Windbreaks constitute a traditional methodology adopted to reduce heat loss from greenhouses. In this study, the influence of a windbreak on the heat loss from a heated greenhouse was investigated. Energy balance and leakage measurements were conducted simultaneously at a plastic-film-covered greenhouse with a forced air heater and a single-layer thermal curtain. A windbreak covered with a plastic net (porosity of 0.60) was built 7.0 m away from the windward side of the greenhouse. The heat transfer coefficients of the greenhouse were derived from the cases where the net was rolled up and deployed on the windbreak. The overall heat transfer coefficient of the entire greenhouse (U value) was reduced by the net-covered windbreak. The reduction increased as the windspeed increased: the windbreak reduced the U value by 11.9% at a windspeed of 0.5 m s^-1, and by 22.4% at a windspeed of 3.0 m s^-1. The effect of the windbreak on the U value was mainly due to the well-known dependency of leakage on windspeed. The windbreak also influenced the U value owing to the reduction of the overall heat transfer coefficient of the greenhouse cover (K value) in windy and radiative cooled conditions.