Energy performance - have your windows got the U-factor?

With an increasing focus on the environment, sustainability and reducing our use of non-renewables, energy performance is a subject close to our heart here at Cheltenham Glass and Glazing.

The British Fenestration Rating Council (BFRC) suggests that the average home in the UK loses around 25% of its heat through its windows.

Windows have traditionally been seen as the weakest link in terms of energy performance in building products. Not as easy to insulate as lofts or cavity walls, advances in technology over recent years have led to significant improvements in the insulative properties of window products.

The installation of windows with a better energy performancerating, coupled with other energy efficiency measures could reduce heat that25% heat loss by up to 75%.

So what is a U-factor?

The U-factor or U-value of a product shows the thermal transmittance – the amount of heat energy lost through a door or window. It is measured in watts per meter squared per degree Kelvin.

The lower the U-factor, the less energy is lost therefore the door or window unit is more thermally efficient. U-factor ratings for windows should be as close to zero as possible. Current UK Building Regulations stipulate a minimum U-factor of 1.6 w/m2K.

Other measures of energy performance include G-value, which measures the amount of solar energy that is gained through the window - the higher the value the more energy gained and the greater the saving, and L-value (effective air leakage), which measures the amount of air that can pass through a window’s weather seals.

The Rating Value

The most widely recognised measure of energy performance is the Rating Value which has been developed to grade the overall energy performance of a window or door unit.

Ranging from A++ to G-rating, the rating value combines the U-value with the G-value and L-value of the same product. The result is an overall rating that allows consumers to clearly see the energy performance of the products they are installing in their home.

It also allows customers to compare the energy performance of products before they buy. At Cheltenham Glass and Glazing we supply and fit products across Gloucestershire that follow the BFRC rating system, which works in the same way as systems from CERTASS and the British Standards Institute

Energy Efficient Glazing

Traditional glazed windows consisted of one pane of glass in a frame. Energy Efficient Glazing (EEG) refers to two or more panes of glass within a sealed unit/frame. The reason that double or even triple glazed units are more energy efficient is that more air or gas can be trapped between the panes.

Glazed units that have a low U-factor (least energy loss), high G-factor (good solar energy gain) and low L-factor (low air loss) will achieve a better energy efficiency rating as they are the most energy efficient.

It’s all about the Argon

Argon gas, an inert, odour and colourless gas with low thermal conductivity properties is used as the industry standard for filling double glazing. An Argon-filled window unit will reduce heat loss by around 30% compared to a window filled with just air.

‘A’ rating as standard

At Cheltenham Glass and Glazing, we supply standard with ‘A’ rated double glazed units as standard. We then offer upgrade options and the choice of higher energy efficiency and specialist glazing coatings.

A++ rated glazing is the most energy efficient available. With a higher energy performance rating, they are typically made with specially coated glass and are available at extra cost – just ask us for more information.

With a higher initial cost, the effect of this is that more warmth is retained in your home which consequently means that your heating system shouldn’t have to work as hard or for as long, meaning lower energy bills. This is often a key consideration for our customers in Cheltenham and the wider Gloucestershire area.

Contact us

For further information or if you have any questions about the energy performance of your windows, give our friendly team a call on 01242 238887 or contact us.


Providing fast, friendly advice and fitting, based on over 10 years experience in the glass fitting industry, our staff take pride in delivering the best possible service to all our customers. So, if you are looking for the best aluminium and UPVC windows & doors Cheltenham has to offer, contact Cheltenham Glass and Glazing today.

01242 238 887

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1 Tremblant Close, Cheltenham, GL52 5FL