What makes a window energy efficient?
Energy Efficient Windows
You’ve paid for your heating – it’s the job of your double glazing to keep the heat in your home. Improvements in energy saving technology is arguably one of the most significant developments over the past few years, not just for the environment but for home comfort and money savings, too.
Up to a third of a home’s heat is lost through single glazing, and on average upgrading to energy efficient windows can save around 20% in energy use. Thermally efficient windows are at least double glazed, with low emissivity glass, and an inert and non-toxic insulating gas like Argon between the panes.
Combined with energy efficient frames, new double glazed windows can achieve up to an A+12 Window Energy Rating (WER), and triple glazing can achieve up to A+32. Upgrading your windows could save you up to £450 on household bills every year.
The British Fenestration Rating Council (BFRC) created the WER rating (Window Energy Rating) in order to simplify how effective a window is. The rating system offers grades from A-G (with A being the best). If the window is rated higher than an A it will be followed by a number, and anything above an A10 becomes A+. In October 2015, the BFRC introduced the A++ rating for windows that achieve higher than A+20. The highest rating currently available within the UK is A+32.
The WER rating is made up of three key elements: U value (low heat loss), solar gain and air loss through ventilation.
1. Thermal efficiency (U value)
A window’s U value is the technical way to measure heat loss through a type of building material, such as a brick wall or tiled roof. U values are calculated by the equation W/m2k, which measures heat loss in watts (W) per square metre of material, when the temperature (k) outside is at least one degree lower.
The lower the U value, the better the insulation provided by the material. Single glazed windows have a typical U value of 4.8-5.6W/m²K, which means that around 5 watts of heat are lost per hour, for every square metre of window. Modern double glazing can achieve a U value as low as 1.4W/m²K, and triple glazing is even more energy efficient, reaching 0.6W/m²K. This is lower than the U value of an external wall!
2. Solar gain (G value)
Solar gain is another factor that can boost a window’s energy rating. Not only are windows now designed to stop heat escaping, but they can also let heat in by capturing the sun’s rays. The idea is to capitalise on solar radiation as a natural or ‘passive’ form of heating, reducing the dependence on carbon energy to heat your home in the winter months.
The factors that influence the solar factor are the number of panes, the type of gas between the panes, and also the type of coatings added to the windows, as they dictate whether the glass absorbs or reflects the heat. It is measured by the G value, on a scale between 0 and 1, with a high number indicating high solar gain.
Contrary to what you might think, solar gain windows do not always lead to overheating – the UK has a relatively cool climate and relatively little sunshine, so overheating is rarely a problem! For rooms where sun streams in during the afternoon, it could be worth considering upgrading to triple glazing – the level of solar gain is actually less than double glazing, due to the extra pane of glass and cavity.
3. Air leakage (L value)
Air leakage occurs when there is a weak point around the window frame, such as the seals. Most modern windows are fully airtight, and should have an air leakage factor or L-value of zero (0.00W/m²K).
Air leakage is not to be mistaken with ventilation. Ventilation is a controlled system, letting in small amounts of fresh air to reduce stuffiness and improve air quality, whereas air leakage will compromise the energy efficiency of your window.
Sound insulating windows
Upgrading to double or even triple glazed windows also has the benefits of improving sound insulation in your home – this means a quieter and more relaxing living space, free from the noises of traffic from roads and flight paths.
The same factors that increase a window’s energy efficiency also work for noise cancellation: more panes of glass, Argon gas to fill the cavities and airtight seals work together to give you more peace and quiet! You can even get ‘silent sealed units’ with double-thickness glass that can cut noise down by up to 36 decibels.
Can’t install double glazing in your property? If you’re restricted by conservation planning rules, secondary glazing can also help to reduce the noise from outside filtering into your home.
Check out our video on the science behind thermal technology
'What makes a window energy efficient?'