Table 1: Carbon Index requirements for various domestic heating fuels for home extensions.
Heating Fuel / Carbon Index
Natural gas, LPG = 8.6
Oil = 7.4
Table 2: Limit factors for exposed structural (2003)
elements and for heating boilers
Element U-value (W/m2K)
Roofs = 0.13
Floors = 0.20
Windows, doors, rooflights (area-weighted average) 1.8
Heating Fuel efficiency (%)
Natural gas = 86
What the limits given in Table 2 mean in practice can be seen from the examples on the following page. These
give typical figures for three common types of dwelling:
• a flat
• a semi-detached house
• a detached house
In each case, the insulation levels needed to reach the Best Practice CI are given for different heating
As has been mentioned, a number of factors influence a building’s Carbon Index, some of them being site specific
and fixed (such as site layout and building orientation). Once these have been determined for a particular
building, it may be necessary to adjust the specification in order to achieve the appropriate CI.
However, some general conclusions can be drawn from the examples about the relationship between the insulation
levels given in the specification and the Carbon Index achieved:
1. For smaller properties such as flats, the Best Practice Carbon Index can usually be met with the insulation
levels given in Table 2 and without any further efficiency measures (except in the case of ground floor flats with
2. For semi-detached properties, additional insulation would be required in the case of electric or LPG heating
systems (and in addition a higher efficiency boiler would be required).
3. Additional insulation will be required irrespective of the choice of heating fuel for detached properties.
These increases need only be small in houses with natural gas or oil heating.
Electrically heated properties will need significantly more insulation. LPG heated houses will need further
efficiency measures in addition to high insulation levels and a very high efficiency boiler.
4. Additional insulation or other energy efficiency measures will be needed for dwellings where the ratio of the
exposed areas, such as walls and roofs, is high compared to the floor area (for example large bungalows).