What rated windows are best?

The rating formula used by the Thermal Rating Register was determined by following the methodology adopted by the European Window Energy Rating Scheme, EWERS, which was developed as part of the EU 'Save' initiative. This is an agreed standard for UK Building Regulation compliance.

The coefficients (weather conditions, solar gain/loss ) 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 objective of the Thermal Rating is to allow a relative comparison of the overall thermal efficiency of different window systems, comparing how the windows perform under standard conditions.

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.

Things to consider:

  1. The better the U-vale, 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. 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. 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 rates is the air loss through the window seals.

Now consider your home and how you heat it!