There are several reasons why you might be considering upgrading your old single-pane windows to new double-glazed units, but for most people, energy efficiency (the ability of windows to keep the heat inside and the cold outside) is high on the list.
Double glazed windows are much more efficient at preventing warmth escaping than traditional windows, but not all units are created equal - some do the job better than others - so how can you tell them apart?
Luckily the insulating performance of a window can be measured and tested scientifically. In the UK and many other countries, the result will be expressed as a number known as a U Value.
The U value is a measurement of how much heat is transferred over a standard area at a standard temperature, humidity and air speed (usually 24 degrees C, 50% and 0 metres / second respectively). Because we want our windows to transfer (lose) as little heat as possible, the smaller the U Value quoted for our windows, the better.
There are many factors that can affect the U values of double glazed windows, including the material from which the frame is made, the quality and attention to detail in design and construction, whether air or argon gas is used in the gap between the panes of glass, whether the window uses energy-saving glass and many more.
When you are buying windows, don't forget to ask your supplier about the U Value of the units they're offering, and compare that with the U Values quoted by the other suppliers you're talking to.
You may find that the more expensive units tend to perform better - though this is not always the case. Suppliers often carry more than one type of window (eg with or without Argon Gas, or high-efficiency glass). When you're negotiating a price, make sure that as you haggle the price down, the spec of windows being offered isn't also being quietly changed.
Finally, the care with which the windows are installed can have a big impact on their efficiency in your home - sloppy installation can make even the highest spec windows much less efficient in practice - so always choose a reputable supplier.
Energy efficiency is one area where it often makes sense to spend a little more up front, in order to get long-term savings on your energy bills for many years to come. Now that you understand what U values are, and their importance when choosing new windows, you're in a great position to make an informed decision.
Click here to find out more about u values and read an independent guide to everything to do with u values and heat loss in double glazing.
Sarah Clark is a freelance author who writes for many popular home & garden publications.
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