Window Efficiency Ratings Explained

So, what makes one type of window better then others, and how can you save energy in your home this coming winter?

To answer that question, we first have to look at BFRC ratings. The higher the BFRC rating your window has, the greater your energy saving potential. In fact, you could make savings of up to £2,000 over 20 years (the general lifespan of windows), if you have the best possible windows fitted.

The BFRC created the Window Energy Rating (WER), which is similar to the energy efficiency ratings you see on your electronics. It uses the rainbow rating system, running from A++, right the way down to G. Modern building regulations require that all windows achieve a rating of at least a C.

The rating of windows is made up of 3 components, the Thermal Transmittance, Solar Factor and Effective Air Leakage.

Thermal Transmittance

Thermal Transmittance is a measure of how quickly heat will leave through the window when it is closed. So, this effectively measures how long it will take from when heat begins to travel through the window, to when it reaches the other side. The higher the rating, the slower heat will travel through the window. Double glazing systems will slow this process, when compared with their single glazed counterparts. However triple glazing is becoming more popular, and is more effective when compared with double glazing.

Solar Factor

Solar Factor measures the amount of heat that windows allow through from the sun. In the UK, this value tends to be high. This is due to the UK being cold for the majority of the year, meaning this value is kept high to ensure that properties remain as warm as possible.

Effective Air Leakage

Effective Air Leakage is fairly straightforward. It is a measurement of how much air can escape from the window unit, including the frame. Having a window with a low effective air leakage is key, as this helps to retain warm air in a property. All windows must have an Effective Air Leakage rating of 0.30 or under.

A Which! survey found that the following efficiency could be expected from different ratings:

“If you were to replace all of the single-glazed windows in a three-bedroom, semi-detached house, the Energy Saving Trust says that you’d save between £75 and £100 per year with B-rated glass and £85 to £110 per year with A-rated glass.”

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