New Housing and home extensions

The Best Practice specification for new housing includes Carbon Index (CI) targets. The Index is an indicator of the carbon dioxide emissions associated with space and water heating and is expressed on a scale of 1-10; the higher the number, the better the performance. The CI for a building is obtained from the Government’s Standard Assessment Procedure (SAP) which is required for all new dwellings in the UK.

The Carbon Index is determined by a number of factors, including:

• insulation levels

• heating system

• ventilation

• orientation and shading

• the exposed surface area relative to floor area

The greater the exposed surface area compared with floor dimensions, the more insulation will be needed to achieve the same CI. For example, a semi-detached dwelling will require more insulation than a mid-terrace property. The Index can be used to compare performance between dwellings as it is expressed in terms of emissions per square metre of floor area.

Changes to one factor will require alterations to others if the same CI is to be achieved.This gives a certain flexibility to the design process, while the inclusion of maximum acceptable U-values for walls, floors, roofs and glazed areas in the specification ensures that good insulation levels are always achieved.Where wet heating systems are used, minimum boiler efficiencies also have to be achieved. It should be noted that there are limit values: to reach a particular CI value in different dwellings, greater efficiencies and lower energy loss through external surfaces may be required.