Double or triple glazing – a matter of choice

Paul Hicks of Velux, joins the debate on triple glazing……..

With an ever-increasing need to think about sustainability in design, the choice of window should be considered very carefully when calculating the performance and impact of a building.

Triple glazing is increasingly being presented as the best solution, but growing evidence shows it might not always be better than double. In fact, switching to triple glazing alone may not have enough of an impact to offer you return on investment or indeed offer the most environmentally friendly option.

U-values versus energy balance – what does it all mean?

Traditionally, the effectiveness of windows is measured by their U-value – the same measure of heat loss we use to measure walls, floors and roofs. A low U-value means good heat retention and triple glazed windows boast a much lower U-value than double. However, it’s important to consider that often the amount of energy used in manufacturing triple glazing outweighs the savings in energy when the window is in use. It leaves us asking if this really is a better solution.

A more effective approach is to look at energy balance – the difference between the amount of heat from sunlight the window transfers to your home and the amount of heat that escapes through the window. Energy balance supports the argument that low U-value is not the only factor to consider. In a climate like ours it’s important to also think about solar gain and insulation.

Climate payback

If we look at carbon emissions, it takes on average an extra 26kg of CO2/m² to manufacture triple glazing than double, this would result in a climate payback period of 20 years. As glazing units currently have a life expectancy of around 20 years, it seems less appropriate for renovation projects where the building fabric will generally have less energy efficiency than a new build.

Small changes, big impact

Using a heat-loss calculator, we are able to determine the energy lost through windows – and compare with the resulting extra CO2 emissions created – when using double glazing instead of triple. An average house with 17m² of double glazing installed creates an extra 68kg of carbon dioxide annually.

This CO2 can be recouped by simply turning down the thermostat on your central heating by less than 0.5 degrees, or by switching off appliances instead of leaving them on standby. Both make much more economic sense for you and your family too. It might seem obvious, but fitting the right the blinds and curtains and closing them at night can also help make a huge difference in cutting down on heat loss.

Do the sums

Triple glazing costs around £75 per m² more than double glazing – that’s around £1,275 extra for an average house. Taking into account its life expectancy of 20 years, triple glazed windows will cost an extra £63.75 per year.

Based on the average homes’ energy use, triple glazing can generally save you an extra £14.75 per year; meaning that you’re not recouping the financial benefits that you thought you would. Even if energy prices double every five years for the next 20 years, most homes won’t make their money back on the initial spend.

Other features

There are many claims that triple glazing provides better acoustic performance than double glazing, but this hasn’t yet been proven. Triple glazing does earn plaudits when it comes to surface temperature. The inner pane’s temperature is generally warmer than that in double glazing, a difference you’ll notice if you regularly sit near a window.

Conclusion

There is certainly a place for triple glazing in some low carbon designs, especially where a project is pushing the boundaries with regard to standards – or indeed in some cases where severe weather conditions dictate its use.

In Britain, we only experience extreme cold temperatures for three months of the year, at most. Therefore, should we really be investing in a product that only performs at its best for less than 25% of its life? And is it worth investing in a product that has a more severe impact on the environment in its production than a high performing double glazed unit?

The average family in Britain moves house every 10 years, so the true value of triple glazing will never be appreciated, as it is likely that people will not live in the house long enough to benefit form the investment.

Before you invest its worth thinking of all the options for both short and long term gain.

For more advice and information visit www.velux.co.uk or call 01592 778 225