Confused about window energy rating

Are you confused by terms such as ‘A’ rated and energy efficient windows? Well that needn’t be the case any longer because we intend to simplify things and clear up the confusion.

Windows are rated using a traffic-light system to illustrate the level of efficiency, rather like that used for ‘white-goods’ such as fridges and washing machines. Rating levels range from the most efficient ‘A’ rating through to ‘G’ rating.

Why are windows rated?????

The rating tells us about the product as an insulator, so ‘A’ rated windows are better insulators than ‘B’ rated windows, ‘B’ rated windows are better insulators than ‘C’ rated windows etc. A higher rated window will also help with the reduction of condensation, for the simple reason, the better a window is at insulating, the warmer it will keep the inside pane, thus reducing the area that would draw moisture from the air and materialises as condensation.

The energy rating is simply an indicator as to how energy efficient a window is. It is not a comparison of how good a window is!

Typical 'A' Rated Double-Glazed uPVC Window

Double-Glazed 'A' Rated uPVC window

The higher ‘A’ rating, where uPVC windows are concerned, is usually achieved through the specification of the glass used in the frame. For example, the exact same window we install achieves a different rating depending on the glass used. Ratings do not make one uPVC window frame better than another, nor do they have any affect on the security of a window, nor do they make any difference to the installation or after care service.

All windows with a rating of A - C are considered good insulators and are recommended by the Energy Saving Trust. To achieve an ‘A’ rating on a window it has to also gain from solar heat, ie. have very low heat loss but a high heat gain when the sun is shining.

To achieve ‘A’ rating ---- Heat gain must be greater than heat loss

The confusing part of it all is that in order to calculate solar heat gain for a replacement window average figures have to be used and are totally dependent on sunshine... A window fitted onto a south elevation has a potential to be classed as A rating however a window fitted on the North side would never fulfil the criteria for A rating.

Sooooooooo, if all your windows are on the south facing side of your house and there are no trees, bushes or houses obstructing them then choose an A rated window. If as most of us your windows are on all sides of the house you have to question is the additional cost of A rating really worth it?????


As you would expect an A rated window will cost more due to the increased specification of the glass but not a massive amount more. Glass is purchased by the m2 so the difference in cost would be relevant to the overall size of the window. It has to be considered that the benefits of solar heat gain are mainly achieved on south facing aspects, windows that do not receive much direct sun will not benefit very little from low iron glass.

To achieve a window rating of C or above a ‘low emissivity’ coating (Low ‘E’) is added to one of the panes of glass in a double glazed unit. This coating reflects heat back into a room. Low ‘E’ is commonly measured using ‘u’ values. To lower the ‘u’ value further an inert gas can be used to fill the cavity of a double glazed unit making it better at insulating.

To achieve an A rating usually a “low iron” glass is used in the manufacture of a sealed unit. The glass is manufactured with reduced iron content giving 2 main benefits, the first is a higher light transmittance than ordinary float glass giving slightly better clarity and secondly a higher solar transmittance allowing more of the suns heat to pentrate through the glass.

How will it make my home more energy efficient?

Simply using a C-Rated or above window reduces the amount of energy lost through windows by up to 90%. That’s enough energy to:

  • Run 9 televisions for a year
  • Drive over 600 miles in an average sized car
  • Make 34,000 cups of tea
Expert window advice, when you want it

Find out more about uPVC and energy efficient windows at a time that’s convenient for you!

To find out more contact us, or call on 0161 339 4982 for a no-obligation quote.