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How to make your windows more energy efficient

Posted by greentomatoenergy . on 10 December 2013 at 12:59 pm

Windows and doors typically account for 20% of all heat loss in a home according to National Insulation Association figures. A leaky single glazed window also has a big impact on comfort thanks both to draughts and the cool temperature adjacent to the window. This is frequently borne out in our energy surveys most of which make some recommendation to improve windows

The means of heat loss from windows are more complex than one might assume. But there are two characteristics which largely determine how well or poorly a window conserves heat; the U-values of the glazing and timber and the degree of air tightness of the window. The latter can be particularly decisive with period sliding sash windows. Research undertaken by English Heritage found that 60% of overall heat loss could be attributed to air leakage.

So in plain terms there are two objectives:

  • improve the insulation values of the window, and

  • reduce any uncontrolled air movement.

But what is the best way of meeting these goals? While there are a good many options available which vary significantly in effectiveness and cost, it boils down to whether you refurbish or replace.

Air tightness

Draught proofing can make an immediate difference and won’t cost the earth. You could try doing it yourself, but unless you are an undercover joiner I’d leave this to the pros. There is a world of difference between a well-installed draught proofing system routed into the wood and some sticky-backed rubber strip.

It can be staggering to see how timber windows that are seemingly beyond repair can be refurbished to look like new. This will also help reduce air leakage and is often done at the same time as draught proofing. Regular maintenance of timber windows is essential to help them realise their full life time. A well-maintained timber window should last longer than most people live. Then it’s someone else’s problem. In contrast a uPVC window can need replacing after twenty years.

Improve the insulation values

An increasingly popular solution for single-glazed sash windows is to keep the box and sashes and install new double glazing. In some cases this is feasible but we would urge caution as many of these windows date from the early 20th century or before and simply weren’t designed to cope with the additional weight of the double glazing.

Secondary glazing will improve the U-value* of the window, although not to the same extent as modern double glazing. If noise is a problem, secondary glazing with a large air gap is the better solution. Also in its favour is that it is a lot cheaper than installing new windows and in some conservation areas and in listed buildings, it’s the only option. Ventilation must also be considered.

Upgrading the glazing to either modern double or triple glazing is the best way to significantly improve the U-values of a window. The typical U-values for a single, double and triple glazed window are 5.0W/m2, 1.7W/m2 and 0.8W/m2 respectively. It will also result in improved comfort.

This is exemplified by a table often seen in Passivhaus presentations. It depicts the internal surface temperature of a window with an external temperature of -5°C and an internal temperature of 21°C. Under this scenario the temperature next to a single glazed window is 1°C, with an older double-glazed window it rises to 11°C, a more modern double-glazed window results in 16°C and if you stand next to a triple-glazed window the temperature is a balmy 18°C.   

Crudely speaking, air tightness can be significantly improved without having to replace a window. However, if you want to make in-roads into the heat loss through the glazing you should consider installing new windows. Incremental improvements can be achieved with the use of curtains, shutters and blinds.

However, there is little point in installing Passivhaus standard triple-glazed windows if other elements of the building have a much poorer thermal standard. It can also seem wasteful to replace windows that are in a good shape. The most natural time to upgrade your windows is when they need replacing anyway, although that may be too long to be cold.

Deciding what to do with your windows is not clear cut, there will be a number of things you’ll need to factor in. Just make sure you know what all the options are. And make sure you choose a supplier you can trust otherwise missed delivery times will only be the start of your worries.

by Inigo Harrison

*U-values are measured in W/m2 - this is the rate of energy loss in watts per square meter.

More information on YouGen

YouGen's energy saving pages - windows and doors

Windows: single, double or triple glazing

How can we stop heat loss from our bay windows?

How does a passivhaus handle the summer heat?


Photo Credit: Ctd 2005 via Flikr

About the author: greentomatoenergy specialises in cost-effective renewable technologies and low carbon building.

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