With carbon emissions high on the global agenda and the current high cost of fuel, we are all looking for ways to make our homes more energy efficient. In addition, installing energy efficient windows is a legal requirement in order to comply with current Building Regulations.

Windows & Doors >

To incorporate thermal performance, the building regulations regarding the replacement of windows and doors in domestic dwellings were changed on the 1st April 2002 and then amended on the 1st October 2010.

Replacement windows means both the frame and glass is replaced at the same time. If you simply replace the glazing unit only, these building regulations do not apply, and therefore, no Building Regulation Compliance Certificate is needed.

As from the 1st April 2002, a special see-through coating must be applied to the glazing unit of all UPVC double glazed windows. This coating is called a “low e coating”, and is sometimes known by the brand name, such as “K GLASS”. This “low e coating” increases the energy efficiency of the window by helping to reduce warm air leaving and cold air entering through the glazing unit. Measuring the rate of heat loss through a material is called the U-value, and in 2002, glazing units needed to achieve a U-value of 1.8 or below.

On 1st October 2010, the minimum U-value allowed was reduced from 1.8 to 1.6. In addition, a new way of rating the energy efficiency of the whole window was introduced called Window Energy Rating (WER).

The Thermal Rating Register rates the energy efficiency of windows and is recognised as a method to show compliance for your installation.

It uses a consumer-friendly traffic-light style A-G ratings guide similar to that used on ‘white’ goods, such as fridges, freezers, washing machines etc.

This ratings label can be used by you to make more informed choices about the energy efficiency of the windows you are looking to purchase.

Each window rated by the Thermal Rating Register has a unique label, which displays the rating level, the energy rating, the window U-value, the effective heat loss due to air penetration, and the solar heat gain.

Simply put, this will help you to determine how well the window or door will perform in terms of:

  • Containing and conserving heat within your house in the winter.
  • Keeping out the wind.
  • Resisting condensation.
  • Contributing to improved sound insulation.

Window Energy Ratings explained >

  • A window’s rating is determined by a mathematical formula. The values used in the rating formula are based on the energy balance of a notional house through an average UK year, with notional window sizes and configurations.

  • The performance of a real window, in a real building, will differ from that of the notional window used for the rating calculation. In some cases the performance will be better and in others it will be worse.

  • The purpose of a Window Energy Rating is to allow one window’s energy performance to be directly compared to another under standard conditions. All new and replacement windows either have to be rated C or better or have a U-value of 1.6 or lower.

  • A window is made up of several separate thermal aspects (the frame material, the glass, the spacer bar) these aspects have differing thermal values. The thermal performance calculation of the window takes all these aspects and combines them to produce a whole window U-Value or WER.

What rated windows are
best for my property?

The use of a window system with a good Thermal Rating does not guarantee that a home will be easy to heat during winter, and may lead to issues with solar overheating in summer unless a proper ventilation strategy is adopted. Here are some things you should consider:

1. U-Values >

The better the U-value, the less heat is lost from inside of the property to the outside.

The lower the U-Value number the less heat energy is lost.

2. G-Values >

The higher the G value the more free solar heat gain is obtained from the sun. 

Some degree of gain will be achieved in both summer and winter; however in the summer months this can lead to the property ‘overheating’ requiring increased ventilation (opening of windows).

3. Air Infiltration >

The lower the air infiltration rate the less heat is lost through leaky windows.

When testing air infiltration rates any trickle vents in windows is excluded. The air infiltration rate is the air loss through the window seals.

Now consider your home and how you heat it!


  • If your home gets too hot in summer, and if you need to use air conditioning units, then, a lower G value may be better, ensure you have adequate opening lights and trickle ventilation.

  • If your home is particularly shaded, say from trees, other building etc. then a high G rating will have little real world benefit.

  • If your home needs excessive heating in winter then look for the best u-value, higher G value and lowest air infiltration rates. The best u-value is likely to have the greatest impact on saving fuel.