The researchers studied the greenhouse-gas emissions produced during the construction of residential "energy-efficient" neighbourhoods and those produced when the buildings were lived in afterwards (the "use" phase). They compared these figures to the emission cost of renovating existing "conventional" buildings to make them as energy efficient as the new builds. They also analysed the emissions producing during the use phase of these homes.

The study, which is reported in Environmental Research Letters, suggests that new residential constructions could be more costly in terms of greenhouse-gas emissions overall. In fact the carbon payback time for the renovated buildings appears to be significantly shorter than that of new constructions, thanks to a much lower carbon spike associated with the beginning of a building's life cycle (see figure).

"We conclude that while increasing the energy efficiency of a residential building is important in the long term, constructing new buildings should not be used as a way to achieve short- to medium-term climate-change mitigation goals – as is often put forward by policymakers," said Säynäjoki. "What is more, since constructing a new building produces a large amount of greenhouse gases in a very short space of time, future mitigation strategies should be setting reduction targets for this phase as well, and not just for the use phase – as is currently the case."

Finally, the emissions produced when constructing energy-efficient buildings appear to be greater than those produced when putting up conventional buildings, so this needs to be taken into account too when assessing a building's energy efficiency over its entire lifecycle.