How do we decide on the most appropriate energy-related measures?

With up-to-date zero-energy-building technologies a comprehensive renovation of an existing building can achieve energy consumption ratings between 15 and 35 kWh/m²a. The heating energy requirement can be reduced by 75 to 95 % if zero-energy-building components (or pas-sive house components, respectively) are used.

What are zero-energy-building (or passive house) components?

The essential elements of passive house technology consist of excellent heat protection, very good airtightness and a highly efficient heat recovery from waste air.

This video shows what thermal insulation can do for older buildings:
http://youtu.be/kfly8kY_Yqw

A simple rule of thumb is: difference between U values ("unrenovated" minus "renovated") x 8.4 = annual savings in litres of oil or cubic metres of natural gas per square metre of the external building component in question.