How is heat loss measured?
It is determined from the U-value (thermal transmittance), which is a measure of the rate at which heat is lost through a product or component (see table overleaf). A lower U-value indicates a more energy efficient product. U-value is measured in W/m2K (watts per square metre, per degree Kelvin). Although each component of the building (i.e. roof, wall, windows) needs to achieve a minimum U-value as stated in the appropriate Building Regulations, by carefully choosing to have certain components achieving better U-values you can improve the energy performance of the whole building.
New Build
- To determine the U-value required for windows and doors, the Whole Building CO2 Target Emission Rate (TER) must be determined using notional values. The Dwelling Emission Rate (DER) is then determined using actual values and must not exceed the TER. Only with these figures can the U-value for windows and doors be reached. The Building Regulations limit the maximum acceptable whole U-value to 2.2 W/m2K.
Replacement Windows & Doors
- Replacement windows should be limited to a maximum whole window U-value of 2.0 W/m2K or use glass with a centre pane U-value of 1.2 W/m2K or better. Alternatively, it may have a Window Energy Rating (WER) performance of E or better. (See point 2 below).
- Replacement doors will only need to meet the recommendation where they have more than 50% glass area (measured overall of frame) which must then be proved by hot-box testing, calculation or by using glass with a centre pane U-value of 1.2 W/m2K.
- When replacement windows and doors are used consideration should also be given to compliance with other parts of the Building Regulations notably parts B, E, F, J and particularly part N. REF:[www.planningportal.gov.uk/approveddocuments]
Extensions
- Windows in extensions must achieve a whole window U-value of 1.8 W/m2K or better or use glass with a centre pane U-value of 1.2 W/m2K or less. Alternatively for the first time, windows must have a WER performance of D or better
© University of Strathclyde 2009
