Techniques and systems are becoming available for use in commercial greenhouses which provide a combination of insulation, thermal storage and overheating protection. The use of these techniques and systems increases the inherent ability of the greenhouse structure to use incident solar radiation to meet the space heating load. A monthly calculation technique for the prediction of heating loads in greenhouses with the use of temperature occurrence data organized into night and day periods and the consideration of solar radiation inputs to the greenhouse and its conversion into sensible heat by the crop is developed and presented. It is an accurate technique for the prediction of heating loads which can use readily available weather data and which can be aided by a hand-held calculator. These predictions can consider the effects of various types of glazing materials, different night and day temperature setpoints, the use of night insulation and different kinds of crops.