If you are thinking of buying new windows for your home, you may think that the only real difference between all those available is the price. A window is just a window, isn’t it?

There is actually a lot more to the modern window than meets the eye. Some of the latest windows are making use of energy efficient technologies which could end up bringing you significant savings in your fuel bills. So before you sign up for the cheapest deal, read our short guide to make sure you get the best lifetime deal.

Energy efficiency vs Style

There will be an energy efficient version of your desired window available – regardless of style. So don’t feel that you are going be to limited in choice if you opt for energy efficiency. Some styles of window are naturally more efficient than others however, and it is very logical when you think about it.

Why bother?

When fuel bills are constantly rising, and scientists have proven that up to a third of heat loss escapes through poorly insulated windows and doors, it makes sense to try to save money. By spending a bit more on energy efficient windows could end up saving you much more in the long run.

Energy flow through windows

To avoid this sounding like a science lesson, I’ll try to keep this nice and simple.

There are 3 main ways in which energy (ie heat or cold) can pass through a window.

1. Non-Solar (the transfer of heat/cold that is not related to the sun).

Non-solar energy transfer occurs when the outside temperature is significantly different to the internal temperature of the room. This is not due to a draught (that would be type 3 – airflow), but due to conduction, convection and radiation.

Scientists have developed a way of measuring the amount of energy that can be transferred through a window in this way, and this is known as the U-factor. The lower the U-factor, the more resistant the window is to heat flow – so the more energy efficient.

The U-factor can be found on the Energy efficiency rating sticker on new windows, and is referred to as "Thermal Transmittance".

Windows with a low U-factor tend to be double or triple paned, and filled with inert gas. These extra layers of insulation provide a buffer between the differing temperatures – helping to minimise heat loss and gain. The frames also play a big part – with metal frames (if not properly insulated) conducting heat or cold and often leading to condensation. Plastic, UPVC, Wood or thermally broken aluminium frames provide the best insulation.

2. Solar (due to the sun)

Solar energy is basically the process by which the sun will warm the room through the window, through radiation. In cooler climates, solar heat gain (free heat from the sun) can be very welcome, but in warmer climates this could be enough to have you reaching for the air conditioning. This ‘Solar Factor’ is measured in ‘G-Value’ – the lower the g-value the lower the amount of solar gain. Depending on the climate, and orientation of your window you may wish to take advantage of this free energy – or opt for a higher G-rating to keep cool.

Special glazing such as Low-E helps to prevent ultraviolet light, which can cause furniture to fade.

3. Air Flow

Casement windows, where a section of the window will open outwards on a hinge, are the most efficient style of window. This is because when the window is closed, a seal is formed around the opening section. Also, driving winds will only serve to improve this seal, leading to a very low draught rate.

Sash windows, or sliding windows require there to be an element of play between the panes to allow you to open and close them. This means that – as there is no ‘seal’ as such, these windows are more likely to let a draught in.

Air leakage is measured as the ‘L-Value’ and the lower the better in terms of energy efficiency.

A to G ratings?

In the UK windows are rated from A to G for energy efficiency, with A being the most efficient. The calculation is based upon the above criteria. It is always best to get the most efficient window you can afford, but if you are struggling to choose between similar windows - you may want to look at the small print of U-Values, G-Values and L-Values.