Rising energy costs and a warming climate create the need for innovative, low-carbon technologies that help cool buildings. We constructed four small buildings and instrumented them to measure the cooling effect of a green façade on their south and west walls. The green façade significantly reduced the temperature of the building’s ambient air, exterior surface, and interior air, and the heat flux through the vegetated wall. Using a mathematical model, we determined that the whole-building cooling load reduction (1.4 to 28.4%) depended on building construction, green façade placement, and especially whether the windows were covered. An emergy analysis of a south-facing green façade revealed that the total emergy consumed could be balanced by the electricity saved from reduced air conditioning if the cooling load was reduced by at least 14%. With thoughtful design and placement of a green façade it can sustainably and effectively help cool buildings.