Green plants affect the temperature and moisture of the microclimate in the surrounding area. The heat flux distribution on a west-facing wall of a two-story building covered with thick ivy was measured experimentally to investigate the cooling effect of the ivy. The shading due to the ivy can effectively reduce the radiation gain of the building in summer. The heat transfer mechanisms were also analyzed theoretically to determine the basis for the cooling effect of the green wall. The green wad reduced the peak-cooling load transferred through the west-facing wad by 28% on a clear summer day.