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Double glazing is common now, standard in new buildings, but this was not the case in the past. Before the current obsession with reducing energy bills due to pressure for sustainable buildings, it was seen as a extra - famously, the Everest advert showed a helicopter landing outside a double glazed window... with no noise.


Double glazed windows have an extra sheet of glass, with a gap which creates an insulating barrier to stop heat moving across by conduction. The gap is sometimes filled with an inert gas or vacuum sealed (no air, less conduction). Vacuum gaps are better for sound insulation as well as heat insulation.

Triple glazed windows have three panes of glass. Often the third pane is set back more with a much larger gap to provide better sound insulation, due to standing waves, a larger gap is more effective at cancelling sound. 4 inches is supposed to be best, so this pane has to be set back and can look quite ugly.


The most obvious visual aspect is the frame, not the window. You should buy high quality wooden or metal (usually steel or aluminium) frames matched to your house and also the local streetscene. Cheap uPVC might save money but will rot, discolour, and reduce the value of your home on resale.

Planning permission might even be required, if you live in a conservation zone.

Insert window or a full frame window?

If just the window need replacing, ie, the frame casing and surround is in good condition, the glass can be replaced. The double glazed unit is made in a factory to exact dimensions and fitted into the existing frame system.

Full frame replacement windows are needed when the window frame is in poor condition, or the sash system is broken and not easily fixed, the casing containing the sash is damaged, or the actual window opening is not aligned correctly due to building movement or damage.

There are also aesthetic reasons to replace the whole window, as windows can be bought easily as ready made units with frames, opening system, security and double glazing all built it.. This is the preferred solution for new build or in an extension or attic / loft conversion.


You get what you pay for so beware of cheap deals sold door to door.

Also be aware you might book a fitting, and then have delays once your deposit is banked. So try and find a good company by recommendation.


Ecotist Homes Building DIY Book cover

Above: double glazed glass and windows from our ebook

In our own eco new build houses, we used double glazed windows with low emissivity glass, sustainable softwood polished frames on inside, and aluminium clad frames on outside.

HOW IS IT RATED? BFRC Energy Efficient Window Ratings Label

It is now rated the same as white goods such as refrigerators and boilers, with a simple Energy Efficient Window Ratings Label with an A-G guide letter. This is organised by the BFRC ratings council. A is the best!

Window Energy Efficiency chart BFRC

Above: the smaller framing at the top of the window is glued to the front of the sealed double glazing unit to give a traditional appearance.


A double glazing company will usually come in and measure up, which means take all the measurements needed for each individual window. They will then get the double glazing panes made exactly, and come back and fit them into your existing, or new, window frames. Making the double glazed panes takes a couple of weeks.

DIY kits are available to add a second layer of glass to an existing window, such that they can be opened and closed independently of the main window, which is left untouched. This is not a great solution but is effective at saving energy and stopping sound, and is also very cheap.

Traditional window and frame decoration (curved mouldings etc) are added to the outside of the rectangular double glazing unit, to giove a traditional appearance.

Traditional window double glazing

Above: the smaller framing at the top of the window is glued to the front of the sealed double glazing unit to give a traditional appearance.


This varies according to the quality of the fitting company. The panes are made by specialist companies and are a set costs which also varies on size, shape, quality, special materials etc.

Get a couple of quotes. Some large companies offer special deals if you sign up immediatly (this selling is often done door to door). But make sure the delivery dates are fixed, and that there are guarantees. Smaller bespoke companies are better for higher quality jobs.

Specialist companies are better than all-rounders, who might also fit conservatories, etc.

A typical price might be 700 UKP per window, if sash. This varies a lot though, with size, frame type, waiting list, etc. Don't expect a cheap job to be any good in the long term.


Many houses with double glazing have uPVC frames. These are ugly and get damaged by the weather, the sun mainly, and also sprout mould and mildew, and warp.

Frames should be replaced with timber, aluminium or metal (often steel) all of which are sustainable materials, if sourced correctly.

uPVC is not allowed in conservation zones due to its inherent ugliness, and nasty chemicals. It is also short-lived and not really the bargain it appears. It will have a negative effect on your house value too.

For more technical details on ratings etc, see also our DIY Building Glazing page >

And our DIY Building advice on types of window frames >

See BFRC Energy Ratings >