This research describes the effects of injecting rigid polyurethane foam into walls and fitting double glazing or inner panes to sash windows as simple renovation methods for houses, based on estimations of CO 2 emissions from heating/cooling and LCCO 2 (Life Cycle CO 2). We estimated CO 2 emissions and LCCO 2 up to 2020 in Japan and obtained the following results. (1) Injecting rigid polyurethane foam into walls in detached houses, where there is little thermal insulation, reduces the LCCO 2 significantly. (2) In regions with mild climates like Tokyo, renovating windows brings higher LCCO 2 reductions in houses insulated to 1992 Criteria for the Rational Use of Energy in Houses 1) than in those insulated to conventional levels or the 1980 criteria. (3) When choosing one of the two methods above, we should take into account the initial insulation level, the house structure, and the region. (4) Domestic CO 2 emissions for houses in Japan can be reduced by 13% to 19% by 2020 with the use of simple renovation methods for thermal insulation.