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This article evaluates the incentive impact of the 2006 increase from 25 % to 40 % in the tax credit on residential energy-efficiency improvement for homeowners who have lived for two years or less in a dwelling built before 1977. We estimate a triple-differences model combined with matching on exhaustive fiscal data. If the credit rate had stayed at 25 %, one in fifteen eligible households among households filing tax returns and living in buildings completed between 1969 and 1976 would not have used this tax credit in 2006; the proportion would have risen to one in eight in 2007 and 2008. Unless we assume substantial CO2 emission reductions after energy conservation investments, our model shows that public spending per tonne of CO2 not emitted would exceed €32, the social cost of carbon in France.