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Conservatories and house extensions could be helping to wipe out the reductions in gas use secured by insulating homes, according to a study that found insulation only provides a short-term fall in energy consumption.

In a surprise finding, the study into the long-term effect of loft and cavity wall insulation in England and Wales showed that the fall in gas consumption for each household was small, with all energy savings disappearing by the fourth year after it had been fitted.

Policy experts at the University of Cambridge said the findings suggested a “rebound effect” in energy use, where changing behaviour cancelled out the reductions in gas use. They also suggested that fitting insulation often happened alongside the building of house extensions, which use extra energy. For households with conservatories, any gains in energy efficiency disappeared after the first year.

The government and opposition parties have championed the retrofitting of homes with insulation as a way of dealing with the energy crisis. Ministers have announced insulation retrofits as a leading part of a programme to reduce the energy consumption of buildings and industry by 15% over the next eight years. Labour has said insulating homes should be a “national mission” that could save people £11bn in three years.

However, researchers said that while insulation was vital for fighting fuel poverty, it was not a “magic bullet” for reducing energy use and should come alongside advice to conserve energy and programmes to install heat pumps in homes.

Researchers said it was hard to identify the exact causes of the rebound effect. However, they stated that turning up the heating, opening windows in stuffy rooms or building extensions could all contribute. They made clear that in circumstances such as the current cost of living crisis, it was possible that energy savings from insulation could be more significant and longer lasting.

“This study does not say that energy efficiency doesn’t work,” said Prof Laura Diaz Anadon, director of the Cambridge Centre for Environment, Energy and Natural Resource Governance, who co-authored the study. “These sorts of efficiency improvements can keep our houses warmer in the winter, cooler in the summer. They might reduce damp, or it might become more affordable to keep your house warm. There are plenty of benefits, especially for low-income households.

“However, home insulation alone is not a magic bullet. These results suggest that to also have the benefits of reduced gas consumption, additional measures are needed. It’s a good opportunity to do even more on things like heat pumps. For short-term reductions in gas consumption, it is also really important to provide consumers with information about what they can do to reduce their usage.”