Direct energy assessors can cit the Royal Institute of Chartered Surveyors (RICS) when they inform clients that insulation is the most cost effective way of reducing the carbon footprint and energy consumption of a household.

The RICS has published a guide which compares the set up costs and payback time of various energy efficiency generation technologies.

Its conclusion is that cavity wall insulation is the best measure as the cost of installing it will be recouped in three years, after which it will create up to £145 in energy bill savings a year.

Joe Martin executive director of the Building Cost Information Service which produced the guide said: "We all have a role to play in helping to reduce our carbon footprint, be it through changes to our behaviour or by choosing greener alternatives."

However, the guide's suggestion that it takes 100 years to pay back on photovoltaics have been slammed.

Photovoltaic company Solarcentury executive chairman said: "RICS's conclusions are a mixture of historic pricing information and the very crudest 'payback' calculations which bear no resemblance to reality."