The building industry plays an important role in economic and social development, but it is also a major source of carbon emissions. However, to date, there is no mature design theory for low-carbon buildings. In this paper, a low-carbon optimization design method is presented for roof insulation using carbon emissions assessment method, comprehensive economic analysis model, and evaluation index. The traditional economic analysis model is extended to cope with carbon emissions of insulation, and a new comprehensiveeconomic analysis model is developed to evaluate economic performances of insulation. Moreover, a balanced index consisting of a carbon reduction factorand an additional economic inputs factor is proposed to assess carbon reduction of insulation. The validation results show that carbon emission cost can apparently improve economic performances of insulation. The economic benefits of insulation increase gradually with an increase in the service life. The ranking of balanced indexes of insulation in the low-temperature granary roof concerned is recycled EPS > recycled XPS > non-recycled EPS > non-recycled XPS > recycled RW > recycled PU > non-recycled RW > non-recycled PU. The low-carbon optimization design method presented can provide a useful tool in the low-carbon design of roof insulation in real buildings conveniently.