will be the cause of most heat-loss in winter and gain in summer. How much is lost (or gained) depends on where you live, the type of windows you have, how big they are, and the direction they face.

Fitting double-glazed or secondary-glazed windows (see 'Glazing options') is likely to improve winter and summer comfort and also reduce noise. It may also add to the house’s resale value. But it’ll take many years to pay for itself through reduced heating (or cooling) bills.

New double-glazed windows are effective. But research has shown that secondary-glazing of existing windows can be just as effective and cheaper – sometimes a lot cheaper.

But before you look at glazing options, you should get your curtains working as well as possible.

A good set of curtains can be as effective as double-glazing at reducing heat loss from windows. But if the curtain is open it's not doing anything – a layer of air must be trapped between the window and curtain.

What’s more, curtains with a gap at the top and the bottom can form a "reverse chimney". That’s when the cooled air from near the window falls out the bottom of the curtains and is replaced at the top by warm air from inside. This could be worse than having no curtain.

Although they’re no longer fashionable, pelmets help seal curtain tops. Sealing at the bottom can be improved by curtains that fall on to the window sills or touch the floor.

“Thermal curtains” have a plasticised coating on the reverse of the cloth. The coating helps stop air passing through the cloth – but that’s not the major cause of heat loss with curtains. The fitting of the curtains matters much more than the cloth. For more information see our Curtains report.

Glazing options

12apr doubleglazing secondary glazing
Above: secondary glazing

Secondary-glazing creates an air-gap by inserting a second pane of glass or other clear sheet behind an existing windowpane.

12apr double glazing doubleglazing
Above: double glazing

Double-glazing has 2 panes of glass (or a pane of glass and another clear sheet) spaced apart by a gap containing air or argon gas, (which is a better insulator than air).

This is then sealed to form an insulated glazing unit (IGU).

IGUs can have different insulation ratings. These depend on:

  • whether standard or low-emissivity (low-e) glass is used
  • the distance between the 2 panes
  • the type of gas (air or argon) in the gap.

The edge-seal of an IGU must not be allowed to remain wet. That means these units must be installed in a frame that allows free drainage of water.

The unit can be fitted either directly into new window frames or retro-fitted into an existing window frame once the original glass is removed.

“Triple” glazing
This is used mostly in Northern-Europe and North America, where winter temperatures get much colder than here. “Triple” glazing uses 3 sheets of glass to create 2 air-gaps. This gives even greater thermal performance – but it needs an extra-wide frame and creates a heavy window.

Secondary glazing

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Double glazing

Costs and payback


We’ve used R-values in our table to show the extent to which different materials or building structures transfer heat.

Insulating materials that have higher R-values reduce the rate of a building’s heat loss in winter (or heat gain in summer). This in turn reduces the amount of heating (or cooling) required for a comfortable indoor temperature.

The R-value depends on the type of material and its density and thickness. So an aluminium single-glazed window has an R-value of 0.15 while an insulated wall’s R-value is 1.99 – more than 10 times greater.

Well-fitting heavy or thermal drapes will add an R-value of 0.26 to a window.

Wooden framed wall
No windows Wooden framed wall (insulated, weatherboard-clad, gib lined) 1.99
Wooden or PVC window frame
Single Original single glazing (clear glass) 0.19
Secondary Secondary glazing using thin plastic film 0.35
Secondary Secondary glazing using magnetically attached acrylic sheet 0.36
Secondary Framed secondary glazing added to inside 0.34
Secondary Framed secondary glazing added to inside (low-e glass) 0.57
Double Clear glass (air-filled IGU) 0.36
Double Low-e glass (argon-filled IGU) 0.53
Standard aluminium window frame
Single Original single glazing (clear glass) 0.15
Double Clear glass (air-filled IGU) 0.26
Double Low-e glass (argon-filled IGU) 0.33
Aluminium-thermal window frame
Double Low-e glass (argon-filled IGU) 0.43

Our advice

Before you get double-glazing installed:

  • Insulate your ceilings and under the floor (plus walls if practical) before considering double-glazing.
  • Use curtains – and make sure they have good seals at the top and bottom.
  • Try temporary secondary-glazing first. It may be all you need.
  • If you want to install permanent secondary-glazing or new double-glazing, look at options such as low-e glass and argon gas between the layers.
  • You want to replace the window frames? Thermal-aluminium, wooden or PVC joinery is more efficient than standard aluminium frames.
  • Make sure you get several quotes for whichever double-glazing system you’re considering.

Building Act changes
Where installing double-glazing alters the weather-tight envelope of a house it may be classified as "restricted building work" and need to be done by a licensed building practitioner.

DIYers can now do restricted building work unsupervised but they must file a statutory declaration with their local council. The declaration will be kept on council property records.

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