What Makes a Window Energy Efficient?

Double glazing Colwyn Bay

So, you’ve paid for your heating - now it’s the job of your double glazed windows to keep the heat in your home. The dramatic improvements made in energy-saving technology have been good for the environment as well as 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 homeowners around 20% in energy use. Thermally efficient windows are usually at least double glazed, featuring 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 result in a saving of up to £450 on household energy bills every year.

WER Rating

The British Fenestration Rating Council (BFRC) created the WER rating in order to simplify how effective a window is. The rating system offers a grading system, 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 then anything above an A10 the becomes an A+. Back in October 2015, the BFRC introduced the A++ rating for windows which achieve higher than an A+20 rating - the highest rating that is currently available in the UK is A+32.

The WER rating is made up of three main elements; U value (low heat loss), solar gain and air loss through ventilation.

Thermal Efficiency (U value)

A window’s U value is the technical way in which heat loss is measured through a type of building material, such as a brick wall or a tiled roof. The lower the U value, the better the insulation provided by the material.

Single glazed windows have a standard U value of 4.8-5.6W/m2K, which means that approximately 5 watts of heat are lost per hour, for every square metre of window.

Modern double glazed windows can achieve a U value that is a 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!

Solar Gain (G value)

Solar gain is another factor that has an influence on a window’s energy rating. Windows are now designed with energy efficiency in mind; so they are designed to stop heat escaping and to also let in heat captured by the sun’s rays. The idea behind this is to capitalise on solar radiation as a natural or ‘passive’ form of heating, therefore reducing the dependence on carbon energy to heat your home in the winter months.

Factors that impact the solar gain are the number of panes, the type of gas between the panes and also the specific type of coatings added to the windows, as these dictate whether the glass absorbs or reflects heat. This is measured by the G value, on a scale between 0 and 1, with the higher the number indicating a high solar gain.

Contrary to popular belief, solar gain windows don’t lead to overheating very often - the UK has a relatively cool climate and relatively little sunshine, so overheating is rarely a concern.

Air Leakage (L value)

Air leakage occurs when there is a weak point around the window frame, this is commonly found at 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 shouldn’t be confused with ventilation. Ventilation is a controlled system, letting in small amounts of fresh air to reduce stuffiness and to improve the air quality; whereas air leakage will compromise the energy efficiency of your window.

Get in touch with the team here at AA Conservatories today to discuss how we can help you upgrade to energy efficient windows.

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