Double Glazing - Energy Efficient Glazing Glossary

Energy Efficient Glazing Glossary
We all know, nowadays, that we’re supposed to be environmentally friendly and energy efficient. Were you are, though, that heat loss through windows and doors in a typical home accounts for 30% of the energy used to heat the building in the first place? In other words, simply by installing highly efficient windows, you could start saving a big chunk of change on your energy bill.

Are you aware, though, of what you should be looking for, and what all the terms manufacturers use in their sales literature mean? To help you understand more, we’ve pit together a short glossary of the most important terms in Energy Efficient Glazing.

BFRC – The British Fenestration Ratings Council (BFRC) are the standards-setting body for energy efficiency in the double glazing industry. They test and rate windows, doors and conservatories, and produce guidelines for homeowners and manufacturers. You may have seen a BFRC label (pictured on the right)

Energy Index Value – The energy index value is a simplified general measure of the energy performance of a given window, including its frame and fitting. The value is calculated by testing a standard sized window (1230mm x 1480mm) for variety of factors, including the U-value, and the amount of air which can leak through the frame. The Energy Index Value is given in kWh/m2/year, and for good, modern windows typically falls between -30 and 0

Energy Rating – The most eye-catching item on the BFRC label is the alphabetical rating given to the window casement as a whole, based on the Energy Index Value. These go from A (Outstanding — Energy Index Value >= 0) to G (Very Poor — Energy Index Value <-70). The BFRC say that replacing standard windows with windows which have an energy rating of C or higher can lead to 90% reductions in energy loss.

Low-E Glass – Low emissivity glass, or ‘Low-E’ as it is often called, is the latest in energy efficient glass manufacturing. The panes have an invisible coating applied to them which allows energy from the sun to pass through the window in one direction, but prevents heat from escaping the house in the other direction.

U-Value/Thermal Transmittance – The U-Value represents the amount heat which can escape your house through your window. Using U-Values to compare windows has been less common in recent times in favour of using more comprehensive measures such as the Energy Index Value, but some salesmen will still mention U-Values. Because U-Values can be measured for the glass pane on its own as well as for the entire window unit including its frame, it’s very important to ensure you clarify what measure people are using when they mention U-Values.

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