Building Regulations

Part L - Conservation of energy fuel and power

Windows need to provide minimal heat loss ensuring lower energy use thereby reducing CO2 emissions. A u-value is a measurement of heat loss and it is this, along with the way windows are fitted, that will ensure compliance. Alternatively, windows are now also given energy ratings from A-G in a similar way to your fridge or freezer, where A rated products give better thermal performance than those rated G. Building Regulations now require replacement windows to be C or better.

U-values and WER’s 

U-values are the standard unit of measurement used in Building Control legislation to specify the thermal insulation requirements of various elements in a building. They indicate the rate of heat loss from inside to outside.

Current building control legislation for windows requires a u-value of 1.4 W/m2K for new build residential properties and 1.6 W/m2K for replacement windows in existing properties. To avoid confusion, these figures relate to the overall system Window Frame and Glass.

As a Rule of thumb a sealed unit which provides a 1.2W/m2K centre pane u-value with the benefit of Low Iron on the external pane (for thermal gain), a Warmedge spacer (to act as a thermal break) and argon gas will generally achieve a 1.4W/m2k as an overall system figure when using a Timber Window with a 70mm Frame. This is in accordance with the Thermal Rating Register. Most joineries these days has access to a Timber calculator which will take into account the frame and glass.

To find out what you require in UPVC or Aluminium frames you need to check with your ‘system’ company who will check on their calculator which will tell you what to use to conform to Building Regulations or to achieve a particular Window Energy Rating, or for particular projects which require higher performance windows and glass to conform to the overall Building Performance.